diff --git a/MQTT_IMPLEMENTATION.md b/MQTT_IMPLEMENTATION.md
index 0df96e676d..f18fed751f 100644
--- a/MQTT_IMPLEMENTATION.md
+++ b/MQTT_IMPLEMENTATION.md
@@ -285,7 +285,7 @@ flash regardless of target.
 
 #### Compile-time fresh-install defaults (`src/helpers/MQTTDefaults.h`)
 
-Optional PlatformIO `build_flags` override defaults written when `/mqtt_prefs` is first created. They do **not** change existing saved prefs on upgrade or reflash (unless `/mqtt_prefs` is erased).
+Optional PlatformIO `build_flags` override defaults used when `/mqtt.json` is first created. They do **not** change existing saved prefs on upgrade or reflash (unless `/mqtt.json` is erased).
 
 | Macro | Default | Notes |
 |-------|---------|-------|
diff --git a/MQTT_INTERNALS.md b/MQTT_INTERNALS.md
index d0de03985d..bc1e13b2c8 100644
--- a/MQTT_INTERNALS.md
+++ b/MQTT_INTERNALS.md
@@ -8,7 +8,8 @@ Developer-facing notes on how the MQTT observer feature is structured in the cod
 - `src/helpers/bridges/MQTTBridge.h` - MQTT bridge class definition
 - `src/helpers/bridges/MQTTBridge.cpp` - MQTT bridge implementation
 - `src/helpers/MQTTPresets.h` - Preset definitions, CA certificates, and lookup functions
-- `src/helpers/MQTTDefaults.h` - Compile-time defaults for fresh `/mqtt_prefs`
+- `src/helpers/MQTTDefaults.h` - Compile-time defaults for fresh `/mqtt.json`
+- `src/helpers/MQTTPrefsSerializer.h` - Versioned, semantic observer JSON schema
 - `src/helpers/MQTTMessageBuilder.h` - JSON message formatting utilities
 - `src/helpers/MQTTMessageBuilder.cpp` - JSON message formatting implementation
 - `src/helpers/JWTHelper.h` - JWT token generation for Ed25519-based authentication
@@ -30,8 +31,9 @@ The observer feature is kept out of upstream-tracked files through three mechani
   `resolveAlertScope`, `beginDeferredOtaUpdate`). The example apps override them
   behind `#ifdef WITH_MQTT_BRIDGE`.
 - **Separate settings file** — observer settings (MQTT slots, WiFi, timezone, SNMP,
-  radio watchdog, fault alerts) live in the `MQTTPrefs` struct persisted to
-  `/mqtt_prefs`, keeping `NodePrefs` / `/com_prefs` aligned with the upstream layout.
+  radio watchdog, fault alerts) live in the runtime `MQTTPrefs` object and are
+  field-serialized to `/mqtt.json`, keeping `NodePrefs` / `/prefs.json` aligned with
+  upstream. The two files are independent transactions, not one atomic snapshot.
 
 Remaining integration points in upstream files:
 - `examples/simple_repeater/MyMesh.{h,cpp}`, `examples/simple_room_server/MyMesh.{h,cpp}` -
@@ -164,75 +166,132 @@ which packet events non-MQTT bridges capture. The MQTT bridge ignores `bridge.so
 favour of independent `mqtt.rx` / `mqtt.tx` controls. Everything MQTT-specific lives under
 `mqtt.*` (shared settings), `mqttN.*` (per-slot broker config), `wifi.*`, and `timezone.*`.
 
-### `/mqtt_prefs` file format
-
-`/mqtt_prefs` is written with an 8-byte `MQTTPrefsHeader` (`magic`, `version`,
-`payload_len`) followed by the raw `MQTTPrefs` payload. The magic is
-`{0xF5, 'M', 'Q', 'P'}` — its leading non-ASCII byte can never collide with the first
-bytes of a legacy (headerless) file, whose payload begins with the `mqtt_origin`
-string. Bump `MQTT_PREFS_VERSION` when the payload layout changes incompatibly; a file
-whose version this firmware doesn't recognize is left untouched and the in-memory prefs
-fall back to defaults (no downgrade, no misread). `saveMQTTPrefs()` also refuses to
-write while such a file is present (`_mqtt_prefs_hold`), so a `set` command after a
-firmware downgrade can't clobber the newer config — observer settings changed in that
-state simply don't persist. The frozen legacy layouts are pinned with `static_assert`s
-in `MQTTPrefsStorage.h`, so every target build re-verifies the fleet's file offsets.
-
-Adding a field to the current version stays backward compatible: append it to the end
-of `MQTTPrefs`, give the older exact payload length an explicit decoder boundary, and
-leave the missing tail at its default. The packet-filter addition follows that rule:
-the prior 2864-byte v1 payload loads with all six filters set to `all`, while the
-full payload is 2876 bytes.
-
-#### The downgrade contract
-
-**Within a version tag the layout is append-only, and a longer payload is always
-readable.** A file written by a later build starts with this binary's exact baseline,
-so `classify()` reads that prefix and ignores the tail. A downgraded node keeps its
-WiFi credentials, broker slots, and every other setting it understands; the only thing
-it loses is the settings the newer build added.
-
-That asymmetry is the whole point. Refusing the file costs the operator the network
-itself — `/mqtt_prefs` holds `wifi_ssid`/`wifi_password` as well as the broker config,
-so a node that falls back to defaults has no WiFi, no portal, and no OTA, recoverable
-only over serial. Reading it costs a feature's settings. Losing later settings is the
-acceptable half of that trade; losing the node is not.
-
-The tail survives until something actually writes. `saveMQTTPrefs()` rewrites at this
-binary's own length, so a rollback that changes no observer setting and is later rolled
-forward keeps the newer fields intact — only an explicit `set` while downgraded drops
-them. The boot log says so when it happens.
-
-**A change that is not a pure append MUST bump `MQTT_PREFS_VERSION`.** The version
-check is what makes the rule above safe: a different tag is still refused outright and
-the file preserved, because the bytes may no longer mean what this binary thinks. Never
-reorder, resize, or repurpose an existing field within a version.
-
-Note the rule is only as old as the build that implements it. Firmware already deployed
-carries the *previous* decoder, which rejects any longer v1 payload — so rolling back
-from this build to one shipped before it still falls back to defaults.
-`MQTTPrefsCodec::payloadLenFor()` covers that gap from the writing side: it returns the
-shortest length that still round-trips the configuration, so a node keeps writing 2864
-bytes until a packet filter actually holds something, and clearing the last non-default
-filter puts it back. That mitigation can be retired once no supported downgrade target
-predates the contract; the contract itself is the durable half.
-
-Shorter payloads keep their existing, stricter treatment: a short length must match a
-boundary that really shipped (`MQTT_PREFS_V1_*_PAYLOAD_SIZE`), because raw prefs have
-no checksum and an arbitrary short size cannot be trusted to mean anything.
+### `/mqtt.json` file format
+
+Observer preferences use the same `ConfigSerializer` object notation as upstream
+`/prefs.json`: semantic unquoted keys, quoted strings, decimal numbers, and nested
+objects. Schema version 1 requires the root `version:1` field, which must be the
+first property of the root object. The main shape is:
+
+```text
+{version:1,
+ wifi:{ssid:"...",password:"...",power_save:1},
+ time:{timezone:"...",utc_offset:0,ntp_server:"..."},
+ mqtt:{origin:"...",iata:"SEA",packets_enabled:1,raw_enabled:0,
+       tx_enabled:2,rx_enabled:1,
+       status:{enabled:1,interval_ms:300000},
+       neighbors:{enabled:0,interval_ms:86400000},
+       owner:{public_key:"...",email:"..."},
+       slot1:{preset:"analyzer-us",host:"",port:0,username:"",password:"",
+              token:"",topic:"",audience:"",packet_filter:65535},
+       ... slot2 through slot6 ...},
+ snmp:{enabled:0,community:"public"},
+ radio:{watchdog_min:5},
+ alert:{enabled:0,psk_hex:"",wifi_minutes:30,mqtt_minutes:240,
+        rate_limit_min:60,hashtag:"",region:""}}
+```
+
+Keys may contain digits after their first character, which permits the readable
+`slot1` ... `slot6` names. Every schema key fits `ConfigSerializer`'s 15-character
+visible-key limit. Known strings and numbers use strict parsing: duplicates, overlong
+strings, malformed decimals, and overflow reject the complete file. A supported file
+with a semantically out-of-range value is repaired to that field's safe default and
+rewritten atomically. Unknown fields are ignored only within the serializer's general
+limits (15-character keys, 127-byte decoded values, and six nested object levels below the root).
+
+Loading always starts with defaults and parses into a separate heap scratch object.
+The live preferences change only after the complete file parses, has an explicit
+supported version, and passes validation. A missing/future version, syntax error,
+overlength value, or allocation/read failure leaves `/mqtt.json` untouched, runs this
+boot on defaults, and holds observer saves so a later CLI/WebConfig write cannot erase
+the opaque source. If an observer setter cannot commit its JSON transaction, it restores
+the pre-command in-memory preferences and does not restart or apply a bridge change as
+though the setting were durable.
+
+Saves stream to `/mqtt.json.tmp` through a sticky short-write detector while computing
+size and checksum. The firmware closes and rereads the temp, verifies size/checksum,
+parses it into another scratch object, then publishes with
+`/mqtt.json` -> `/mqtt.json.bak` and temp -> primary. Failure safety is paid for in
+transient heap: a CLI setter holds one `MQTTPrefs` rollback snapshot (2876 bytes) for the
+whole command, and the save allocates one more for normalization defaults (released
+before file I/O) and then one for verification — about 5.6 KiB peak above baseline.
+Each allocation is checked, so exhaustion refuses the setting rather than crashing.
+
+If publishing fails partway, the transaction is rolled back rather than left for boot
+recovery: the backup is renamed back over the empty primary and the verified temp is
+discarded. This is what makes the setter's "change rolled back" reply true — without it
+the next boot would promote that temp and activate the value the CLI just refused. When
+the rollback itself cannot complete, the save reports an indeterminate outcome instead,
+the artifacts stay for recovery, and the CLI says the flash state is unresolved. Further
+saves are refused until then, because a transaction cannot start while a temp or backup
+exists. Boot recovery selects the usable
+primary/temp/backup without overwriting an opaque future or corrupt primary. A valid
+future-version temp that reached the rename phase wins over the stale backup and is held
+for newer firmware. If a temp claims a future version but uses grammar this firmware
+cannot parse, or cannot be classified because scratch allocation fails, recovery renames
+nothing at all: it reads the last usable backup straight out of `/mqtt.json.bak`, leaves
+the primary name empty, and holds all observer writes. The empty primary name is the only
+record that the candidate had already passed the backup rename, so publishing the backup
+would make the candidate indistinguishable from a stale artifact — and the next boot, the
+one with enough heap or new enough firmware to finally read it, would delete it. Leaving
+the names alone means that boot promotes the candidate through the ordinary temp rule, or
+falls back to the backup if it turns out to be definitively corrupt. A
+definitively corrupt/incomplete current-version temp may be discarded for that backup.
+If power fails during the very first migration, there is no JSON primary or backup yet;
+recovery discards a definitively invalid temp so the intact `/mqtt_prefs` source can be
+loaded and the migration retried. An uncertain/future temp is still preserved and held.
+
+Unknown slot preset names are repaired to `none`, never to a build-specific default
+broker. Duplicate known presets from historical firmware are preserved on load; current
+CLI and WebConfig setters prevent creating new duplicates without silently changing an
+existing deployment during migration.
+
+Schema changes that reinterpret existing names or values must increment `version`.
+Additive fields may remain in version 1 only when old readers can safely ignore them
+within the limits above. A future version is treated as opaque rather than partially
+loading its known-looking fields. Literal schema keys are compile-time checked against
+the 15-character visible-key limit so a new version-1 field cannot accidentally violate
+that downgrade contract.
+
+Every future version must also keep `version` as the first root property. Older firmware
+detects a future file by probing that field with its own grammar, and the probe stops at
+the first construct it cannot tokenize (an array, an overlong key, a value type it does
+not expect). Syntax a newer schema introduces *before* the version field therefore makes
+its files look corrupt rather than future, which costs them the preservation guarantee
+above: as the temp of an interrupted commit such a file is discardable instead of
+retained. `test_mqtt_prefs_serializer` pins both the ordering and that consequence.
+
+#### Downgrade and rollback
+
+The old `/mqtt_prefs` binary is read only for one-time migration and is deliberately
+not updated or deleted. When it exists and is usable, it is the exact pre-migration
+rollback snapshot. Firmware older than this JSON change will therefore see stale
+observer settings after a downgrade. A fresh JSON-only install has no binary observer
+configuration to recover on downgrade.
+
+The two files are intentionally not reconciled. If an operator changes observer settings
+while running old firmware, those changes update only `/mqtt_prefs`. Rolling forward to
+this firmware makes the existing `/mqtt.json` authoritative again, so rollback-era edits
+are ignored unless the operator exports and reapplies them. Conversely, JSON-only settings
+cannot be recovered by old firmware. This is a rollback snapshot, not bidirectional sync.
+
+`LegacyV1MQTTPrefs` and the older pre-slot/3-slot/6-slot structs are frozen migration
+ABIs with size/offset assertions. The runtime `MQTTPrefs` layout is not an on-flash ABI.
 
 ### Settings upgrade / migration
 
 `loadPrefs()` handles every historical on-device format one-time at boot:
-- **`/mqtt_prefs`** — if the file has the version header it is read directly. Otherwise
+- **`/mqtt_prefs` -> `/mqtt.json`** — if the legacy file has the version header its
+  frozen v1 layout is field-copied. Otherwise
   it is a legacy headerless file and its layout is detected by size: pre-slot
   (`OldMQTTPrefs`), 3-slot (`ThreeSlotMQTTPrefs`), or the 6-slot layout shipped on
   `observer-firmware` back when it was named `mqtt-bridge-implementation-flex`
   (`Legacy6SlotMQTTPrefs`). Each is field-copied into
-  the current compact `MQTTPrefs` and re-saved with the version header — which also
+  the runtime `MQTTPrefs` and saved as schema-versioned JSON — which also
   drops the vestigial `_legacy_*` fields the flex layout carried mid-struct. This is a
   one-time rewrite; every deployed device performs it on its first boot of versioned
-  firmware, after which all reads take the header path.
+  firmware, after which `/mqtt.json` is authoritative. The binary source remains as
+  a rollback snapshot and is never dual-written.
   The pre-slot (`OldMQTTPrefs`) copy maps the old single-broker keys onto slots:
   `mqtt.analyzer.us = on` → slot 1 `analyzer-us`, `mqtt.analyzer.eu = on` → slot 2
   `analyzer-eu`, and a configured `mqtt.server` / `mqtt.port` / `mqtt.username` /
@@ -241,7 +300,7 @@ no checksum and an arbitrary short size cannot be trusted to mean anything.
 - **`/com_prefs`** — a file written by fork firmware that predates the `MQTTPrefs` split
   (a zero-filled MQTT gap plus a trailing observer block) is detected by size; the
   trailing SNMP / radio-watchdog / fault-alert settings and the `rx_boosted_gain` /
-  `flood_max_*` fields are recovered, carried into `/mqtt_prefs`, and both files are
+  `flood_max_*` fields are recovered, carried into `/mqtt.json`, and the active files are
   rewritten in the current formats.
 - Settings the pre-split firmware stored *inside* the `/com_prefs` MQTT gap (the MQTT
   slot/WiFi config itself) are **not** recovered — users upgrading from firmware that
diff --git a/examples/simple_repeater/MyMesh.cpp b/examples/simple_repeater/MyMesh.cpp
index 7e209cfb2c..226455033e 100644
--- a/examples/simple_repeater/MyMesh.cpp
+++ b/examples/simple_repeater/MyMesh.cpp
@@ -1050,7 +1050,7 @@ MyMesh::MyMesh(mesh::MainBoard &board, mesh::Radio &radio, mesh::MillisecondCloc
   // _prefs.agc_reset_interval = 7;  // 28 seconds (secs/4)
 #endif
   // Observer defaults (radio_watchdog, alert.*, snmp.*) moved to applyMQTTDefaults()
-  // in MQTTDefaults.h — they live in /mqtt_prefs now, not NodePrefs.
+  // in MQTTDefaults.h — they live in /mqtt.json now, not NodePrefs.
 
   // bridge defaults
   _prefs.bridge_enabled = 1;    // enabled
@@ -1066,7 +1066,7 @@ MyMesh::MyMesh(mesh::MainBoard &board, mesh::Radio &radio, mesh::MillisecondCloc
   _prefs.gps_interval = 0;
   _prefs.advert_loc_policy = ADVERT_LOC_PREFS;
 
-  // MQTT/WiFi/timezone/radio_watchdog defaults live in /mqtt_prefs now (see applyMQTTDefaults).
+  // MQTT/WiFi/timezone/radio_watchdog defaults live in /mqtt.json now (see applyMQTTDefaults).
 
   _prefs.adc_multiplier = 0.0f; // 0.0f means use default board multiplier
 
diff --git a/examples/simple_repeater/MyMesh.h b/examples/simple_repeater/MyMesh.h
index a232ef5202..e4c3a3f7d9 100644
--- a/examples/simple_repeater/MyMesh.h
+++ b/examples/simple_repeater/MyMesh.h
@@ -285,6 +285,11 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks
   void savePrefs() override {
     _cli.savePrefs(_fs);
   }
+#ifdef WITH_MQTT_BRIDGE
+  bool saveObserverPrefs() override {
+    return _cli.saveObserverPrefs(_fs);
+  }
+#endif
 
   void sendFloodScoped(const TransportKey& scope, mesh::Packet* pkt, uint32_t delay_millis, uint8_t path_hash_size);
 
diff --git a/examples/simple_room_server/MyMesh.cpp b/examples/simple_room_server/MyMesh.cpp
index 9fe5a9c8a8..02efbd43d8 100644
--- a/examples/simple_room_server/MyMesh.cpp
+++ b/examples/simple_room_server/MyMesh.cpp
@@ -891,7 +891,7 @@ MyMesh::MyMesh(mesh::MainBoard &board, mesh::Radio &radio, mesh::MillisecondCloc
   _prefs.radio_fem_txgain = 0;
 
   // Observer defaults (alert.*, etc.) moved to applyMQTTDefaults() — they live
-  // in /mqtt_prefs now, not NodePrefs.
+  // in /mqtt.json now, not NodePrefs.
 
   // bridge defaults (same as repeater)
   _prefs.bridge_enabled = 1;    // enabled
@@ -900,7 +900,7 @@ MyMesh::MyMesh(mesh::MainBoard &board, mesh::Radio &radio, mesh::MillisecondCloc
   _prefs.bridge_baud = 115200;  // baud rate
   _prefs.bridge_channel = 1;    // channel 1
 
-  // MQTT/WiFi/timezone defaults live in /mqtt_prefs now (see applyMQTTDefaults).
+  // MQTT/WiFi/timezone defaults live in /mqtt.json now (see applyMQTTDefaults).
 
   next_post_idx = 0;
   next_client_idx = 0;
diff --git a/examples/simple_room_server/MyMesh.h b/examples/simple_room_server/MyMesh.h
index b0eb4e8f33..65de77ede9 100644
--- a/examples/simple_room_server/MyMesh.h
+++ b/examples/simple_room_server/MyMesh.h
@@ -289,6 +289,11 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks
   void savePrefs() override {
     _cli.savePrefs(_fs);
   }
+#ifdef WITH_MQTT_BRIDGE
+  bool saveObserverPrefs() override {
+    return _cli.saveObserverPrefs(_fs);
+  }
+#endif
 
   void sendFloodScoped(const TransportKey& scope, mesh::Packet* pkt, uint32_t delay_millis, uint8_t path_hash_size);
 
diff --git a/src/helpers/CommonCLI.cpp b/src/helpers/CommonCLI.cpp
index 427d456bfd..bf01552778 100644
--- a/src/helpers/CommonCLI.cpp
+++ b/src/helpers/CommonCLI.cpp
@@ -6,6 +6,7 @@
 #include "MQTTPrefsAtomicStore.h"
 #include <RTClib.h>
 #include <Utils.h>
+#include <new>
 
 #ifndef BRIDGE_MAX_BAUD
 #define BRIDGE_MAX_BAUD 115200
@@ -23,6 +24,7 @@
 #include "MQTTDefaults.h"
 #include "MQTTPrefsCodec.h"
 #include "MQTTPrefsRecovery.h"
+#include "MQTTPrefsSerializer.h"
 #endif
 
 // Believe it or not, this std C function is busted on some platforms!
@@ -94,7 +96,8 @@ void CommonCLI::loadPrefs(FILESYSTEM* fs) {
     _prefs->bridge_pkt_src = 1;  // Default to RX (logRx) for new installs
   }
 #ifdef WITH_MQTT_BRIDGE
-  // Load observer preferences (MQTT/WiFi/timezone/SNMP/alert) from /mqtt_prefs.
+  // Load observer preferences (MQTT/WiFi/timezone/SNMP/alert) from /mqtt.json,
+  // migrating the old /mqtt_prefs binary when JSON does not exist yet.
   // Readers (MQTTBridge, AlertReporter, observer CLI) use _mqtt_prefs directly —
   // these fields no longer exist in NodePrefs, so there is nothing to sync.
   MQTTPrefsAtomicStore::LegacyUpgradeGate legacy_upgrade(
@@ -119,13 +122,13 @@ void CommonCLI::loadPrefs(FILESYSTEM* fs) {
     }
   }
   // mqtt_rx_enabled: new field appended to end of MQTTPrefs. On upgrade from older firmware,
-  // the shorter /mqtt_prefs file won't contain it, so it keeps the default value (1 = on)
+  // a shorter legacy /mqtt_prefs file won't contain it, so it keeps the default value (1 = on)
   // set by setMQTTPrefsDefaults(). No explicit migration needed.
 #endif
 
   // Republish legacy binary prefs as /prefs.json. Old-format files also carried a
   // trailing observer block, which loadPrefsInt() recovered into _legacy_tail; wait
-  // for loadMQTTPrefs() to commit that to /mqtt_prefs first. The legacy file is left
+  // for loadMQTTPrefs() to commit that to /mqtt.json first. The legacy file is left
   // on flash either way, so a deferred or failed save just retries on the next boot.
 #ifdef WITH_MQTT_BRIDGE
   if (loaded_from_legacy || _com_prefs_needs_upgrade) {
@@ -134,7 +137,7 @@ void CommonCLI::loadPrefs(FILESYSTEM* fs) {
       legacy_upgrade.recordComPrefsRewrite();
       _com_prefs_needs_upgrade = false;
     } else {
-      MESH_DEBUG_PRINTLN("Prefs: deferring /prefs.json migration until /mqtt_prefs commits");
+      MESH_DEBUG_PRINTLN("Prefs: deferring /prefs.json migration until /mqtt.json commits");
     }
   }
 #else
@@ -376,13 +379,17 @@ bool CommonCLI::savePrefs(FILESYSTEM* fs, bool save_mqtt) {
 #ifdef WITH_MQTT_BRIDGE
   // Observer config (MQTT/WiFi/timezone/SNMP/alert) is persisted separately. The
   // observer CLI writes _mqtt_prefs directly, so no NodePrefs->MQTTPrefs sync runs.
-  // Runs regardless of the NodePrefs result so a failed JSON write cannot strand it.
+  // Ordinary NodePrefs callers leave save_mqtt false; migration is the only
+  // workflow that may explicitly combine these independent transactions.
   if (save_mqtt) saveMQTTPrefs(fs);
 #endif
   return success;
 }
 
 #ifdef WITH_MQTT_BRIDGE
+static const uint32_t MQTT_JSON_FNV1A_OFFSET_BASIS = 2166136261u;
+static const uint32_t MQTT_JSON_FNV1A_PRIME = 16777619u;
+
 // Set default values for MQTT preferences (used when file doesn't exist or is corrupted)
 static void setMQTTPrefsDefaults(MQTTPrefs* prefs) {
   applyMQTTDefaults(prefs);
@@ -396,6 +403,170 @@ static File openMqttPrefsRead(FILESYSTEM* fs, const char* path = "/mqtt_prefs")
 #endif
 }
 
+enum class JsonPrefsLoadResult : uint8_t {
+  Loaded,
+  LoadedWithRepairs,
+  UnsupportedVersion,
+  FutureClaimed,
+  Invalid,
+  NoMemory,
+};
+
+static bool jsonPrefsLoaded(JsonPrefsLoadResult result) {
+  return result == JsonPrefsLoadResult::Loaded ||
+         result == JsonPrefsLoadResult::LoadedWithRepairs;
+}
+
+static JsonPrefsLoadResult loadMqttJsonFile(FILESYSTEM* fs, const char* path,
+                                             MQTTPrefs* output) {
+  if (output == nullptr) return JsonPrefsLoadResult::Invalid;
+  applyMQTTDefaults(output);
+
+  // Probe the root version without applying the v1 schema. A future version
+  // may legitimately change an existing field's type, and must still be held
+  // opaquely rather than misclassified as corrupt and rolled back.
+  File version_file = openMqttPrefsRead(fs, path);
+  if (!version_file) return JsonPrefsLoadResult::Invalid;
+  MQTTPrefsVersionProbe version_probe;
+  const bool version_parsed = version_probe.loadSerial(version_file);
+  version_file.close();
+  if (version_probe.hasFutureVersion()) {
+    return version_parsed ? JsonPrefsLoadResult::UnsupportedVersion
+                          : JsonPrefsLoadResult::FutureClaimed;
+  }
+
+  File file = openMqttPrefsRead(fs, path);
+  if (!file) return JsonPrefsLoadResult::Invalid;
+  MQTTPrefsSerializer serializer(output);
+  const bool parsed = serializer.loadSerial(file);
+  file.close();
+  if (!parsed) return JsonPrefsLoadResult::Invalid;
+  if (serializer.hasFutureVersion()) return JsonPrefsLoadResult::UnsupportedVersion;
+  bool repaired = false;
+  if (!serializer.apply(&repaired)) return JsonPrefsLoadResult::Invalid;
+  return repaired ? JsonPrefsLoadResult::LoadedWithRepairs : JsonPrefsLoadResult::Loaded;
+}
+
+// Parse `path` through a heap scratch object and publish it over `dest` only
+// once the whole file is known good, so a late failure cannot leave the live
+// preferences half-loaded.
+static JsonPrefsLoadResult adoptMqttJsonFile(FILESYSTEM* fs, const char* path,
+                                              MQTTPrefs* dest) {
+  MQTTPrefs* scratch = new (std::nothrow) MQTTPrefs;
+  if (scratch == nullptr) return JsonPrefsLoadResult::NoMemory;
+  const JsonPrefsLoadResult result = loadMqttJsonFile(fs, path, scratch);
+  if (jsonPrefsLoaded(result)) memcpy(dest, scratch, sizeof(*dest));
+  delete scratch;
+  return result;
+}
+
+static MQTTPrefsRecovery::FileState mqttJsonFileState(FILESYSTEM* fs, const char* path) {
+  if (!fs->exists(path)) return MQTTPrefsRecovery::FileState::Missing;
+  MQTTPrefs* scratch = new (std::nothrow) MQTTPrefs;
+  if (scratch == nullptr) return MQTTPrefsRecovery::FileState::Indeterminate;
+  const JsonPrefsLoadResult result = loadMqttJsonFile(fs, path, scratch);
+  delete scratch;
+  if (result == JsonPrefsLoadResult::UnsupportedVersion) {
+    // Syntax and the mandatory version field were valid, so this can be a
+    // fully verified transaction written by newer firmware. Keep it distinct
+    // from a torn/corrupt temp during recovery.
+    return MQTTPrefsRecovery::FileState::FutureUsable;
+  }
+  if (result == JsonPrefsLoadResult::FutureClaimed) {
+    return MQTTPrefsRecovery::FileState::FutureClaimed;
+  }
+  return result == JsonPrefsLoadResult::Loaded ||
+         result == JsonPrefsLoadResult::LoadedWithRepairs
+      ? MQTTPrefsRecovery::FileState::Usable
+      : MQTTPrefsRecovery::FileState::Preserve;
+}
+
+// hold: observer writes must not replace whatever was left on disk.
+// run_from_backup: nothing was renamed and /mqtt.json does not exist; this boot
+// reads the last committed image straight out of /mqtt.json.bak.
+struct MqttJsonRecovery {
+  bool hold;
+  bool run_from_backup;
+};
+
+static MqttJsonRecovery recoverMqttJsonFiles(FILESYSTEM* fs) {
+  const MQTTPrefsRecovery::FileState primary = mqttJsonFileState(fs, "/mqtt.json");
+  const MQTTPrefsRecovery::FileState temp = mqttJsonFileState(fs, "/mqtt.json.tmp");
+  const MQTTPrefsRecovery::FileState backup = mqttJsonFileState(fs, "/mqtt.json.bak");
+  const MQTTPrefsRecovery::Action action = MQTTPrefsRecovery::select(primary, temp, backup);
+
+  if (action == MQTTPrefsRecovery::Action::KeepPrimary) {
+    if (primary == MQTTPrefsRecovery::FileState::Usable) {
+      if (!MQTTPrefsRecovery::uncertain(temp) &&
+          temp != MQTTPrefsRecovery::FileState::Missing) {
+        fs->remove("/mqtt.json.tmp");
+      }
+      if (!MQTTPrefsRecovery::uncertain(backup) &&
+          backup != MQTTPrefsRecovery::FileState::Missing) {
+        fs->remove("/mqtt.json.bak");
+      }
+    }
+    return {MQTTPrefsRecovery::uncertain(primary) ||
+                MQTTPrefsRecovery::uncertain(temp) ||
+                MQTTPrefsRecovery::uncertain(backup),
+            false};
+  }
+  if (action == MQTTPrefsRecovery::Action::DiscardTemp) {
+    if (fs->remove("/mqtt.json.tmp")) {
+      MESH_DEBUG_PRINTLN("MQTT: discarded incomplete first-migration JSON temp");
+      return {false, false};
+    }
+    MESH_DEBUG_PRINTLN("MQTT: could not discard incomplete /mqtt.json temp; source held");
+    return {true, false};
+  }
+  if (action == MQTTPrefsRecovery::Action::PromoteTemp) {
+    if (fs->rename("/mqtt.json.tmp", "/mqtt.json")) {
+      if (temp == MQTTPrefsRecovery::FileState::Usable &&
+          backup != MQTTPrefsRecovery::FileState::Missing) {
+        fs->remove("/mqtt.json.bak");
+      }
+      MESH_DEBUG_PRINTLN("MQTT: recovered /mqtt.json from transaction temp");
+      return {MQTTPrefsRecovery::uncertain(temp) ||
+                  MQTTPrefsRecovery::uncertain(backup),
+              false};
+    }
+    MESH_DEBUG_PRINTLN("MQTT: could not recover /mqtt.json temp; files preserved");
+    return {true, false};
+  }
+  if (action == MQTTPrefsRecovery::Action::UseBackupHeld) {
+    // Deliberately rename nothing. The empty primary name is what records that
+    // the interrupted commit had already moved the old image aside, and a boot
+    // that cannot classify the candidate must not spend that record: a later
+    // boot with more heap, or firmware that understands the candidate, promotes
+    // it through the ordinary rule instead of deleting it as a stale artifact.
+    MESH_DEBUG_PRINTLN("MQTT: unresolved /mqtt.json candidate; running the backup in place and holding writes");
+    return {true, true};
+  }
+  if (action == MQTTPrefsRecovery::Action::RunDefaultsHeld) {
+    // Same reasoning, with no image this firmware can run: renaming either file
+    // would spend transaction state that a later boot still needs.
+    MESH_DEBUG_PRINTLN("MQTT: unresolved /mqtt.json candidate and no runnable backup; "
+                       "using defaults (files preserved)");
+    return {true, false};
+  }
+  if (action == MQTTPrefsRecovery::Action::PromoteBackup) {
+    if (fs->rename("/mqtt.json.bak", "/mqtt.json")) {
+      if (backup == MQTTPrefsRecovery::FileState::Usable &&
+          !MQTTPrefsRecovery::uncertain(temp) &&
+          temp != MQTTPrefsRecovery::FileState::Missing) {
+        fs->remove("/mqtt.json.tmp");
+      }
+      MESH_DEBUG_PRINTLN("MQTT: recovered /mqtt.json from transaction backup");
+      return {MQTTPrefsRecovery::uncertain(temp) ||
+                  MQTTPrefsRecovery::uncertain(backup),
+              false};
+    }
+    MESH_DEBUG_PRINTLN("MQTT: could not recover /mqtt.json backup; files preserved");
+    return {true, false};
+  }
+  return {false, false};
+}
+
 static MQTTPrefsRecovery::FileState mqttPrefsFileState(FILESYSTEM* fs, const char* path) {
   if (!fs->exists(path)) return MQTTPrefsRecovery::FileState::Missing;
   File file = openMqttPrefsRead(fs, path);
@@ -432,6 +603,14 @@ static bool recoverMqttPrefsFiles(FILESYSTEM* fs) {
     }
     return false;
   }
+  if (action == MQTTPrefsRecovery::Action::DiscardTemp) {
+    if (fs->remove("/mqtt_prefs.tmp")) {
+      MESH_DEBUG_PRINTLN("MQTT: discarded incomplete legacy transaction temp");
+      return false;
+    }
+    MESH_DEBUG_PRINTLN("MQTT: could not discard incomplete /mqtt_prefs temp; source held");
+    return true;
+  }
   if (action == MQTTPrefsRecovery::Action::PromoteTemp) {
     if (fs->rename("/mqtt_prefs.tmp", "/mqtt_prefs")) {
       // A usable temp is now the committed primary. Its backup is necessarily
@@ -446,6 +625,13 @@ static bool recoverMqttPrefsFiles(FILESYSTEM* fs) {
     MESH_DEBUG_PRINTLN("MQTT: could not recover /mqtt_prefs temp; files preserved");
     return true;
   }
+  if (action == MQTTPrefsRecovery::Action::UseBackupHeld ||
+      action == MQTTPrefsRecovery::Action::RunDefaultsHeld) {
+    // Unreachable for the binary format, whose classifier never reports an
+    // uncertain state. Hold rather than fall through to "nothing to do" if a
+    // later classifier change makes it reachable.
+    return true;
+  }
   if (action == MQTTPrefsRecovery::Action::PromoteBackup) {
     if (fs->rename("/mqtt_prefs.bak", "/mqtt_prefs")) {
       // Symmetric case: a usable backup is now primary, so any interrupted
@@ -463,30 +649,33 @@ static bool recoverMqttPrefsFiles(FILESYSTEM* fs) {
   return false;
 }
 
-// Filesystem adapter for MQTTPrefsAtomicStore. It writes the new image to
-// /mqtt_prefs.tmp and verifies its size. Publishing is a recoverable SPIFFS
+// Filesystem adapter for the ConfigSerializer image. It writes to
+// /mqtt.json.tmp and verifies its size and checksum. Publishing is a recoverable SPIFFS
 // transaction: primary -> .bak, then tmp -> primary, then best-effort backup
 // cleanup. A power loss at every boundary leaves at least one recoverable file.
-class MQTTPrefsFileStore {
+class MQTTPrefsJsonFileStore {
 public:
-  explicit MQTTPrefsFileStore(FILESYSTEM* fs) : _fs(fs) {}
+  explicit MQTTPrefsJsonFileStore(FILESYSTEM* fs) : _fs(fs) {}
 
   bool begin() {
     _finished = false;
     _open = false;
     _owns_temp = false;
+    _owns_backup = false;
     _bytes_written = 0;
-    // Recovery owns stale artifacts. Do not delete them here: a failed commit
-    // may have moved the old primary to .bak and left a verified temp that the
-    // next boot must choose between. Refusing the save is safer than erasing an
-    // image this firmware cannot decode.
-    if (_fs->exists("/mqtt_prefs.tmp") || _fs->exists("/mqtt_prefs.bak")) return false;
+    _expected_crc = MQTT_JSON_FNV1A_OFFSET_BASIS;
+    // Recovery owns stale artifacts. Do not delete them here: a power cut, or a
+    // failed commit that could not be rolled back, may have moved the old
+    // primary to .bak and left a verified temp that the next boot must choose
+    // between. Refusing the save is safer than erasing an image this firmware
+    // cannot decode.
+    if (_fs->exists("/mqtt.json.tmp") || _fs->exists("/mqtt.json.bak")) return false;
 #if defined(NRF52_PLATFORM) || defined(STM32_PLATFORM)
-    _file = _fs->open("/mqtt_prefs.tmp", FILE_O_WRITE);
+    _file = _fs->open("/mqtt.json.tmp", FILE_O_WRITE);
 #elif defined(RP2040_PLATFORM)
-    _file = _fs->open("/mqtt_prefs.tmp", "w");
+    _file = _fs->open("/mqtt.json.tmp", "w");
 #else
-    _file = _fs->open("/mqtt_prefs.tmp", "w", true);
+    _file = _fs->open("/mqtt.json.tmp", "w", true);
 #endif
     _open = _file;
     _owns_temp = _open;
@@ -496,6 +685,9 @@ class MQTTPrefsFileStore {
   size_t write(const uint8_t* bytes, size_t size) {
     if (!_open) return 0;
     const size_t written = _file.write(bytes, size);
+    for (size_t i = 0; i < written; ++i) {
+      _expected_crc = (_expected_crc ^ bytes[i]) * MQTT_JSON_FNV1A_PRIME;
+    }
     _bytes_written += written;
     return written;
   }
@@ -505,12 +697,33 @@ class MQTTPrefsFileStore {
     _file.close();
     _open = false;
 #if defined(RP2040_PLATFORM)
-    File verify = _fs->open("/mqtt_prefs.tmp", "r");
+    File verify = _fs->open("/mqtt.json.tmp", "r");
 #else
-    File verify = _fs->open("/mqtt_prefs.tmp");
+    File verify = _fs->open("/mqtt.json.tmp");
 #endif
     if (!verify) return false;
-    const bool complete = verify.size() == _bytes_written;
+    uint32_t actual_crc = MQTT_JSON_FNV1A_OFFSET_BASIS;
+    size_t actual_size = 0;
+    bool read_failed = false;
+    uint8_t buf[64];
+    while (verify.available() > 0) {
+      // Arduino File implementations normally return a byte count, but some
+      // Stream implementations use -1 for a read error. Keep that sentinel
+      // signed so it cannot become a huge size_t and overrun this buffer.
+      const int count = static_cast<int>(verify.read(buf, sizeof(buf)));
+      if (count <= 0) {
+        read_failed = count < 0;
+        break;
+      }
+      actual_size += static_cast<size_t>(count);
+      for (int i = 0; i < count; ++i) {
+        actual_crc = (actual_crc ^ buf[i]) * MQTT_JSON_FNV1A_PRIME;
+      }
+    }
+    const bool complete = !read_failed &&
+                          verify.size() == _bytes_written &&
+                          actual_size == _bytes_written &&
+                          actual_crc == _expected_crc;
     verify.close();
     if (!complete) return false;
     _finished = true;
@@ -521,30 +734,76 @@ class MQTTPrefsFileStore {
     if (!_finished) return false;
     // SPIFFS refuses rename(tmp, existing_dest). Move the existing image to a
     // recoverable backup first, then publish temp into the now-empty primary.
-    // Never remove either image after a failed boundary; boot recovery selects
-    // the completed temp or restores the backup.
-    if (_fs->exists("/mqtt_prefs.bak")) return false;
-    if (_fs->exists("/mqtt_prefs") && !_fs->rename("/mqtt_prefs", "/mqtt_prefs.bak")) {
-      return false;
+    // A power cut at either boundary is resolved by boot recovery; a failure
+    // that returns here is undone by rollbackFailedCommit().
+    if (_fs->exists("/mqtt.json.bak")) return false;
+    if (_fs->exists("/mqtt.json")) {
+      if (!_fs->rename("/mqtt.json", "/mqtt.json.bak")) return false;
+      _owns_backup = true;
     }
-    if (!_fs->rename("/mqtt_prefs.tmp", "/mqtt_prefs")) return false;
+    if (!_fs->rename("/mqtt.json.tmp", "/mqtt.json")) return false;
     // Cleanup failure is non-fatal: the new primary is published and recovery
     // will remove a known-good stale backup on a later boot.
-    if (_fs->exists("/mqtt_prefs.bak")) _fs->remove("/mqtt_prefs.bak");
+    if (_fs->exists("/mqtt.json.bak")) _fs->remove("/mqtt.json.bak");
+    return true;
+  }
+
+  // Undo a commit that failed midway. Without this, a failed publish can leave
+  // the old primary parked in .bak and the verified temp still holding the new
+  // image, which boot recovery then promotes — so a setter that told the
+  // operator the change was rolled back would be wrong after the next reset.
+  //
+  // Republishing the backup is the operation that matters: once the primary
+  // name is occupied, recovery keeps it and treats the temp as stale, so the
+  // temp removal below is only housekeeping. Returns false when the
+  // pre-transaction state could not be restored and the outcome of the next
+  // boot is therefore uncertain.
+  bool rollbackFailedCommit() {
+    // Only ever restore the backup this transaction made. Anything else under
+    // that name predates the transaction and is recovery's to resolve.
+    if (_owns_backup && !_fs->exists("/mqtt.json") &&
+        !_fs->rename("/mqtt.json.bak", "/mqtt.json")) {
+      return false;
+    }
+    _owns_backup = false;
+    if (_owns_temp && _fs->exists("/mqtt.json.tmp") && !_fs->remove("/mqtt.json.tmp")) {
+      // A verified temp still outranks a missing primary during recovery, so
+      // this is only harmless if the primary name is occupied again.
+      return _fs->exists("/mqtt.json");
+    }
     return true;
   }
 
-  void abort() {
+  // Drop a temp that was written and byte-verified but then rejected. Returns
+  // false when it is still on flash and no primary outranks it: recovery
+  // promotes a lone temp, so the caller cannot claim the change was undone.
+  bool discardFinishedTemp() {
+    if (_open) _file.close();
+    _open = false;
+    bool removed = true;
+    if (_owns_temp && _fs->exists("/mqtt.json.tmp")) {
+      removed = _fs->remove("/mqtt.json.tmp");
+    }
+    _finished = false;
+    _owns_temp = false;
+    return removed || _fs->exists("/mqtt.json");
+  }
+
+  // Same disposition contract as discardFinishedTemp(). Only unfinished staging
+  // is disposable here: once finish() has verified the temp,
+  // rollbackFailedCommit() has already decided its fate, and a temp that
+  // survived that is one recovery must resolve after reset.
+  bool abort() {
     if (_open) _file.close();
     _open = false;
-    // Once finish() has verified the temp, commit may already have moved the
-    // primary to .bak. Keep the temp on a commit failure so recovery can
-    // publish it (or fall back to .bak) after reset.
-    if (_owns_temp && !_finished && _fs->exists("/mqtt_prefs.tmp")) {
-      _fs->remove("/mqtt_prefs.tmp");
+    bool removed = true;
+    if (_owns_temp && !_finished && _fs->exists("/mqtt.json.tmp")) {
+      removed = _fs->remove("/mqtt.json.tmp");
     }
     _finished = false;
     _owns_temp = false;
+    _owns_backup = false;
+    return removed || _fs->exists("/mqtt.json");
   }
 
 private:
@@ -553,32 +812,105 @@ class MQTTPrefsFileStore {
   bool _open = false;
   bool _finished = false;
   bool _owns_temp = false;
+  // This transaction moved the old primary to /mqtt.json.bak, so a failed
+  // publish may put it back. Never true for a backup it did not create.
+  bool _owns_backup = false;
   size_t _bytes_written = 0;
+  uint32_t _expected_crc = MQTT_JSON_FNV1A_OFFSET_BASIS;
+};
+
+class MQTTPrefsStoreStream : public Stream {
+public:
+  explicit MQTTPrefsStoreStream(MQTTPrefsJsonFileStore* store) : _store(store) {}
+
+  size_t write(uint8_t byte) override { return write(&byte, 1); }
+  size_t write(const uint8_t* buffer, size_t size) override {
+    if (!_ok || _store == nullptr) return 0;
+    const size_t written = _store->write(buffer, size);
+    if (written != size) _ok = false;
+    return written;
+  }
+  int available() override { return 0; }
+  int read() override { return -1; }
+  int peek() override { return -1; }
+  bool ok() const { return _ok; }
+
+private:
+  MQTTPrefsJsonFileStore* _store;
+  bool _ok = true;
 };
 
 #endif  // WITH_MQTT_BRIDGE
 
 #ifdef WITH_MQTT_BRIDGE
 
-static const char* mqttPrefsSaveResultName(MQTTPrefsAtomicStore::Result result) {
-  switch (result) {
-    case MQTTPrefsAtomicStore::Result::BeginFailed: return "begin";
-    case MQTTPrefsAtomicStore::Result::HeaderWriteFailed: return "header write";
-    case MQTTPrefsAtomicStore::Result::PayloadWriteFailed: return "payload write";
-    case MQTTPrefsAtomicStore::Result::FinishFailed: return "close";
-    case MQTTPrefsAtomicStore::Result::CommitFailed: return "rename";
-    case MQTTPrefsAtomicStore::Result::Committed: return "committed";
-  }
-  return "unknown";
-}
-
 void CommonCLI::loadMQTTPrefs(
     FILESYSTEM* fs, MQTTPrefsAtomicStore::LegacyUpgradeGate* legacy_upgrade) {
   setMQTTPrefsDefaults(&_mqtt_prefs);
+  const MqttJsonRecovery recovery = recoverMqttJsonFiles(fs);
+  _mqtt_prefs_hold = recovery.hold;
+
+  // An interrupted commit left a candidate this boot cannot classify, so
+  // recovery renamed nothing. Read the last committed image out of the backup
+  // rather than publishing it: the transaction filenames must survive this boot
+  // intact for a later one to promote the candidate.
+  if (recovery.run_from_backup) {
+    _legacy_tail.valid = false;
+    const JsonPrefsLoadResult backup_result =
+        adoptMqttJsonFile(fs, "/mqtt.json.bak", &_mqtt_prefs);
+    if (jsonPrefsLoaded(backup_result)) {
+      MESH_DEBUG_PRINTLN("MQTT: running the /mqtt.json backup; candidate preserved and writes held");
+    } else {
+      setMQTTPrefsDefaults(&_mqtt_prefs);
+      MESH_DEBUG_PRINTLN("MQTT: /mqtt.json backup became unreadable; using defaults (files preserved)");
+    }
+    return;
+  }
+
+  if (_mqtt_prefs_hold && !fs->exists("/mqtt.json") &&
+      (fs->exists("/mqtt.json.tmp") || fs->exists("/mqtt.json.bak"))) {
+    _legacy_tail.valid = false;
+    MESH_DEBUG_PRINTLN("MQTT: unresolved /mqtt.json recovery files; using defaults (files preserved)");
+    return;
+  }
+
+  // The JSON file is authoritative once it exists. Never fall back to the
+  // stale binary snapshot when JSON is corrupt, unreadable, or from a future
+  // schema: preserve it and run defaults until an operator resolves it.
+  if (fs->exists("/mqtt.json")) {
+    const JsonPrefsLoadResult json_result =
+        adoptMqttJsonFile(fs, "/mqtt.json", &_mqtt_prefs);
+    if (jsonPrefsLoaded(json_result)) {
+      _legacy_tail.valid = false;
+      if (json_result == JsonPrefsLoadResult::LoadedWithRepairs) {
+        MESH_DEBUG_PRINTLN("MQTT: repaired out-of-range values in /mqtt.json");
+        if (!_mqtt_prefs_hold && !saveMQTTPrefs(fs)) {
+          _mqtt_prefs_hold = true;
+          MESH_DEBUG_PRINTLN("MQTT: could not persist /mqtt.json repairs; source held");
+        }
+      }
+      return;
+    }
+    _mqtt_prefs_hold = true;
+    _legacy_tail.valid = false;
+    if (json_result == JsonPrefsLoadResult::NoMemory) {
+      MESH_DEBUG_PRINTLN("MQTT: no memory to validate /mqtt.json; source preserved");
+    } else if (json_result == JsonPrefsLoadResult::UnsupportedVersion) {
+      MESH_DEBUG_PRINTLN("MQTT: /mqtt.json uses a future version; using defaults (file preserved)");
+    } else if (json_result == JsonPrefsLoadResult::FutureClaimed) {
+      MESH_DEBUG_PRINTLN(
+          "MQTT: /mqtt.json claims a future version but uses unknown grammar; "
+          "using defaults (file preserved)");
+    } else {
+      MESH_DEBUG_PRINTLN("MQTT: /mqtt.json is invalid or unreadable; using defaults (file preserved)");
+    }
+    return;
+  }
+
   // Complete or preserve an interrupted SPIFFS transaction before decoding.
   // A failed recovery leaves the artifacts untouched and blocks this boot from
   // replacing them with defaults through a later CLI save.
-  _mqtt_prefs_hold = recoverMqttPrefsFiles(fs);
+  _mqtt_prefs_hold = _mqtt_prefs_hold || recoverMqttPrefsFiles(fs);
   bool has_observer_fields = false;
   bool mqtt_rewrite_pending = false;
   bool migrated_legacy_mqtt = false;
@@ -600,24 +932,34 @@ void CommonCLI::loadMQTTPrefs(
       } else if (plan.source == MQTTPrefsCodec::Source::Current) {
         file = openMqttPrefsRead(fs);
         MQTTPrefsHeader header;
-        if (!file || file.read((uint8_t *)&header, sizeof(header)) != sizeof(header) ||
-            file.read((uint8_t *)&_mqtt_prefs, plan.payload_len) != plan.payload_len) {
+        LegacyV1MQTTPrefs* old_prefs = new (std::nothrow) LegacyV1MQTTPrefs;
+        if (old_prefs) memset(old_prefs, 0, sizeof(*old_prefs));
+        if (!old_prefs || !file || file.read((uint8_t *)&header, sizeof(header)) != sizeof(header) ||
+            file.read((uint8_t *)old_prefs, plan.payload_len) != plan.payload_len) {
           setMQTTPrefsDefaults(&_mqtt_prefs);
           _mqtt_prefs_hold = true;
           MESH_DEBUG_PRINTLN("MQTT: /mqtt_prefs read failed, using defaults (file preserved)");
+        } else if (!MQTTPrefsCodec::isPlausibleV1(*old_prefs, plan.payload_len)) {
+          setMQTTPrefsDefaults(&_mqtt_prefs);
+          _mqtt_prefs_hold = true;
+          MESH_DEBUG_PRINTLN("MQTT: /mqtt_prefs v1 content failed plausibility checks; source preserved");
         } else {
+          MQTTPrefsCodec::migrateV1(*old_prefs, plan.payload_len, &_mqtt_prefs);
           has_observer_fields = plan.observer_fields_present;
+          mqtt_rewrite_pending = true;
+          migrated_legacy_mqtt = true;
           // Written by a later build with appended fields. Everything this
           // binary knows loaded normally; say so, because the next `set` will
           // rewrite the file at this length and drop the newer settings.
           if (file_size - sizeof(MQTTPrefsHeader) > plan.payload_len) {
             MESH_DEBUG_PRINTLN(
                 "MQTT: /mqtt_prefs written by newer firmware (%u > %u bytes); "
-                "config loaded, newer settings ignored and dropped on next save",
+                "known settings loaded; newer binary fields remain in the rollback snapshot",
                 (unsigned)(file_size - sizeof(MQTTPrefsHeader)),
                 (unsigned)plan.payload_len);
           }
         }
+        delete old_prefs;
         if (file) file.close();
       } else if (plan.rewrite_legacy) {
         bool migrated = false;
@@ -674,16 +1016,18 @@ void CommonCLI::loadMQTTPrefs(
             case MQTTPrefsCodec::Source::LegacySixSlotAudience:
             case MQTTPrefsCodec::Source::LegacySixSlotAudienceRx:
             case MQTTPrefsCodec::Source::LegacySixSlot: {
-              Legacy6SlotMQTTPrefs old_prefs = {};
-              if (file.read((uint8_t *)&old_prefs, plan.payload_len) == plan.payload_len) {
+              Legacy6SlotMQTTPrefs* old_prefs = new (std::nothrow) Legacy6SlotMQTTPrefs;
+              if (old_prefs) memset(old_prefs, 0, sizeof(*old_prefs));
+              if (old_prefs && file.read((uint8_t *)old_prefs, plan.payload_len) == plan.payload_len) {
                 if (MQTTPrefsCodec::isPlausibleLegacy(plan.source,
-                                                       (const uint8_t *)&old_prefs, plan.payload_len)) {
-                  MQTTPrefsCodec::migrateLegacySixSlot(old_prefs, plan.source, &_mqtt_prefs);
+                                                       (const uint8_t *)old_prefs, plan.payload_len)) {
+                  MQTTPrefsCodec::migrateLegacySixSlot(*old_prefs, plan.source, &_mqtt_prefs);
                   migrated = true;
                 } else {
                   MESH_DEBUG_PRINTLN("MQTT: /mqtt_prefs legacy content failed plausibility checks");
                 }
               }
+              delete old_prefs;
               break;
             }
             default:
@@ -742,9 +1086,9 @@ void CommonCLI::loadMQTTPrefs(
   if (mqtt_rewrite_pending) {
     legacy_upgrade->requireMqttRewrite();
     if (migrated_legacy_mqtt) {
-      MESH_DEBUG_PRINTLN("MQTT: Migrating headerless /mqtt_prefs to versioned layout");
+      MESH_DEBUG_PRINTLN("MQTT: Migrating binary /mqtt_prefs to /mqtt.json");
     } else {
-      MESH_DEBUG_PRINTLN("MQTT: Persisting observer tail into /mqtt_prefs before /com_prefs compaction");
+      MESH_DEBUG_PRINTLN("MQTT: Persisting observer tail into /mqtt.json before /com_prefs compaction");
     }
     if (saveMQTTPrefs(fs)) {
       legacy_upgrade->recordMqttSave(true);
@@ -754,7 +1098,7 @@ void CommonCLI::loadMQTTPrefs(
       // untouched; the next boot can recover the tail and retry the transaction.
       _mqtt_prefs_hold = true;
       legacy_upgrade->recordMqttSave(false);
-      MESH_DEBUG_PRINTLN("MQTT: /mqtt_prefs migration save failed; legacy files preserved and held");
+      MESH_DEBUG_PRINTLN("MQTT: /mqtt.json migration save failed; legacy files preserved and held");
     }
   }
 }
@@ -763,26 +1107,85 @@ bool CommonCLI::saveMQTTPrefs(FILESYSTEM* fs) {
   if (_mqtt_prefs_hold) {
     // Loading deliberately preserved the source file. Do not replace it with this
     // boot's defaults after an unsupported, corrupt, or temporarily failed read.
-    MESH_DEBUG_PRINTLN("MQTT: /mqtt_prefs held, not overwriting");
+    MESH_DEBUG_PRINTLN("MQTT: observer preference source held, not overwriting /mqtt.json");
     return false;
   }
 
-  // Write header and payload sequentially so the transaction needs no second
-  // full-size (2.8 KiB) staging buffer on constrained targets. The length is
-  // the shortest that still round-trips this config, so a node with default
-  // packet filters keeps writing a payload older firmware can read.
-  const size_t payload_len = MQTTPrefsCodec::payloadLenFor(_mqtt_prefs);
-  const MQTTPrefsHeader header = MQTTPrefsCodec::makeHeader(payload_len);
-  MQTTPrefsFileStore store(fs);
-  const MQTTPrefsAtomicStore::Result result = MQTTPrefsAtomicStore::write(
-      store, (const uint8_t *)&header, sizeof(header),
-      (const uint8_t *)&_mqtt_prefs, payload_len);
-  if (!MQTTPrefsAtomicStore::committed(result)) {
-    MESH_DEBUG_PRINTLN("MQTT: atomic /mqtt_prefs save failed at %s; source preserved",
-                       mqttPrefsSaveResultName(result));
+  MQTTPrefs* repair_defaults = new (std::nothrow) MQTTPrefs;
+  if (repair_defaults == nullptr) {
+    MESH_DEBUG_PRINTLN("MQTT: no memory to normalize observer settings before save");
     return false;
   }
-  return true;
+  setMQTTPrefsDefaults(repair_defaults);
+  MQTTPrefsSerializer serializer(&_mqtt_prefs, repair_defaults);
+  // The serializer hierarchy copies the default values it needs. Release this
+  // large temporary before file I/O and the independent verification scratch.
+  delete repair_defaults;
+  bool repaired = false;
+  if (!serializer.normalize(&repaired)) return false;
+  if (repaired) MESH_DEBUG_PRINTLN("MQTT: normalized out-of-range observer settings before save");
+
+  MQTTPrefsJsonFileStore store(fs);
+  bool verify_oom = false;
+  const MQTTPrefsAtomicStore::VerifiedImageResult result =
+      MQTTPrefsAtomicStore::writeVerifiedImage(
+          store,
+          [&]() -> bool {
+            MQTTPrefsStoreStream stream(&store);
+            return serializer.saveSerial(stream) && stream.ok();
+          },
+          [&]() -> bool {
+            MQTTPrefs* verify = new (std::nothrow) MQTTPrefs;
+            if (verify == nullptr) {
+              verify_oom = true;
+              return false;
+            }
+            const JsonPrefsLoadResult verify_result =
+                loadMqttJsonFile(fs, "/mqtt.json.tmp", verify);
+            delete verify;
+            // saveSerial() emits already-normalized values. Needing another
+            // repair here means save/load is not idempotent.
+            return verify_result == JsonPrefsLoadResult::Loaded;
+          });
+
+  // A recovery file this firmware may still promote is holding the new image,
+  // so the change cannot be reported as rolled back. No further save can start
+  // until it is resolved: begin() refuses while either the temp or the backup
+  // exists, which is exactly the state that gets us here.
+  if (MQTTPrefsAtomicStore::saveOutcomeUnresolved(result)) _observer_save_indeterminate = true;
+
+  switch (result) {
+    case MQTTPrefsAtomicStore::VerifiedImageResult::Committed:
+      return true;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::BeginFailed:
+      MESH_DEBUG_PRINTLN("MQTT: atomic /mqtt.json save failed at begin; source preserved");
+      break;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::WriteFailed:
+      MESH_DEBUG_PRINTLN("MQTT: atomic /mqtt.json save failed during write; source preserved");
+      break;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::FinishFailed:
+      MESH_DEBUG_PRINTLN("MQTT: atomic /mqtt.json save failed during checksum verification; source preserved");
+      break;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::VerifyFailed:
+      if (verify_oom) {
+        MESH_DEBUG_PRINTLN("MQTT: no memory to validate /mqtt.json temp; source preserved");
+      } else {
+        MESH_DEBUG_PRINTLN("MQTT: generated /mqtt.json temp failed schema validation; source preserved");
+      }
+      break;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::CleanupIndeterminate:
+      MESH_DEBUG_PRINTLN("MQTT: rejected /mqtt.json temp could not be removed and no primary outranks it; "
+                         "recovery files preserved");
+      break;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::CommitFailed:
+      MESH_DEBUG_PRINTLN("MQTT: atomic /mqtt.json save failed during rename; transaction rolled back");
+      break;
+    case MQTTPrefsAtomicStore::VerifiedImageResult::CommitIndeterminate:
+      MESH_DEBUG_PRINTLN("MQTT: atomic /mqtt.json save failed during rename and could not be rolled back; "
+                         "recovery files preserved");
+      break;
+  }
+  return false;
 }
 
 #endif
@@ -1419,6 +1822,15 @@ void CommonCLI::handleSetCmd(uint32_t sender_timestamp, char* command, char* rep
       strcpy(reply, "Error: delay must be between 0-10000 ms");
     }
   } else if (memcmp(config, "bridge.source ", 14) == 0) {
+#ifdef WITH_MQTT_BRIDGE
+    MQTTPrefs* observer_rollback = new (std::nothrow) MQTTPrefs;
+    if (observer_rollback == nullptr) {
+      strcpy(reply, "Error: insufficient memory to update observer setting");
+      return;
+    }
+    memcpy(observer_rollback, &_mqtt_prefs, sizeof(*observer_rollback));
+    const uint8_t old_bridge_pkt_src = _prefs->bridge_pkt_src;
+#endif
     _prefs->bridge_pkt_src = memcmp(&config[14], "rx", 2) == 0;
 #ifdef WITH_MQTT_BRIDGE
     if (_prefs->bridge_pkt_src == 1) {
@@ -1428,6 +1840,15 @@ void CommonCLI::handleSetCmd(uint32_t sender_timestamp, char* command, char* rep
       _mqtt_prefs.mqtt_rx_enabled = 0;
       _mqtt_prefs.mqtt_tx_enabled = 1;
     }
+    _observer_prefs_rollback = observer_rollback;
+    if (!persistObserverPrefs(reply)) {
+      _prefs->bridge_pkt_src = old_bridge_pkt_src;
+      _observer_prefs_rollback = nullptr;
+      delete observer_rollback;
+      return;
+    }
+    _observer_prefs_rollback = nullptr;
+    delete observer_rollback;
 #endif
     savePrefs();
     strcpy(reply, "OK");
diff --git a/src/helpers/CommonCLI.h b/src/helpers/CommonCLI.h
index e0a0a71b86..42287ffbfd 100644
--- a/src/helpers/CommonCLI.h
+++ b/src/helpers/CommonCLI.h
@@ -73,7 +73,7 @@ class NodePrefs : public ConfigSerializer {
   uint8_t extra_sf[4];
 
   // NOTE: observer settings (MQTT/WiFi/timezone/SNMP/alert) are not in NodePrefs.
-  // They live in MQTTPrefs, persisted separately to /mqtt_prefs, so this struct
+  // They live in MQTTPrefs, persisted separately to /mqtt.json, so this struct
   // stays aligned with upstream. See struct MQTTPrefs below.
 
 private:
@@ -228,6 +228,13 @@ struct LegacyObserverTail {
 class CommonCLICallbacks {
 public:
   virtual void savePrefs() = 0;
+#ifdef WITH_MQTT_BRIDGE
+  virtual bool saveObserverPrefs() = 0;
+#else
+  virtual bool saveObserverPrefs() {
+    return false;
+  }
+#endif
   virtual const char* getFirmwareVer() = 0;
   virtual const char* getBuildDate() = 0;
   virtual const char* getRole() = 0;
@@ -359,10 +366,18 @@ class CommonCLI {
   char tmp[PRV_KEY_SIZE*2 + 4];
 #ifdef WITH_MQTT_BRIDGE
   MQTTPrefs _mqtt_prefs;
+  // Points at a per-command snapshot only while an observer setter is running.
+  // persistObserverPrefs() uses it to undo RAM mutations when flash commit fails.
+  const MQTTPrefs* _observer_prefs_rollback = nullptr;
   LegacyObserverTail _legacy_tail;
-  // /mqtt_prefs is newer, corrupt, or temporarily unreadable. The in-memory prefs
+  // /mqtt.json is newer, corrupt, or temporarily unreadable. The in-memory prefs
   // run on defaults and saveMQTTPrefs() must not overwrite the source file.
   bool _mqtt_prefs_hold = false;
+  // A failed publish could not be undone, so the next boot may still come up
+  // with the value that was refused. persistObserverPrefs() must not call that
+  // a rollback. Latched for the boot: the artifact left behind also makes every
+  // later transaction fail to begin, so the condition cannot clear itself.
+  bool _observer_save_indeterminate = false;
 #endif
   bool _com_prefs_needs_upgrade = false;  // old-format legacy prefs detected; rewrite once after load
 
@@ -384,6 +399,7 @@ class CommonCLI {
   // false to fall through to the base get/set parsing.
   bool handleObserverSetCmd(uint32_t sender_timestamp, const char* config, char* reply);
   bool handleObserverGetCmd(uint32_t sender_timestamp, const char* config, char* reply);
+  bool persistObserverPrefs(char* reply);
   // Observer-only top-level commands (ota check/update, tls.bundletest, alert test)
   // also live in CommonCLI_Observer.cpp; returns true if it handled the command.
   bool handleObserverCommand(uint32_t sender_timestamp, char* command, char* reply);
@@ -393,14 +409,17 @@ class CommonCLI {
       : _board(&board), _rtc(&rtc), _sensors(&sensors), _region_map(&region_map), _acl(&acl), _prefs(prefs), _callbacks(callbacks) { }
 
   void loadPrefs(FILESYSTEM* _fs);
-  bool savePrefs(FILESYSTEM* _fs, bool save_mqtt = true);
+  // Node preferences and observer preferences are separate transactions.
+  // Callers must explicitly request an observer save when they changed it.
+  bool savePrefs(FILESYSTEM* _fs, bool save_mqtt = false);
   void handleCommand(uint32_t sender_timestamp, char* command, char* reply);
   mesh::MainBoard* getBoard() { return _board; }
   uint8_t buildAdvertData(uint8_t node_type, uint8_t* app_data);
 #ifdef WITH_MQTT_BRIDGE
-  // Observer config (MQTT/WiFi/timezone/SNMP/alert), persisted to /mqtt_prefs.
+  // Observer config (MQTT/WiFi/timezone/SNMP/alert), persisted to /mqtt.json.
   // Exposed so the app can hand it to MQTTBridge/AlertReporter, which read these
   // fields directly (they no longer live in NodePrefs).
   MQTTPrefs* getObserverPrefs() const { return const_cast<MQTTPrefs*>(&_mqtt_prefs); }
+  bool saveObserverPrefs(FILESYSTEM* fs) { return saveMQTTPrefs(fs); }
 #endif
 };
diff --git a/src/helpers/CommonCLI_Observer.cpp b/src/helpers/CommonCLI_Observer.cpp
index c1b7e74a11..c6fcb60651 100644
--- a/src/helpers/CommonCLI_Observer.cpp
+++ b/src/helpers/CommonCLI_Observer.cpp
@@ -16,6 +16,7 @@
 #include "AlertReporter.h"  // for alertReporterBannedChannelMatch[Hex]()
 #include "MQTTObserverValidation.h"  // pure input validators (host-testable)
 #include <Utils.h>
+#include <new>
 #ifdef ESP_PLATFORM
 #include <WiFi.h>
 #include <WiFiClientSecure.h>
@@ -109,6 +110,46 @@ static bool valueTooLong(const char* val, size_t bufsize, char* reply, const cha
   return false;
 }
 
+static bool isObserverPrefsSetCommand(const char* config) {
+  return strncmp(config, "snmp", 4) == 0 ||
+      strncmp(config, "radio.watchdog", 14) == 0 ||
+      strncmp(config, "mqtt", 4) == 0 ||
+      strncmp(config, "wifi.", 5) == 0 ||
+      strncmp(config, "timezone", 8) == 0 ||
+      strncmp(config, "alert", 5) == 0;
+}
+
+// Keep observer setters atomic from the caller's perspective. The live object
+// is restored if its JSON transaction fails, and the snapshot pointer is valid
+// only for this synchronous command dispatch.
+class ObserverPrefsRollbackScope {
+public:
+  ObserverPrefsRollbackScope(const MQTTPrefs** active_snapshot,
+                             const MQTTPrefs& live,
+                             bool capture)
+      : _active_snapshot(active_snapshot) {
+    *_active_snapshot = nullptr;
+    if (capture) {
+      _snapshot = new (std::nothrow) MQTTPrefs;
+      if (_snapshot != nullptr) {
+        memcpy(_snapshot, &live, sizeof(*_snapshot));
+        *_active_snapshot = _snapshot;
+      }
+    }
+  }
+
+  ~ObserverPrefsRollbackScope() {
+    *_active_snapshot = nullptr;
+    delete _snapshot;
+  }
+
+  bool available() const { return _snapshot != nullptr; }
+
+private:
+  const MQTTPrefs** _active_snapshot;
+  MQTTPrefs* _snapshot = nullptr;
+};
+
 static const char* getMQTTPresetNameByIndex(int index) {
   if (index < MQTT_PRESET_COUNT) return MQTT_PRESETS[index].name;
   if (index == MQTT_PRESET_COUNT) return MQTT_PRESET_CUSTOM;
@@ -161,17 +202,44 @@ static void formatMQTTPresetListReply(char* reply, size_t reply_size, int start)
 }
 #endif
 
+bool CommonCLI::persistObserverPrefs(char* reply) {
+#ifdef WITH_MQTT_BRIDGE
+  if (_callbacks->saveObserverPrefs()) return true;
+  if (_observer_prefs_rollback != nullptr) {
+    memcpy(&_mqtt_prefs, _observer_prefs_rollback, sizeof(_mqtt_prefs));
+  }
+  // The running node is back on the old value either way, but only claim the
+  // change is gone when the write really was undone on flash.
+  if (_observer_save_indeterminate) {
+    strcpy(reply, "Error: setting not persisted; flash state unresolved, recheck after reboot");
+  } else {
+    strcpy(reply, "Error: setting not persisted; change rolled back");
+  }
+  return false;
+#else
+  (void)reply;
+  return false;
+#endif
+}
+
 bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* config, char* reply) {
 #ifdef WITH_MQTT_BRIDGE
+  const bool needs_snapshot = isObserverPrefsSetCommand(config);
+  ObserverPrefsRollbackScope rollback_scope(
+      &_observer_prefs_rollback, _mqtt_prefs, needs_snapshot);
+  if (needs_snapshot && !rollback_scope.available()) {
+    strcpy(reply, "Error: insufficient memory to update observer setting");
+    return true;
+  }
   bool handled = true;
   if (memcmp(config, "snmp.community ", 15) == 0) {
     if (valueTooLong(&config[15], sizeof(_mqtt_prefs.snmp_community), reply, "snmp.community")) return true;
     StrHelper::strncpy(_mqtt_prefs.snmp_community, &config[15], sizeof(_mqtt_prefs.snmp_community));
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK - restart to apply");
   } else if (memcmp(config, "snmp ", 5) == 0) {
     _mqtt_prefs.snmp_enabled = memcmp(&config[5], "on", 2) == 0;
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK - restart to apply");
   } else if (memcmp(config, "radio.watchdog ", 15) == 0) {
     const char* val = &config[15];
@@ -189,7 +257,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
         strcpy(reply, "Error: radio.watchdog must be 0-120 minutes");
       } else {
         _mqtt_prefs.radio_watchdog_minutes = (uint8_t)mins;
-        savePrefs();
+        if (!persistObserverPrefs(reply)) return true;
         if (mins == 0) {
           strcpy(reply, "OK - radio watchdog disabled");
         } else {
@@ -200,13 +268,13 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
 #ifdef WITH_MQTT_BRIDGE
   } else if (strcmp(config, "mqtt.origin") == 0) {
     _mqtt_prefs.mqtt_origin[0] = '\0';
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.origin ", 12) == 0) {
     if (valueTooLong(&config[12], sizeof(_mqtt_prefs.mqtt_origin), reply, "origin")) return true;
     StrHelper::strncpy(_mqtt_prefs.mqtt_origin, &config[12], sizeof(_mqtt_prefs.mqtt_origin));
     StrHelper::stripSurroundingQuotes(_mqtt_prefs.mqtt_origin, sizeof(_mqtt_prefs.mqtt_origin));
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.iata ", 10) == 0) {
     const char* iata = &config[10];
@@ -215,7 +283,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       // Empty clears the region code (meshcore-topic publishing stays disabled
       // until one is set). This keeps the pre-existing "clear IATA" capability.
       _mqtt_prefs.mqtt_iata[0] = '\0';
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridge();
       strcpy(reply, "OK - IATA cleared");
     } else {
@@ -228,22 +296,22 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
         for (int i = 0; _mqtt_prefs.mqtt_iata[i]; i++) {
           _mqtt_prefs.mqtt_iata[i] = toupper(_mqtt_prefs.mqtt_iata[i]);
         }
-        savePrefs();
+        if (!persistObserverPrefs(reply)) return true;
         _callbacks->restartBridge();
         strcpy(reply, "OK");
       }
     }
   } else if (memcmp(config, "mqtt.status ", 12) == 0) {
     _mqtt_prefs.mqtt_status_enabled = memcmp(&config[12], "on", 2) == 0;
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.packets ", 13) == 0) {
     _mqtt_prefs.mqtt_packets_enabled = memcmp(&config[13], "on", 2) == 0;
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.raw ", 9) == 0) {
     _mqtt_prefs.mqtt_raw_enabled = memcmp(&config[9], "on", 2) == 0;
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.tx ", 8) == 0) {
     if (memcmp(&config[8], "advert", 6) == 0) {
@@ -251,17 +319,17 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     } else {
       _mqtt_prefs.mqtt_tx_enabled = memcmp(&config[8], "on", 2) == 0 ? 1 : 0;
     }
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.rx ", 8) == 0) {
     _mqtt_prefs.mqtt_rx_enabled = memcmp(&config[8], "on", 2) == 0 ? 1 : 0;
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.interval ", 14) == 0) {
     uint32_t minutes = _atoi(&config[14]);
     if (minutes >= 1 && minutes <= 60) {
       _mqtt_prefs.mqtt_status_interval = minutes * 60000;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridge();
       sprintf(reply, "OK - interval set to %u minutes (%lu ms), bridge restarted", minutes, (unsigned long)_mqtt_prefs.mqtt_status_interval);
     } else {
@@ -274,7 +342,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     uint32_t hours = _atoi(&config[24]);
     if (hours >= MQTT_NEIGHBORS_MIN_INTERVAL_HOURS && hours <= MQTT_NEIGHBORS_MAX_INTERVAL_HOURS) {
       _mqtt_prefs.mqtt_neighbors_interval = hours * 3600000UL;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       sprintf(reply, "OK - neighbors interval set to %u hours (%lu ms)", (unsigned)hours,
               (unsigned long)_mqtt_prefs.mqtt_neighbors_interval);
     } else {
@@ -284,7 +352,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     // The mesh loop reads this live, so no bridge restart is needed; enabling it
     // triggers a discovery on the next eligible loop pass.
     _mqtt_prefs.mqtt_neighbors_enabled = memcmp(&config[15], "on", 2) == 0;
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
 #elif defined(WITH_MQTT_BRIDGE)
   } else if (memcmp(config, "mqtt.neighbors.interval ", 24) == 0 ||
@@ -303,7 +371,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       } else {
         StrHelper::strncpy(_mqtt_prefs.mqtt_ntp_server, host, sizeof(_mqtt_prefs.mqtt_ntp_server));
       }
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
 #ifdef ESP_PLATFORM
       // Queue a sync on the MQTT task (Core 0) but do NOT block: this handler
       // runs on the Arduino loop task, shared with mesh/radio processing and the
@@ -325,12 +393,12 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
   } else if (memcmp(config, "wifi.ssid ", 10) == 0) {
     if (valueTooLong(&config[10], sizeof(_mqtt_prefs.wifi_ssid), reply, "wifi.ssid")) return true;
     StrHelper::strncpy(_mqtt_prefs.wifi_ssid, &config[10], sizeof(_mqtt_prefs.wifi_ssid));
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "wifi.pwd ", 9) == 0) {
     if (valueTooLong(&config[9], sizeof(_mqtt_prefs.wifi_password), reply, "wifi.pwd")) return true;
     StrHelper::strncpy(_mqtt_prefs.wifi_password, &config[9], sizeof(_mqtt_prefs.wifi_password));
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "wifi.powersave ", 15) == 0) {
     const char* value = &config[15];
@@ -350,7 +418,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       strcpy(reply, "Error: must be none, min, or max");
     } else {
       _mqtt_prefs.wifi_power_save = ps_value;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
 #ifdef ESP_PLATFORM
       if (WiFi.status() == WL_CONNECTED) {
         wifi_ps_type_t ps_mode = (ps_value == 1) ? WIFI_PS_NONE :
@@ -374,13 +442,13 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
   } else if (memcmp(config, "timezone ", 9) == 0) {
     if (valueTooLong(&config[9], sizeof(_mqtt_prefs.timezone_string), reply, "timezone")) return true;
     StrHelper::strncpy(_mqtt_prefs.timezone_string, &config[9], sizeof(_mqtt_prefs.timezone_string));
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
   } else if (memcmp(config, "timezone.offset ", 16) == 0) {
     int8_t offset = _atoi(&config[16]);
     if (offset >= -12 && offset <= 14) {
       _mqtt_prefs.timezone_offset = offset;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       strcpy(reply, "OK");
     } else {
       strcpy(reply, "Error: timezone offset must be between -12 and +14");
@@ -397,20 +465,14 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
           strcmp(preset_name, MQTT_PRESET_CUSTOM) == 0 ||
           strcmp(preset_name, MQTT_PRESET_NONE) == 0) {
         // Reject duplicate presets (except "none" and "custom")
-        int dup_slot = -1;
-        if (findMQTTPreset(preset_name) != nullptr) {
-          for (int s = 0; s < MAX_MQTT_SLOTS; s++) {
-            if (s != slot && strcmp(_mqtt_prefs.mqtt_slot_preset[s], preset_name) == 0) {
-              dup_slot = s;
-              break;
-            }
-          }
-        }
+        const int dup_slot = findMQTTPreset(preset_name) == nullptr ? -1 :
+            mqttAssignedPresetSlot(_mqtt_prefs.mqtt_slot_preset,
+                                   preset_name, slot);
         if (dup_slot >= 0) {
           sprintf(reply, "Error: preset '%s' is already assigned to slot %d", preset_name, dup_slot + 1);
         } else {
           StrHelper::strncpy(_mqtt_prefs.mqtt_slot_preset[slot], preset_name, sizeof(_mqtt_prefs.mqtt_slot_preset[slot]));
-          savePrefs();
+          if (!persistObserverPrefs(reply)) return true;
           _callbacks->restartBridgeSlot(slot);
           // Check if the slot has everything it needs to connect
           const MQTTPresetDef* p = findMQTTPreset(preset_name);
@@ -469,7 +531,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     } else if (memcmp(subcmd, "server ", 7) == 0) {
       if (valueTooLong(&subcmd[7], sizeof(_mqtt_prefs.mqtt_slot_host[slot]), reply, "server")) return true;
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_host[slot], &subcmd[7], sizeof(_mqtt_prefs.mqtt_slot_host[slot]));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       // Reconfigure the slot so the new host reaches the live connection (other
       // custom-slot setters do the same; without it the change only applies on
       // the next reboot/bridge restart).
@@ -479,7 +541,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       int port = atoi(&subcmd[5]);
       if (port > 0 && port <= 65535) {
         _mqtt_prefs.mqtt_slot_port[slot] = port;
-        savePrefs();
+        if (!persistObserverPrefs(reply)) return true;
         _callbacks->restartBridgeSlot(slot);
         strcpy(reply, "OK");
       } else {
@@ -488,19 +550,19 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     } else if (memcmp(subcmd, "username ", 9) == 0) {
       if (valueTooLong(&subcmd[9], sizeof(_mqtt_prefs.mqtt_slot_username[slot]), reply, "username")) return true;
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_username[slot], &subcmd[9], sizeof(_mqtt_prefs.mqtt_slot_username[slot]));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridgeSlot(slot);
       strcpy(reply, "OK");
     } else if (memcmp(subcmd, "password ", 9) == 0) {
       if (valueTooLong(&subcmd[9], sizeof(_mqtt_prefs.mqtt_slot_password[slot]), reply, "password")) return true;
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_password[slot], &subcmd[9], sizeof(_mqtt_prefs.mqtt_slot_password[slot]));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridgeSlot(slot);
       strcpy(reply, "OK");
     } else if (memcmp(subcmd, "token ", 6) == 0) {
       if (valueTooLong(&subcmd[6], sizeof(_mqtt_prefs.mqtt_slot_token[slot]), reply, "token")) return true;
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_token[slot], &subcmd[6], sizeof(_mqtt_prefs.mqtt_slot_token[slot]));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridgeSlot(slot);
       sprintf(reply, "OK - slot %d token set", slot + 1);
     } else if (memcmp(subcmd, "topic ", 6) == 0) {
@@ -510,14 +572,14 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
         return true;
       } else {
         StrHelper::strncpy(_mqtt_prefs.mqtt_slot_topic[slot], &subcmd[6], sizeof(_mqtt_prefs.mqtt_slot_topic[slot]));
-        savePrefs();
+        if (!persistObserverPrefs(reply)) return true;
         _callbacks->restartBridgeSlot(slot);
         sprintf(reply, "OK - slot %d topic: %s", slot + 1, _mqtt_prefs.mqtt_slot_topic[slot]);
       }
     } else if (memcmp(subcmd, "audience ", 9) == 0) {
       if (valueTooLong(&subcmd[9], sizeof(_mqtt_prefs.mqtt_slot_audience[slot]), reply, "audience")) return true;
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_audience[slot], &subcmd[9], sizeof(_mqtt_prefs.mqtt_slot_audience[slot]));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridgeSlot(slot);
       if (_mqtt_prefs.mqtt_slot_audience[slot][0] != '\0') {
         sprintf(reply, "OK - slot %d JWT audience: %s", slot + 1, _mqtt_prefs.mqtt_slot_audience[slot]);
@@ -527,7 +589,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     } else if (memcmp(subcmd, "audience", 8) == 0 && subcmd[8] == '\0') {
       // "set mqttN.audience" with no value — clear the audience
       _mqtt_prefs.mqtt_slot_audience[slot][0] = '\0';
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->restartBridgeSlot(slot);
       sprintf(reply, "OK - slot %d JWT audience cleared (using username/password auth)", slot + 1);
     } else if (strcmp(subcmd, "filter") == 0 ||
@@ -539,20 +601,18 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
         strcpy(reply, "Error: filter must be all, none, or a CSV of types 0-15 / names (advert,txt_msg,...)");
       } else {
         _mqtt_prefs.mqtt_slot_packet_filter[slot] = filter_mask;
-        savePrefs();
+        if (!persistObserverPrefs(reply)) return true;
         char filter_text[MQTTPacketFilter::kFilterTextSize];
         MQTTPacketFilter::format(filter_mask, filter_text, sizeof(filter_text));
         snprintf(reply, 160, "OK - slot %d packet types: %s", slot + 1, filter_text);
-        // A non-default filter extends /mqtt_prefs past what pre-filter
-        // firmware can read (see MQTTPrefsCodec::payloadLenFor), so say when
-        // that cost buys nothing: slots beyond the runtime array are never
-        // published to on this board, the same warning `preset` gives.
+        // Slots beyond the runtime array are never published to on this board,
+        // so retain the same inactive-hardware warning `preset` gives.
         if (slot >= RUNTIME_MQTT_SLOTS &&
             filter_mask != MQTTPacketFilter::kAllPacketTypes) {
           size_t used = strlen(reply);
           if (used < 158) {
             snprintf(reply + used, 160 - used,
-                     " (slot inactive on this hardware; blocks firmware rollback)");
+                     " (slot inactive on this hardware)");
           }
         }
       }
@@ -562,21 +622,35 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
   } else if (memcmp(config, "mqtt.analyzer.us ", 17) == 0) {
     const int slot = 0;
     if (memcmp(&config[17], "on", 2) == 0) {
+      const int dup_slot = mqttAssignedPresetSlot(
+          _mqtt_prefs.mqtt_slot_preset, "analyzer-us", slot);
+      if (dup_slot >= 0) {
+        sprintf(reply, "Error: preset 'analyzer-us' is already assigned to slot %d",
+                dup_slot + 1);
+        return true;
+      }
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_preset[slot], "analyzer-us", sizeof(_mqtt_prefs.mqtt_slot_preset[slot]));
     } else {
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_preset[slot], MQTT_PRESET_NONE, sizeof(_mqtt_prefs.mqtt_slot_preset[slot]));
     }
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     _callbacks->restartBridgeSlot(slot);
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.analyzer.eu ", 17) == 0) {
     const int slot = 1;
     if (memcmp(&config[17], "on", 2) == 0) {
+      const int dup_slot = mqttAssignedPresetSlot(
+          _mqtt_prefs.mqtt_slot_preset, "analyzer-eu", slot);
+      if (dup_slot >= 0) {
+        sprintf(reply, "Error: preset 'analyzer-eu' is already assigned to slot %d",
+                dup_slot + 1);
+        return true;
+      }
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_preset[slot], "analyzer-eu", sizeof(_mqtt_prefs.mqtt_slot_preset[slot]));
     } else {
       StrHelper::strncpy(_mqtt_prefs.mqtt_slot_preset[slot], MQTT_PRESET_NONE, sizeof(_mqtt_prefs.mqtt_slot_preset[slot]));
     }
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     _callbacks->restartBridgeSlot(slot);
     strcpy(reply, "OK");
   } else if (memcmp(config, "mqtt.owner ", 11) == 0) {
@@ -585,11 +659,11 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       // Owner key is optional — empty clears it (previously this errored, so a
       // set key could never be removed via the portal/CLI).
       _mqtt_prefs.mqtt_owner_public_key[0] = '\0';
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       strcpy(reply, "OK - owner key cleared");
     } else if (mqttOwnerKeyValid(owner_key)) {
       StrHelper::strncpy(_mqtt_prefs.mqtt_owner_public_key, owner_key, sizeof(_mqtt_prefs.mqtt_owner_public_key));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       strcpy(reply, "OK");
     } else {
       strcpy(reply, "Error: public key must be 64 hex characters (32 bytes)");
@@ -597,7 +671,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
   } else if (memcmp(config, "mqtt.email ", 11) == 0) {
     if (valueTooLong(&config[11], sizeof(_mqtt_prefs.mqtt_email), reply, "email")) return true;
     StrHelper::strncpy(_mqtt_prefs.mqtt_email, &config[11], sizeof(_mqtt_prefs.mqtt_email));
-    savePrefs();
+    if (!persistObserverPrefs(reply)) return true;
     strcpy(reply, "OK");
 #endif
   } else if (memcmp(config, "alert ", 6) == 0) {
@@ -605,12 +679,12 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     const char* val = &config[6];
     if (memcmp(val, "on", 2) == 0 && (val[2] == 0 || val[2] == ' ')) {
       _mqtt_prefs.alert_enabled = 1;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->onAlertConfigChanged();
       strcpy(reply, "OK - alerts on");
     } else if (memcmp(val, "off", 3) == 0 && (val[3] == 0 || val[3] == ' ')) {
       _mqtt_prefs.alert_enabled = 0;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->onAlertConfigChanged();
       strcpy(reply, "OK - alerts off");
     } else {
@@ -625,7 +699,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     if (len == 0) {
       _mqtt_prefs.alert_psk_hex[0] = '\0';
       _mqtt_prefs.alert_hashtag[0] = '\0';
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->onAlertConfigChanged();
       strcpy(reply, "OK - alert.psk cleared (alerts disabled until configured)");
     } else if (val[0] == '#') {
@@ -659,7 +733,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
           // The new PSK is operator-supplied, so any previously-derived
           // hashtag name is no longer accurate provenance — drop it.
           _mqtt_prefs.alert_hashtag[0] = '\0';
-          savePrefs();
+          if (!persistObserverPrefs(reply)) return true;
           _callbacks->onAlertConfigChanged();
           strcpy(reply, "OK - alert.psk updated");
         }
@@ -672,7 +746,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     if (in_len == 0) {
       _mqtt_prefs.alert_psk_hex[0] = '\0';
       _mqtt_prefs.alert_hashtag[0] = '\0';
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->onAlertConfigChanged();
       strcpy(reply, "OK - alert.hashtag cleared (alerts disabled until configured)");
     } else {
@@ -708,7 +782,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
           mesh::Utils::toHex(hex, digest, 16);
           StrHelper::strncpy(_mqtt_prefs.alert_hashtag, hashtag, sizeof(_mqtt_prefs.alert_hashtag));
           StrHelper::strncpy(_mqtt_prefs.alert_psk_hex, hex, sizeof(_mqtt_prefs.alert_psk_hex));
-          savePrefs();
+          if (!persistObserverPrefs(reply)) return true;
           _callbacks->onAlertConfigChanged();
           sprintf(reply, "OK - alert.hashtag: %s", _mqtt_prefs.alert_hashtag);
         }
@@ -726,7 +800,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     size_t len = strlen(val);
     if (len == 0) {
       _mqtt_prefs.alert_region[0] = '\0';
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->onAlertConfigChanged();
       strcpy(reply, "OK - alert.region cleared (using default scope)");
     } else if (len >= sizeof(_mqtt_prefs.alert_region)) {
@@ -734,7 +808,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
     } else {
       StrHelper::strncpy(_mqtt_prefs.alert_region, val, sizeof(_mqtt_prefs.alert_region));
       StrHelper::stripSurroundingQuotes(_mqtt_prefs.alert_region, sizeof(_mqtt_prefs.alert_region));
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       _callbacks->onAlertConfigChanged();
       sprintf(reply, "OK - alert.region: %s", _mqtt_prefs.alert_region);
     }
@@ -744,7 +818,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       strcpy(reply, "Error: alert.wifi must be 0-1440 minutes (0=off)");
     } else {
       _mqtt_prefs.alert_wifi_minutes = (uint16_t)mins;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       sprintf(reply, "OK - alert.wifi %d min%s", mins, mins == 0 ? " (disabled)" : "");
     }
   } else if (memcmp(config, "alert.mqtt ", 11) == 0) {
@@ -753,7 +827,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       strcpy(reply, "Error: alert.mqtt must be 0-10080 minutes (0=off)");
     } else {
       _mqtt_prefs.alert_mqtt_minutes = (uint16_t)mins;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       sprintf(reply, "OK - alert.mqtt %d min%s", mins, mins == 0 ? " (disabled)" : "");
     }
   } else if (memcmp(config, "alert.interval ", 15) == 0) {
@@ -764,7 +838,7 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
       strcpy(reply, "Error: alert.interval must be 60-10080 minutes");
     } else {
       _mqtt_prefs.alert_min_interval_min = (uint16_t)mins;
-      savePrefs();
+      if (!persistObserverPrefs(reply)) return true;
       sprintf(reply, "OK - alert.interval %d min", mins);
     }
   } else {
diff --git a/src/helpers/ConfigSerializer.cpp b/src/helpers/ConfigSerializer.cpp
index adff147f47..b8788bf5f9 100644
--- a/src/helpers/ConfigSerializer.cpp
+++ b/src/helpers/ConfigSerializer.cpp
@@ -1,5 +1,9 @@
 #include "ConfigSerializer.h"
 
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+
 bool ConfigSerializer::saveSerial(Stream& s) {
   Context context(&s, OP::WRITE);
   _context = &context;  // set the context for structure() call
@@ -22,9 +26,12 @@ bool ConfigSerializer::saveSerial(Stream& s) {
 static bool is_whitespace(char c) {
   return c == ' ' || c == '\t' || c == '\r' || c == '\n';
 }
-static bool is_key_char(char c) {
+static bool is_key_start_char(char c) {
   return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '_';
 }
+static bool is_key_char(char c) {
+  return is_key_start_char(c) || (c >= '0' && c <= '9');
+}
 static bool is_value_char(char c) {
   return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || c == '-' || c == '.';
 }
@@ -64,14 +71,26 @@ int ConfigSerializer::Context::readNext() {
     case EXPECT_KEY:
       if (rd_len > 0 && c == ':') { rd_buf[rd_len] = 0; rd_len = 0; rd_mode = EXPECT_VAL_OR_OBJ; return TOK_KEY; }
       if (rd_len == 0 && is_whitespace(c)) return TOK_WHITESPACE;
-      if (rd_len < CONFIG_MAX_KEYLEN-1 && is_key_char(c)) { rd_buf[rd_len++] = c; return TOK_WHITESPACE; }
+      if (rd_len < CONFIG_MAX_KEYLEN-1 &&
+          ((rd_len == 0 && is_key_start_char(c)) || (rd_len > 0 && is_key_char(c)))) {
+        rd_buf[rd_len++] = c;
+        return TOK_WHITESPACE;
+      }
       return TOK_ERROR;
 
     case EXPECT_VAL_OR_OBJ:
       if (rd_len == 0 && is_whitespace(c)) return TOK_WHITESPACE;
-      if (rd_len == 0 && c == '"') { rd_mode = EXPECT_STRING_VAL; return TOK_WHITESPACE; }
+      if (rd_len == 0 && c == '"') {
+        rd_token_quoted = true;
+        rd_mode = EXPECT_STRING_VAL;
+        return TOK_WHITESPACE;
+      }
       if (rd_len == 0 && c == '{') { rd_mode = EXPECT_KEY; return TOK_START_OBJ; }
-      if (is_value_char(c) && rd_len < CONFIG_MAX_TOKEN_LEN-1) { rd_buf[rd_len++] = c; return TOK_WHITESPACE; }
+      if (is_value_char(c) && rd_len < CONFIG_MAX_TOKEN_LEN-1) {
+        if (rd_len == 0) rd_token_quoted = false;
+        rd_buf[rd_len++] = c;
+        return TOK_WHITESPACE;
+      }
       if (rd_len > 0 && (c == ',' || c == '}' || is_whitespace(c))) { pending = c; rd_buf[rd_len] = 0; rd_len = 0; rd_mode = EXPECT_COMMA_OR_CLOSE; return TOK_VALUE;  }
       return TOK_ERROR;
 
@@ -107,9 +126,19 @@ bool ConfigSerializer::loadSerial(Stream& s) {
     if (next_tok == TOK_KEY) {
       context.setKey(sp, context.getToken());
     } else if (next_tok == TOK_VALUE) {
+      context.setValueEvent(sp, false);
       _depth = 1;  // re-run the structure() hierarchy again (looking for specific key, at specific depth)
       structure();
     } else if (next_tok == TOK_START_OBJ) {
+      // The root has no key. For every nested object, rerun the schema at the
+      // parent depth so strict scalar fields can reject an object value and
+      // object fields can confirm that their value has the expected shape.
+      if (sp > 0) {
+        context.setValueEvent(sp, true);
+        _depth = 1;
+        structure();
+        if (!context.success) break;
+      }
       if (sp < CONFIG_MAX_DEPTH - 1) {
         sp++;
       } else {
@@ -153,6 +182,10 @@ void ConfigSerializer::def(const char* key, void* value, size_t len) {
     _context->file()->print("\"");
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       memset(value, 0, len);
       mesh::Utils::fromHex((uint8_t *)value, len, _context->getToken());
     }
@@ -181,6 +214,10 @@ void ConfigSerializer::def(const char* key, char* value, size_t max_len) {
     _context->file()->print("\"");
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       strncpy(value, _context->getToken(), max_len - 1);
       value[max_len - 1] = 0;
     }
@@ -195,6 +232,10 @@ void ConfigSerializer::def(const char* key, int32_t& value) {
     _context->file()->print(value);
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atol(_context->getToken());
     }
   }
@@ -208,6 +249,10 @@ void ConfigSerializer::def(const char* key, uint32_t& value) {
     _context->file()->print(value);
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atol(_context->getToken());
     }
   }
@@ -221,6 +266,10 @@ void ConfigSerializer::def(const char* key, int16_t& value) {
     _context->file()->print((int32_t) value, 10);
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atol(_context->getToken());
     }
   }
@@ -234,6 +283,10 @@ void ConfigSerializer::def(const char* key, uint16_t& value) {
     _context->file()->print((uint32_t) value, 10);
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atoi(_context->getToken());
     }
   }
@@ -247,6 +300,10 @@ void ConfigSerializer::def(const char* key, uint8_t& value) {
     _context->file()->print((uint32_t) value, 10);
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atoi(_context->getToken());
     }
   }
@@ -260,6 +317,10 @@ void ConfigSerializer::def(const char* key, int8_t& value) {
     _context->file()->print((int32_t) value, 10);
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atoi(_context->getToken());
     }
   }
@@ -273,6 +334,10 @@ void ConfigSerializer::def(const char* key, bool& value) {
     _context->file()->print(value ? "true" : "false");
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = strcmp(_context->getToken(), "true") == 0 || atoi(_context->getToken()) != 0;  // 'true' or a non-zero number
     }
   }  
@@ -290,6 +355,10 @@ void ConfigSerializer::def(const char* key, double& value) {
     }
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = atof(_context->getToken());
     }
   }
@@ -307,6 +376,10 @@ void ConfigSerializer::def(const char* key, float& value) {
     }
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() || _context->valueDepth() != _depth) {
+        _context->success = false;
+        return;
+      }
       value = (float) atof(_context->getToken());
     }
   }
@@ -323,9 +396,64 @@ void ConfigSerializer::def(const char* key, ConfigSerializer& sub_obj) {
     if (_context->file()->print("}") != 1) _context->success = false;  // failure detect
   } else {
     if (_context->keyMatch(_depth, key)) {
+      if (_context->objectStart() && _context->valueDepth() == _depth) {
+        return;  // the object itself; descendant values recurse below
+      }
+      if (_context->valueDepth() <= _depth) {
+        _context->success = false;  // known object supplied as a scalar
+        return;
+      }
       sub_obj._context = _context;  // inherit the Context
       sub_obj._depth = _depth + 1;
       sub_obj.structure();   // recurse into sub object
     }
   }
 }
+
+bool ConfigSerializer::defStrict(const char* key, char* value, size_t max_len, bool& seen) {
+  if (_context->op() == OP::WRITE) {
+    def(key, value, max_len);
+    return true;
+  }
+  if (!_context->keyMatch(_depth, key)) return false;
+  if (seen || max_len == 0 || _context->objectStart() ||
+      _context->valueDepth() != _depth || !_context->tokenQuoted()) {
+    _context->success = false;
+    return false;
+  }
+  seen = true;
+  const char* token = _context->getToken();
+  const size_t len = strlen(token);
+  if (len >= max_len) {
+    _context->success = false;
+    return false;
+  }
+  memcpy(value, token, len + 1);
+  return true;
+}
+
+bool ConfigSerializer::defStrict(const char* key, int32_t& value, bool& seen) {
+  if (_context->op() == OP::WRITE) {
+    def(key, value);
+    return true;
+  }
+  if (!_context->keyMatch(_depth, key)) return false;
+  if (seen || _context->objectStart() || _context->valueDepth() != _depth ||
+      _context->tokenQuoted()) {
+    _context->success = false;
+    return false;
+  }
+  seen = true;
+
+  const char* token = _context->getToken();
+  char* end = nullptr;
+  errno = 0;
+  const long long parsed = strtoll(token, &end, 10);
+  if (token[0] == '\0' || end == token || *end != '\0' || errno == ERANGE ||
+      parsed < INT32_MIN || parsed > INT32_MAX) {
+    _context->success = false;
+    return false;
+  }
+  value = static_cast<int32_t>(parsed);
+  return true;
+}
diff --git a/src/helpers/ConfigSerializer.h b/src/helpers/ConfigSerializer.h
index 7e6d6f2a69..e889e6379d 100644
--- a/src/helpers/ConfigSerializer.h
+++ b/src/helpers/ConfigSerializer.h
@@ -25,17 +25,35 @@ class ConfigSerializer {
     OP _op;
     uint8_t rd_len;
     uint8_t rd_mode;
+    uint8_t rd_value_depth;
     char pending;
+    bool rd_token_quoted;
+    bool rd_object_start;
     char rd_buf[CONFIG_MAX_TOKEN_LEN];
     char _keys[CONFIG_MAX_DEPTH][CONFIG_MAX_KEYLEN];
 
   public:
     bool success = true;
-    Context(Stream* f, OP op) : _f(f), _op(op) { rd_buf[rd_len = 0] = 0; rd_mode = 0; pending = 0; }
+    Context(Stream* f, OP op) : _f(f), _op(op) {
+      rd_buf[rd_len = 0] = 0;
+      rd_mode = 0;
+      rd_value_depth = 0;
+      pending = 0;
+      rd_token_quoted = false;
+      rd_object_start = false;
+      memset(_keys, 0, sizeof(_keys));
+    }
     OP op() const { return _op; }
     Stream* file() const { return _f; }
     int readNext();
     const char* getToken() const { return rd_buf; }
+    bool tokenQuoted() const { return rd_token_quoted; }
+    uint8_t valueDepth() const { return rd_value_depth; }
+    bool objectStart() const { return rd_object_start; }
+    void setValueEvent(uint8_t depth, bool object_start) {
+      rd_value_depth = depth;
+      rd_object_start = object_start;
+    }
     bool keyMatch(int8_t depth, const char* key) { return strcmp(key, _keys[depth]) == 0; }
     void setKey(uint8_t depth, const char* key) { strcpy(_keys[depth], key);  }
   };
@@ -60,6 +78,67 @@ class ConfigSerializer {
   void def(const char* key, bool& value);
   void def(const char* key, ConfigSerializer& sub_obj);
 
+  // Strict read helpers for schemas whose values may be edited outside the
+  // firmware. Unlike the legacy def() overloads, these reject duplicate keys,
+  // truncated strings, malformed integers, and integer overflow. `seen` must
+  // be a field owned by the schema object and initialized false before load.
+  bool defStrict(const char* key, char* value, size_t max_len, bool& seen);
+  bool defStrict(const char* key, int32_t& value, bool& seen);
+
+  // Literal keys are checked where a schema defines them, so adding a field
+  // that an older ConfigSerializer cannot tokenize is a build failure rather
+  // than a silent downgrade trap. Dynamic keys retain the pointer overloads.
+  template <size_t N> static void checkKey(const char (&)[N]) {
+    static_assert(N <= CONFIG_MAX_KEYLEN,
+                  "ConfigSerializer key exceeds the visible-key limit");
+  }
+  template <size_t N> void def(const char (&key)[N], char* value, size_t max_len) {
+    checkKey(key); def(static_cast<const char*>(key), value, max_len);
+  }
+  template <size_t N> void def(const char (&key)[N], void* value, size_t len) {
+    checkKey(key); def(static_cast<const char*>(key), value, len);
+  }
+  template <size_t N> void def(const char (&key)[N], int32_t& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], int16_t& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], int8_t& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], uint32_t& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], uint16_t& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], uint8_t& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], float& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], double& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], bool& value) {
+    checkKey(key); def(static_cast<const char*>(key), value);
+  }
+  template <size_t N> void def(const char (&key)[N], ConfigSerializer& sub_obj) {
+    checkKey(key); def(static_cast<const char*>(key), sub_obj);
+  }
+  template <size_t N>
+  bool defStrict(const char (&key)[N], char* value, size_t max_len, bool& seen) {
+    checkKey(key);
+    return defStrict(static_cast<const char*>(key), value, max_len, seen);
+  }
+  template <size_t N>
+  bool defStrict(const char (&key)[N], int32_t& value, bool& seen) {
+    checkKey(key);
+    return defStrict(static_cast<const char*>(key), value, seen);
+  }
+
   virtual void structure() = 0;
 
 public:
diff --git a/src/helpers/MQTTDefaults.h b/src/helpers/MQTTDefaults.h
index 5072e4229f..21c53bc46b 100644
--- a/src/helpers/MQTTDefaults.h
+++ b/src/helpers/MQTTDefaults.h
@@ -8,7 +8,7 @@
 #include "MQTTPacketFilter.h"
 #include "MQTTPresets.h"
 
-// Compile-time defaults for fresh /mqtt_prefs (override via platformio build_flags).
+// Compile-time defaults for fresh /mqtt.json (override via platformio build_flags).
 // Example:
 //   -D MQTT_DEFAULT_SLOT1_PRESET='"meshcore-ca-1"'
 //   -D MQTT_DEFAULT_IATA='"YYZ"'
diff --git a/src/helpers/MQTTObserverValidation.h b/src/helpers/MQTTObserverValidation.h
index 2042f2267c..a8d465a77a 100644
--- a/src/helpers/MQTTObserverValidation.h
+++ b/src/helpers/MQTTObserverValidation.h
@@ -58,3 +58,20 @@ static inline bool mqttNtpHostnameValid(const char* host) {
 static inline bool mqttValueFits(const char* s, size_t bufsize) {
   return s != NULL && bufsize > 0 && strlen(s) < bufsize;
 }
+
+// Find a preset already assigned to another slot. The caller decides which
+// names are singleton presets ("none" and "custom" intentionally are not).
+template <size_t SlotCount, size_t PresetSize>
+static inline int mqttAssignedPresetSlot(
+    const char (&presets)[SlotCount][PresetSize],
+    const char* preset_name,
+    int ignored_slot) {
+  if (preset_name == NULL) return -1;
+  for (size_t slot = 0; slot < SlotCount; ++slot) {
+    if (static_cast<int>(slot) != ignored_slot &&
+        strcmp(presets[slot], preset_name) == 0) {
+      return static_cast<int>(slot);
+    }
+  }
+  return -1;
+}
diff --git a/src/helpers/MQTTPacketFilter.h b/src/helpers/MQTTPacketFilter.h
index df81182827..47fbfe5c21 100644
--- a/src/helpers/MQTTPacketFilter.h
+++ b/src/helpers/MQTTPacketFilter.h
@@ -71,8 +71,8 @@ inline bool resolveToken(const char* begin, size_t len, uint8_t* type_out) {
 }
 
 // Parse an allowlist value. Empty input means "all" so WebConfig can clear a
-// field and retain the same backwards-compatible default as an older
-// /mqtt_prefs file. Keywords and type names are deliberately lowercase; "all"
+// field and retain the same all-packet-types default as an older preference
+// file. Keywords and type names are deliberately lowercase; "all"
 // and "none" cannot be mixed into a list. Entries may be decimal (0..15) or
 // named, may carry surrounding ASCII whitespace, and may be repeated.
 inline bool parse(const char* input, uint16_t* mask_out) {
@@ -186,8 +186,8 @@ inline uint16_t enabledUnion(const uint16_t* masks, const bool* enabled, size_t
   return combined;
 }
 
-// True when every slot still carries the default all-types mask, i.e. nothing
-// depends on the packet-filter tail of /mqtt_prefs being written.
+// True when every slot still carries the default all-types mask. The legacy
+// binary encoder uses this while producing migration fixtures.
 inline bool allMasksDefault(const uint16_t* masks, size_t count) {
   if (masks == nullptr) return true;
   for (size_t i = 0; i < count; ++i) {
diff --git a/src/helpers/MQTTPrefsAtomicStore.h b/src/helpers/MQTTPrefsAtomicStore.h
index be77bf99e1..373dfa3b58 100644
--- a/src/helpers/MQTTPrefsAtomicStore.h
+++ b/src/helpers/MQTTPrefsAtomicStore.h
@@ -3,12 +3,82 @@
 #include <stddef.h>
 #include <stdint.h>
 
-// Transactional writer for /mqtt_prefs. The Store interface is intentionally
-// narrow so host tests can exercise every failure boundary without an Arduino
-// filesystem: begin(), write(), finish(), commit(), and abort(). The caller
-// supplies header and payload separately, avoiding a second full-size buffer.
+// Transactional writer policy for preferences. The Store interface is
+// intentionally narrow so host tests can exercise every failure boundary
+// without an Arduino filesystem.
 namespace MQTTPrefsAtomicStore {
 
+// Production /mqtt.json flow. finish() verifies the exact bytes written,
+// verify_image reparses the finished temp through the schema, and only then is
+// commit() allowed to publish it. A schema-verification failure discards the
+// finished temp; a commit failure is undone by rollbackFailedCommit().
+//
+// A failed save that is really gone and one that may still surface at the next
+// boot are distinct outcomes, not shades of the same one. Publishing moves the
+// old primary aside before the verified temp takes its name, so a half-done
+// commit leaves an image that boot recovery would promote; CommitFailed means
+// that was undone, CommitIndeterminate means it could not be. The same applies
+// before the commit: a rejected temp that cleanup could not delete is still an
+// image recovery may promote when no primary outranks it, which is
+// CleanupIndeterminate. Callers must not report these as a rollback.
+enum class VerifiedImageResult : uint8_t {
+  Committed,
+  BeginFailed,
+  WriteFailed,
+  FinishFailed,
+  VerifyFailed,
+  CleanupIndeterminate,
+  CommitFailed,
+  CommitIndeterminate,
+};
+
+// The change may still be on flash at the next boot, so the caller must not
+// tell the operator it was rolled back.
+inline bool saveOutcomeUnresolved(VerifiedImageResult result) {
+  return result == VerifiedImageResult::CleanupIndeterminate ||
+         result == VerifiedImageResult::CommitIndeterminate;
+}
+
+template <typename Store, typename ImageWriter, typename ImageVerifier>
+inline VerifiedImageResult writeVerifiedImage(Store& store,
+                                               ImageWriter write_image,
+                                               ImageVerifier verify_image) {
+  if (!store.begin()) {
+    store.abort();
+    return VerifiedImageResult::BeginFailed;
+  }
+  if (!write_image()) {
+    // A short write leaves a structurally incomplete image that recovery
+    // classifies as invalid, so a temp that survives cleanup here cannot be
+    // promoted and the change really is gone.
+    store.abort();
+    return VerifiedImageResult::WriteFailed;
+  }
+  if (!store.finish()) {
+    // Unlike a short write, this does not prove the bytes on disk are
+    // unusable: finish() also fails when a complete temp cannot be read back.
+    return store.abort() ? VerifiedImageResult::FinishFailed
+                         : VerifiedImageResult::CleanupIndeterminate;
+  }
+  if (!verify_image()) {
+    // The temp is complete and byte-verified, only rejected by the schema. If
+    // it cannot be deleted and no primary outranks it, boot recovery promotes
+    // the very image this call is about to report as not saved.
+    return store.discardFinishedTemp() ? VerifiedImageResult::VerifyFailed
+                                       : VerifiedImageResult::CleanupIndeterminate;
+  }
+  if (!store.commit()) {
+    // Undo the partial publish before answering. Only the store knows whether
+    // the pre-transaction state was actually restored, so take its word for
+    // which failure this was.
+    const bool rolled_back = store.rollbackFailedCommit();
+    store.abort();
+    return rolled_back ? VerifiedImageResult::CommitFailed
+                       : VerifiedImageResult::CommitIndeterminate;
+  }
+  return VerifiedImageResult::Committed;
+}
+
 enum class Result : uint8_t {
   Committed,
   BeginFailed,
@@ -59,7 +129,7 @@ inline ImageResult writeImage(Store& store, ImageWriter write_image) {
 }
 
 // Coordinates a two-file legacy upgrade. /com_prefs must not be compacted
-// until the observer tail it carries has been published into /mqtt_prefs.
+// until the observer tail it carries has been published into /mqtt.json.
 // Keeping this state in a tiny pure helper lets host tests cover power-cut
 // boundaries without an Arduino filesystem.
 class LegacyUpgradeGate {
diff --git a/src/helpers/MQTTPrefsCodec.h b/src/helpers/MQTTPrefsCodec.h
index 7bbfe30aeb..5ac073c083 100644
--- a/src/helpers/MQTTPrefsCodec.h
+++ b/src/helpers/MQTTPrefsCodec.h
@@ -53,6 +53,9 @@ static const size_t kEncodedSize = sizeof(MQTTPrefsHeader) + kV1BaselinePayloadS
 // refuses to overwrite the file, so the node cannot be recovered over the air.
 // Touching any filter opts that node into the longer payload — a deliberate,
 // operator-initiated trade rather than a side effect of upgrading.
+// The encoder below is retained for host migration fixtures and downgrade
+// compatibility tests. Production firmware reads legacy binary files but only
+// writes the JSON schema.
 inline size_t payloadLenFor(const MQTTPrefs& prefs) {
   return MQTTPacketFilter::allMasksDefault(prefs.mqtt_slot_packet_filter,
                                            MQTT_PREFS_SLOT_COUNT)
@@ -68,13 +71,60 @@ inline MQTTPrefsHeader makeHeader(size_t payload_len) {
   return header;
 }
 
+inline void freezeV1(const MQTTPrefs& prefs, LegacyV1MQTTPrefs* frozen) {
+  if (frozen == nullptr) return;
+  memset(frozen, 0, sizeof(*frozen));
+  memcpy(frozen->mqtt_origin, prefs.mqtt_origin, sizeof(frozen->mqtt_origin));
+  memcpy(frozen->mqtt_iata, prefs.mqtt_iata, sizeof(frozen->mqtt_iata));
+  frozen->mqtt_status_enabled = prefs.mqtt_status_enabled;
+  frozen->mqtt_packets_enabled = prefs.mqtt_packets_enabled;
+  frozen->mqtt_raw_enabled = prefs.mqtt_raw_enabled;
+  frozen->mqtt_tx_enabled = prefs.mqtt_tx_enabled;
+  frozen->mqtt_status_interval = prefs.mqtt_status_interval;
+  memcpy(frozen->wifi_ssid, prefs.wifi_ssid, sizeof(frozen->wifi_ssid));
+  memcpy(frozen->wifi_password, prefs.wifi_password, sizeof(frozen->wifi_password));
+  frozen->wifi_power_save = prefs.wifi_power_save;
+  memcpy(frozen->timezone_string, prefs.timezone_string, sizeof(frozen->timezone_string));
+  frozen->timezone_offset = prefs.timezone_offset;
+  memcpy(frozen->mqtt_slot_preset, prefs.mqtt_slot_preset, sizeof(frozen->mqtt_slot_preset));
+  memcpy(frozen->mqtt_slot_host, prefs.mqtt_slot_host, sizeof(frozen->mqtt_slot_host));
+  memcpy(frozen->mqtt_slot_port, prefs.mqtt_slot_port, sizeof(frozen->mqtt_slot_port));
+  memcpy(frozen->mqtt_slot_username, prefs.mqtt_slot_username, sizeof(frozen->mqtt_slot_username));
+  memcpy(frozen->mqtt_slot_password, prefs.mqtt_slot_password, sizeof(frozen->mqtt_slot_password));
+  memcpy(frozen->mqtt_owner_public_key, prefs.mqtt_owner_public_key,
+         sizeof(frozen->mqtt_owner_public_key));
+  memcpy(frozen->mqtt_email, prefs.mqtt_email, sizeof(frozen->mqtt_email));
+  memcpy(frozen->mqtt_slot_token, prefs.mqtt_slot_token, sizeof(frozen->mqtt_slot_token));
+  memcpy(frozen->mqtt_slot_topic, prefs.mqtt_slot_topic, sizeof(frozen->mqtt_slot_topic));
+  memcpy(frozen->mqtt_slot_audience, prefs.mqtt_slot_audience,
+         sizeof(frozen->mqtt_slot_audience));
+  frozen->mqtt_rx_enabled = prefs.mqtt_rx_enabled;
+  memcpy(frozen->mqtt_ntp_server, prefs.mqtt_ntp_server, sizeof(frozen->mqtt_ntp_server));
+  frozen->snmp_enabled = prefs.snmp_enabled;
+  memcpy(frozen->snmp_community, prefs.snmp_community, sizeof(frozen->snmp_community));
+  frozen->radio_watchdog_minutes = prefs.radio_watchdog_minutes;
+  frozen->alert_enabled = prefs.alert_enabled;
+  memcpy(frozen->alert_psk_hex, prefs.alert_psk_hex, sizeof(frozen->alert_psk_hex));
+  frozen->alert_wifi_minutes = prefs.alert_wifi_minutes;
+  frozen->alert_mqtt_minutes = prefs.alert_mqtt_minutes;
+  frozen->alert_min_interval_min = prefs.alert_min_interval_min;
+  memcpy(frozen->alert_hashtag, prefs.alert_hashtag, sizeof(frozen->alert_hashtag));
+  memcpy(frozen->alert_region, prefs.alert_region, sizeof(frozen->alert_region));
+  frozen->mqtt_neighbors_enabled = prefs.mqtt_neighbors_enabled;
+  frozen->mqtt_neighbors_interval = prefs.mqtt_neighbors_interval;
+  memcpy(frozen->mqtt_slot_packet_filter, prefs.mqtt_slot_packet_filter,
+         sizeof(frozen->mqtt_slot_packet_filter));
+}
+
 inline size_t encode(const MQTTPrefs& prefs, uint8_t* output, size_t output_size) {
   const size_t payload_len = payloadLenFor(prefs);
   const size_t encoded_size = sizeof(MQTTPrefsHeader) + payload_len;
   if (output == nullptr || output_size < encoded_size) return 0;
   const MQTTPrefsHeader header = makeHeader(payload_len);
+  LegacyV1MQTTPrefs frozen;
+  freezeV1(prefs, &frozen);
   memcpy(output, &header, sizeof(header));
-  memcpy(output + sizeof(header), &prefs, payload_len);
+  memcpy(output + sizeof(header), &frozen, payload_len);
   return encoded_size;
 }
 
@@ -346,6 +396,120 @@ inline bool isPlausibleLegacy(Source source, const uint8_t* input, size_t size)
   }
 }
 
+// A valid v1 header proves the intended layout, not that a torn or corrupted
+// payload still contains bounded C strings. Validate every field present at a
+// shipped v1 boundary before copying it into the runtime object; otherwise the
+// later semantic validators and JSON writer could scan beyond a fixed array.
+inline bool isPlausibleV1(const LegacyV1MQTTPrefs& prefs, size_t payload_len) {
+  if (payload_len != kV1PreObserverPayloadSize &&
+      payload_len != kV1PreNeighborsPayloadSize &&
+      payload_len != kV1PreFilterPayloadSize &&
+      payload_len != kV1BaselinePayloadSize) {
+    return false;
+  }
+  const uint8_t* input = reinterpret_cast<const uint8_t*>(&prefs);
+  // A v1 header already identifies the layout. At this stage only reject
+  // fields that could make later C-string validation/serialization read past a
+  // fixed array; numeric values are safely repaired by MQTTPrefsSerializer.
+  if (!hasTerminatedText(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, mqtt_origin), sizeof(prefs.mqtt_origin)) ||
+      !hasTerminatedText(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, mqtt_iata), sizeof(prefs.mqtt_iata)) ||
+      !hasTerminatedText(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, wifi_ssid), sizeof(prefs.wifi_ssid)) ||
+      !hasTerminatedText(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, wifi_password), sizeof(prefs.wifi_password)) ||
+      !hasTerminatedText(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, timezone_string), sizeof(prefs.timezone_string)) ||
+      !hasPlausibleSharedAuth(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, mqtt_owner_public_key),
+          offsetof(LegacyV1MQTTPrefs, mqtt_email)) ||
+      !hasPlausibleSlotText(input, payload_len, MQTT_PREFS_SLOT_COUNT,
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_preset),
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_host),
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_username),
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_password),
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_token),
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_topic),
+          offsetof(LegacyV1MQTTPrefs, mqtt_slot_audience)) ||
+      !hasTerminatedText(input, payload_len,
+          offsetof(LegacyV1MQTTPrefs, mqtt_ntp_server),
+          sizeof(prefs.mqtt_ntp_server))) {
+    return false;
+  }
+  if (payload_len >= kV1PreNeighborsPayloadSize) {
+    if (!hasTerminatedText(input, payload_len,
+            offsetof(LegacyV1MQTTPrefs, snmp_community),
+            sizeof(prefs.snmp_community)) ||
+        !hasTerminatedText(input, payload_len,
+            offsetof(LegacyV1MQTTPrefs, alert_psk_hex),
+            sizeof(prefs.alert_psk_hex)) ||
+        !hasTerminatedText(input, payload_len,
+            offsetof(LegacyV1MQTTPrefs, alert_hashtag),
+            sizeof(prefs.alert_hashtag)) ||
+        !hasTerminatedText(input, payload_len,
+            offsetof(LegacyV1MQTTPrefs, alert_region),
+            sizeof(prefs.alert_region))) {
+      return false;
+    }
+  }
+  return true;
+}
+
+inline void migrateV1(const LegacyV1MQTTPrefs& old_prefs, size_t payload_len,
+                      MQTTPrefs* prefs) {
+  if (prefs == nullptr || payload_len < kV1PreObserverPayloadSize) return;
+  memcpy(prefs->mqtt_origin, old_prefs.mqtt_origin, sizeof(prefs->mqtt_origin));
+  memcpy(prefs->mqtt_iata, old_prefs.mqtt_iata, sizeof(prefs->mqtt_iata));
+  prefs->mqtt_status_enabled = old_prefs.mqtt_status_enabled;
+  prefs->mqtt_packets_enabled = old_prefs.mqtt_packets_enabled;
+  prefs->mqtt_raw_enabled = old_prefs.mqtt_raw_enabled;
+  prefs->mqtt_tx_enabled = old_prefs.mqtt_tx_enabled;
+  prefs->mqtt_status_interval = old_prefs.mqtt_status_interval;
+  memcpy(prefs->wifi_ssid, old_prefs.wifi_ssid, sizeof(prefs->wifi_ssid));
+  memcpy(prefs->wifi_password, old_prefs.wifi_password, sizeof(prefs->wifi_password));
+  prefs->wifi_power_save = old_prefs.wifi_power_save;
+  memcpy(prefs->timezone_string, old_prefs.timezone_string, sizeof(prefs->timezone_string));
+  prefs->timezone_offset = old_prefs.timezone_offset;
+  memcpy(prefs->mqtt_slot_preset, old_prefs.mqtt_slot_preset, sizeof(prefs->mqtt_slot_preset));
+  memcpy(prefs->mqtt_slot_host, old_prefs.mqtt_slot_host, sizeof(prefs->mqtt_slot_host));
+  memcpy(prefs->mqtt_slot_port, old_prefs.mqtt_slot_port, sizeof(prefs->mqtt_slot_port));
+  memcpy(prefs->mqtt_slot_username, old_prefs.mqtt_slot_username, sizeof(prefs->mqtt_slot_username));
+  memcpy(prefs->mqtt_slot_password, old_prefs.mqtt_slot_password, sizeof(prefs->mqtt_slot_password));
+  memcpy(prefs->mqtt_owner_public_key, old_prefs.mqtt_owner_public_key,
+         sizeof(prefs->mqtt_owner_public_key));
+  memcpy(prefs->mqtt_email, old_prefs.mqtt_email, sizeof(prefs->mqtt_email));
+  memcpy(prefs->mqtt_slot_token, old_prefs.mqtt_slot_token, sizeof(prefs->mqtt_slot_token));
+  memcpy(prefs->mqtt_slot_topic, old_prefs.mqtt_slot_topic, sizeof(prefs->mqtt_slot_topic));
+  memcpy(prefs->mqtt_slot_audience, old_prefs.mqtt_slot_audience,
+         sizeof(prefs->mqtt_slot_audience));
+  prefs->mqtt_rx_enabled = old_prefs.mqtt_rx_enabled;
+  memcpy(prefs->mqtt_ntp_server, old_prefs.mqtt_ntp_server, sizeof(prefs->mqtt_ntp_server));
+
+  if (payload_len >= kV1PreNeighborsPayloadSize) {
+    prefs->snmp_enabled = old_prefs.snmp_enabled;
+    memcpy(prefs->snmp_community, old_prefs.snmp_community, sizeof(prefs->snmp_community));
+    prefs->radio_watchdog_minutes = old_prefs.radio_watchdog_minutes;
+    prefs->alert_enabled = old_prefs.alert_enabled;
+    memcpy(prefs->alert_psk_hex, old_prefs.alert_psk_hex, sizeof(prefs->alert_psk_hex));
+    prefs->alert_wifi_minutes = old_prefs.alert_wifi_minutes;
+    prefs->alert_mqtt_minutes = old_prefs.alert_mqtt_minutes;
+    prefs->alert_min_interval_min = old_prefs.alert_min_interval_min;
+    memcpy(prefs->alert_hashtag, old_prefs.alert_hashtag, sizeof(prefs->alert_hashtag));
+    memcpy(prefs->alert_region, old_prefs.alert_region, sizeof(prefs->alert_region));
+    // The pre-neighbors payload includes the old zero padding byte at this
+    // offset; copying it preserves the deployed format's disabled default.
+    prefs->mqtt_neighbors_enabled = old_prefs.mqtt_neighbors_enabled;
+  }
+  if (payload_len >= kV1PreFilterPayloadSize) {
+    prefs->mqtt_neighbors_interval = old_prefs.mqtt_neighbors_interval;
+  }
+  if (payload_len >= kV1BaselinePayloadSize) {
+    memcpy(prefs->mqtt_slot_packet_filter, old_prefs.mqtt_slot_packet_filter,
+           sizeof(prefs->mqtt_slot_packet_filter));
+  }
+}
+
 inline void migratePreSlot(const OldMQTTPrefs& old_prefs, MQTTPrefs* prefs) {
   memcpy(prefs->mqtt_origin, old_prefs.mqtt_origin, sizeof(prefs->mqtt_origin));
   memcpy(prefs->mqtt_iata, old_prefs.mqtt_iata, sizeof(prefs->mqtt_iata));
diff --git a/src/helpers/MQTTPrefsRecovery.h b/src/helpers/MQTTPrefsRecovery.h
index 7534678eb3..15eb014e4a 100644
--- a/src/helpers/MQTTPrefsRecovery.h
+++ b/src/helpers/MQTTPrefsRecovery.h
@@ -2,27 +2,53 @@
 
 #include <stdint.h>
 
-// Pure recovery policy for the three MQTT preference transaction files.  The
+// Pure recovery policy for the three MQTT preference transaction files. The
 // writer first moves the old primary to .bak, then moves the verified .tmp to
-// the primary name.  On a reset, the loader uses this policy before decoding
-// /mqtt_prefs.  "Preserve" is deliberately distinct from "Usable": it covers
-// an unsupported newer layout, corruption, or an unreadable file and must
-// never be replaced by an older image.
+// the primary name. On a reset, the loader uses this policy before decoding
+// the primary. FutureUsable is syntactically valid but belongs to newer
+// firmware. FutureClaimed and Indeterminate cannot be safely classified by
+// this firmware, so recovery may run a known-good backup but must retain the
+// uncertain image and hold further writes. Preserve is definitively invalid or
+// unsupported and may be discarded only where the transaction policy permits.
+//
+// The filenames are the transaction state. A temp that exists while the primary
+// name is empty says the commit had already passed the backup rename, and that
+// is the only record of it — so an uncertain temp is answered with
+// UseBackupHeld, which renames nothing. Publishing the backup instead would
+// make the candidate look like an ordinary stale artifact to the next boot,
+// which would delete it exactly when it finally became readable. When neither
+// file can run this boot, RunDefaultsHeld keeps both names untouched for the
+// same reason.
 namespace MQTTPrefsRecovery {
 
 enum class FileState : uint8_t {
   Missing,
   Usable,
+  FutureUsable,
+  FutureClaimed,
+  Indeterminate,
   Preserve,
 };
 
 enum class Action : uint8_t {
   None,
   KeepPrimary,
+  DiscardTemp,
   PromoteTemp,
   PromoteBackup,
+  // Run the backup where it lies, changing nothing on disk, and hold writes.
+  // Every later boot re-runs this policy against the same three names until one
+  // of them can classify the candidate.
+  UseBackupHeld,
+  // Neither the candidate nor the backup can run this boot. Change nothing,
+  // come up on defaults, and hold writes until a boot that can classify them.
+  RunDefaultsHeld,
 };
 
+inline bool uncertain(FileState state) {
+  return state == FileState::FutureClaimed || state == FileState::Indeterminate;
+}
+
 inline Action select(FileState primary, FileState temp, FileState backup) {
   // A primary of any kind owns the name. In particular, do not roll a newer
   // or corrupt primary back to an older backup just because it cannot be read
@@ -30,14 +56,36 @@ inline Action select(FileState primary, FileState temp, FileState backup) {
   if (primary != FileState::Missing) return Action::KeepPrimary;
 
   // A completed temp is the new image and wins over the old backup.
-  if (temp == FileState::Usable) return Action::PromoteTemp;
+  if (temp == FileState::Usable || temp == FileState::FutureUsable) {
+    return Action::PromoteTemp;
+  }
 
-  // If temp is opaque but a known-good backup exists, boot from the backup.
-  // The caller may discard the opaque temp once that usable backup has become
-  // primary. Otherwise, rename the opaque temp into the empty primary name so
-  // the normal loader can hold it.
+  // Preserve is definitively invalid, so it was never a committed image. If
+  // no backup exists, discard the interrupted temp and leave the primary name
+  // absent; this is what lets a first JSON migration retry from /mqtt_prefs.
+  // If a backup exists, it is the prior committed image and wins regardless
+  // of whether this firmware understands its schema.
   if (temp == FileState::Preserve) {
-    return backup == FileState::Usable ? Action::PromoteBackup : Action::PromoteTemp;
+    return backup == FileState::Missing ? Action::DiscardTemp : Action::PromoteBackup;
+  }
+
+  // FutureClaimed and Indeterminate may be a completed image this firmware
+  // cannot classify. A known-good backup can run this boot, but it must run
+  // from its own name: promoting it would spend the empty primary name that
+  // marks the candidate as mid-commit.
+  //
+  // A backup this firmware cannot run is still the previous committed image, so
+  // the same argument applies to it: promoting the candidate would give it the
+  // authoritative name, and the "any primary owns the name" rule above would
+  // then keep it even on a boot that proves it corrupt. Spend the transaction
+  // state recorded in the filenames only once the backup is definitively no
+  // longer a usable fallback.
+  if (uncertain(temp)) {
+    if (backup == FileState::Usable) return Action::UseBackupHeld;
+    if (backup == FileState::FutureUsable || uncertain(backup)) {
+      return Action::RunDefaultsHeld;
+    }
+    return Action::PromoteTemp;
   }
 
   // No temp survived. The backup is the only recoverable image, even when it
diff --git a/src/helpers/MQTTPrefsSerializer.h b/src/helpers/MQTTPrefsSerializer.h
new file mode 100644
index 0000000000..4d182c442a
--- /dev/null
+++ b/src/helpers/MQTTPrefsSerializer.h
@@ -0,0 +1,421 @@
+#pragma once
+
+#include <helpers/ConfigSerializer.h>
+#include <helpers/MQTTObserverValidation.h>
+#include <helpers/MQTTPresets.h>
+#include <helpers/MQTTPrefsStorage.h>
+
+#ifdef WITH_MQTT_BRIDGE
+
+static const int32_t MQTT_PREFS_JSON_FORMAT_VERSION = 1;
+
+// Format invariant, binding on every future schema version: `version` is the
+// FIRST property of the root object. The probe below reads it with this
+// firmware's grammar and stops at the first construct that grammar cannot
+// tokenize, so a newer file that introduces unknown syntax ahead of its version
+// field is indistinguishable from a corrupt one here. That costs it the
+// opaque-future preservation guarantee: as a temp left by an interrupted commit
+// it would be classified as discardable rather than retained. Keep the version
+// first when adding fields or bumping the version; a host test pins it.
+//
+// Reads only the mandatory root version while ignoring every other schema
+// field. Recovery uses this before the v1 decoder so a syntactically valid
+// future file remains opaque even when that schema changes a v1 field's type.
+class MQTTPrefsVersionProbe : public ConfigSerializer {
+  int32_t _version = 0;
+  bool _seen_version = false;
+protected:
+  void structure() override { defStrict("version", _version, _seen_version); }
+public:
+  bool hasFutureVersion() const {
+    return _seen_version && _version > MQTT_PREFS_JSON_FORMAT_VERSION;
+  }
+};
+
+// ConfigSerializer adapter for the observer preference POD. Keeping the
+// serializer separate avoids making the runtime object layout part of the JSON
+// format and adds no permanent per-slot serializer state to CommonCLI.
+class MQTTPrefsSerializer : public ConfigSerializer {
+
+  class WifiPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _power_save;
+    bool _seen_ssid = false, _seen_password = false, _seen_power_save = false;
+  protected:
+    void structure() override {
+      defStrict("ssid", _prefs->wifi_ssid, sizeof(_prefs->wifi_ssid), _seen_ssid);
+      defStrict("password", _prefs->wifi_password, sizeof(_prefs->wifi_password), _seen_password);
+      defStrict("power_save", _power_save, _seen_power_save);
+    }
+  public:
+    explicit WifiPrefs(MQTTPrefs* prefs) : _prefs(prefs), _power_save(prefs->wifi_power_save) {}
+    bool apply(bool* repaired) {
+      if (_power_save < 0 || _power_save > 2) {
+        _power_save = 1;
+        *repaired = true;
+      }
+      _prefs->wifi_power_save = static_cast<uint8_t>(_power_save);
+      return true;
+    }
+  };
+
+  class TimePrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _utc_offset, _default_utc_offset;
+    char _default_ntp_server[sizeof(MQTTPrefs::mqtt_ntp_server)];
+    bool _seen_timezone = false, _seen_utc_offset = false, _seen_ntp_server = false;
+  protected:
+    void structure() override {
+      defStrict("timezone", _prefs->timezone_string, sizeof(_prefs->timezone_string), _seen_timezone);
+      defStrict("utc_offset", _utc_offset, _seen_utc_offset);
+      defStrict("ntp_server", _prefs->mqtt_ntp_server, sizeof(_prefs->mqtt_ntp_server), _seen_ntp_server);
+    }
+  public:
+    TimePrefs(MQTTPrefs* prefs, const MQTTPrefs* defaults)
+        : _prefs(prefs), _utc_offset(prefs->timezone_offset),
+          _default_utc_offset(defaults->timezone_offset) {
+      if (_default_utc_offset < -12 || _default_utc_offset > 14) {
+        _default_utc_offset = 0;
+      }
+      memcpy(_default_ntp_server, defaults->mqtt_ntp_server, sizeof(_default_ntp_server));
+      if (_default_ntp_server[0] != '\0' &&
+          !mqttNtpHostnameValid(_default_ntp_server)) {
+        _default_ntp_server[0] = '\0';
+      }
+    }
+    bool apply(bool* repaired) {
+      if (_utc_offset < -12 || _utc_offset > 14) {
+        _utc_offset = _default_utc_offset;
+        *repaired = true;
+      }
+      if (_prefs->mqtt_ntp_server[0] != '\0' &&
+          !mqttNtpHostnameValid(_prefs->mqtt_ntp_server)) {
+        memcpy(_prefs->mqtt_ntp_server, _default_ntp_server, sizeof(_default_ntp_server));
+        *repaired = true;
+      }
+      _prefs->timezone_offset = static_cast<int8_t>(_utc_offset);
+      return true;
+    }
+  };
+
+  class StatusPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _enabled, _interval_ms;
+    bool _seen_enabled = false, _seen_interval = false;
+  protected:
+    void structure() override {
+      defStrict("enabled", _enabled, _seen_enabled);
+      defStrict("interval_ms", _interval_ms, _seen_interval);
+    }
+  public:
+    explicit StatusPrefs(MQTTPrefs* prefs)
+        : _prefs(prefs), _enabled(prefs->mqtt_status_enabled),
+          _interval_ms(static_cast<int32_t>(prefs->mqtt_status_interval)) {}
+    void apply(bool* repaired) {
+      if (_enabled < 0 || _enabled > 1) { _enabled = 1; *repaired = true; }
+      if (_interval_ms < 60000 || _interval_ms > 3600000) {
+        _interval_ms = 300000;
+        *repaired = true;
+      }
+      _prefs->mqtt_status_enabled = static_cast<uint8_t>(_enabled);
+      _prefs->mqtt_status_interval = static_cast<uint32_t>(_interval_ms);
+    }
+  };
+
+  class NeighborPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _enabled, _interval_ms;
+    bool _seen_enabled = false, _seen_interval = false;
+  protected:
+    void structure() override {
+      defStrict("enabled", _enabled, _seen_enabled);
+      defStrict("interval_ms", _interval_ms, _seen_interval);
+    }
+  public:
+    explicit NeighborPrefs(MQTTPrefs* prefs)
+        : _prefs(prefs), _enabled(prefs->mqtt_neighbors_enabled),
+          _interval_ms(static_cast<int32_t>(prefs->mqtt_neighbors_interval)) {}
+    void apply(bool* repaired) {
+      if (_enabled < 0 || _enabled > 1) { _enabled = 0; *repaired = true; }
+      if (_interval_ms < static_cast<int32_t>(MQTT_NEIGHBORS_MIN_INTERVAL_MS) ||
+          _interval_ms > static_cast<int32_t>(MQTT_NEIGHBORS_MAX_INTERVAL_MS)) {
+        _interval_ms = static_cast<int32_t>(MQTT_NEIGHBORS_DEFAULT_INTERVAL_MS);
+        *repaired = true;
+      }
+      _prefs->mqtt_neighbors_enabled = static_cast<uint8_t>(_enabled);
+      _prefs->mqtt_neighbors_interval = static_cast<uint32_t>(_interval_ms);
+    }
+  };
+
+  class OwnerPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    bool _seen_public_key = false, _seen_email = false;
+  protected:
+    void structure() override {
+      defStrict("public_key", _prefs->mqtt_owner_public_key,
+                sizeof(_prefs->mqtt_owner_public_key), _seen_public_key);
+      defStrict("email", _prefs->mqtt_email, sizeof(_prefs->mqtt_email), _seen_email);
+    }
+  public:
+    explicit OwnerPrefs(MQTTPrefs* prefs) : _prefs(prefs) {}
+    void apply(bool* repaired) {
+      if (_prefs->mqtt_owner_public_key[0] != '\0' &&
+          !mqttOwnerKeyValid(_prefs->mqtt_owner_public_key)) {
+        _prefs->mqtt_owner_public_key[0] = '\0';
+        *repaired = true;
+      }
+    }
+  };
+
+  class SlotPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int _index;
+    int32_t _port, _filter;
+    bool _seen_preset = false, _seen_host = false, _seen_port = false;
+    bool _seen_username = false, _seen_password = false, _seen_token = false;
+    bool _seen_topic = false, _seen_audience = false, _seen_filter = false;
+  protected:
+    void structure() override {
+      defStrict("preset", _prefs->mqtt_slot_preset[_index],
+                sizeof(_prefs->mqtt_slot_preset[_index]), _seen_preset);
+      defStrict("host", _prefs->mqtt_slot_host[_index],
+                sizeof(_prefs->mqtt_slot_host[_index]), _seen_host);
+      defStrict("port", _port, _seen_port);
+      defStrict("username", _prefs->mqtt_slot_username[_index],
+                sizeof(_prefs->mqtt_slot_username[_index]), _seen_username);
+      defStrict("password", _prefs->mqtt_slot_password[_index],
+                sizeof(_prefs->mqtt_slot_password[_index]), _seen_password);
+      defStrict("token", _prefs->mqtt_slot_token[_index],
+                sizeof(_prefs->mqtt_slot_token[_index]), _seen_token);
+      defStrict("topic", _prefs->mqtt_slot_topic[_index],
+                sizeof(_prefs->mqtt_slot_topic[_index]), _seen_topic);
+      defStrict("audience", _prefs->mqtt_slot_audience[_index],
+                sizeof(_prefs->mqtt_slot_audience[_index]), _seen_audience);
+      defStrict("packet_filter", _filter, _seen_filter);
+    }
+  public:
+    SlotPrefs(MQTTPrefs* prefs, int index)
+        : _prefs(prefs), _index(index), _port(prefs->mqtt_slot_port[index]),
+          _filter(prefs->mqtt_slot_packet_filter[index]) {}
+    void apply(bool* repaired) {
+      if (_port < 0 || _port > 65535) { _port = 0; *repaired = true; }
+      if (_filter < 0 || _filter > 65535) { _filter = 0xffff; *repaired = true; }
+      const char* preset = _prefs->mqtt_slot_preset[_index];
+      if (strcmp(preset, MQTT_PRESET_NONE) != 0 &&
+          strcmp(preset, MQTT_PRESET_CUSTOM) != 0 && findMQTTPreset(preset) == nullptr) {
+        strcpy(_prefs->mqtt_slot_preset[_index], MQTT_PRESET_NONE);
+        *repaired = true;
+      }
+      _prefs->mqtt_slot_port[_index] = static_cast<uint16_t>(_port);
+      _prefs->mqtt_slot_packet_filter[_index] = static_cast<uint16_t>(_filter);
+    }
+  };
+
+  class MqttPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _packets_enabled, _raw_enabled, _tx_enabled, _rx_enabled;
+    char _default_iata[sizeof(MQTTPrefs::mqtt_iata)];
+    bool _seen_origin = false, _seen_iata = false, _seen_packets = false;
+    bool _seen_raw = false, _seen_tx = false, _seen_rx = false;
+    StatusPrefs _status;
+    NeighborPrefs _neighbors;
+    OwnerPrefs _owner;
+    static_assert(MQTT_PREFS_SLOT_COUNT == 6,
+                  "MQTTPrefsSerializer slot members and keys must be updated");
+    SlotPrefs _slot1, _slot2, _slot3, _slot4, _slot5, _slot6;
+  protected:
+    void structure() override {
+      defStrict("origin", _prefs->mqtt_origin, sizeof(_prefs->mqtt_origin), _seen_origin);
+      defStrict("iata", _prefs->mqtt_iata, sizeof(_prefs->mqtt_iata), _seen_iata);
+      defStrict("packets_enabled", _packets_enabled, _seen_packets);
+      defStrict("raw_enabled", _raw_enabled, _seen_raw);
+      defStrict("tx_enabled", _tx_enabled, _seen_tx);
+      defStrict("rx_enabled", _rx_enabled, _seen_rx);
+      def("status", _status);
+      def("neighbors", _neighbors);
+      def("owner", _owner);
+      def("slot1", _slot1);
+      def("slot2", _slot2);
+      def("slot3", _slot3);
+      def("slot4", _slot4);
+      def("slot5", _slot5);
+      def("slot6", _slot6);
+    }
+  public:
+    MqttPrefs(MQTTPrefs* prefs, const MQTTPrefs* defaults)
+        : _prefs(prefs), _packets_enabled(prefs->mqtt_packets_enabled),
+          _raw_enabled(prefs->mqtt_raw_enabled), _tx_enabled(prefs->mqtt_tx_enabled),
+          _rx_enabled(prefs->mqtt_rx_enabled), _status(prefs), _neighbors(prefs),
+          _owner(prefs), _slot1(prefs, 0), _slot2(prefs, 1),
+          _slot3(prefs, 2), _slot4(prefs, 3),
+          _slot5(prefs, 4), _slot6(prefs, 5) {
+      memcpy(_default_iata, defaults->mqtt_iata, sizeof(_default_iata));
+      if (_default_iata[0] != '\0' && !mqttIataValid(_default_iata)) {
+        _default_iata[0] = '\0';
+      }
+      for (char* p = _default_iata; *p; ++p) {
+        if (*p >= 'a' && *p <= 'z') *p = static_cast<char>(*p - ('a' - 'A'));
+      }
+    }
+    void apply(bool* repaired) {
+      if (_packets_enabled < 0 || _packets_enabled > 1) { _packets_enabled = 1; *repaired = true; }
+      if (_raw_enabled < 0 || _raw_enabled > 1) { _raw_enabled = 0; *repaired = true; }
+      if (_tx_enabled < 0 || _tx_enabled > 2) { _tx_enabled = 2; *repaired = true; }
+      if (_rx_enabled < 0 || _rx_enabled > 1) { _rx_enabled = 1; *repaired = true; }
+      _prefs->mqtt_packets_enabled = static_cast<uint8_t>(_packets_enabled);
+      _prefs->mqtt_raw_enabled = static_cast<uint8_t>(_raw_enabled);
+      _prefs->mqtt_tx_enabled = static_cast<uint8_t>(_tx_enabled);
+      _prefs->mqtt_rx_enabled = static_cast<uint8_t>(_rx_enabled);
+      if (_prefs->mqtt_iata[0] != '\0') {
+        if (!mqttIataValid(_prefs->mqtt_iata)) {
+          memcpy(_prefs->mqtt_iata, _default_iata, sizeof(_default_iata));
+          *repaired = true;
+        } else {
+          for (char* p = _prefs->mqtt_iata; *p; ++p) {
+            if (*p >= 'a' && *p <= 'z') {
+              *p = static_cast<char>(*p - ('a' - 'A'));
+              *repaired = true;
+            }
+          }
+        }
+      }
+      _status.apply(repaired);
+      _neighbors.apply(repaired);
+      _owner.apply(repaired);
+      _slot1.apply(repaired); _slot2.apply(repaired); _slot3.apply(repaired);
+      _slot4.apply(repaired); _slot5.apply(repaired); _slot6.apply(repaired);
+    }
+  };
+
+  class SnmpPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _enabled;
+    bool _seen_enabled = false, _seen_community = false;
+  protected:
+    void structure() override {
+      defStrict("enabled", _enabled, _seen_enabled);
+      defStrict("community", _prefs->snmp_community,
+                sizeof(_prefs->snmp_community), _seen_community);
+    }
+  public:
+    explicit SnmpPrefs(MQTTPrefs* prefs) : _prefs(prefs), _enabled(prefs->snmp_enabled) {}
+    void apply(bool* repaired) {
+      if (_enabled < 0 || _enabled > 1) { _enabled = 0; *repaired = true; }
+      _prefs->snmp_enabled = static_cast<uint8_t>(_enabled);
+    }
+  };
+
+  class RadioPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _watchdog_min;
+    bool _seen_watchdog = false;
+  protected:
+    void structure() override { defStrict("watchdog_min", _watchdog_min, _seen_watchdog); }
+  public:
+    explicit RadioPrefs(MQTTPrefs* prefs)
+        : _prefs(prefs), _watchdog_min(prefs->radio_watchdog_minutes) {}
+    void apply(bool* repaired) {
+      if (_watchdog_min < 0 || _watchdog_min > 120) { _watchdog_min = 5; *repaired = true; }
+      _prefs->radio_watchdog_minutes = static_cast<uint8_t>(_watchdog_min);
+    }
+  };
+
+  class AlertPrefs : public ConfigSerializer {
+    MQTTPrefs* _prefs;
+    int32_t _enabled, _wifi_minutes, _mqtt_minutes, _rate_limit_min;
+    bool _seen_enabled = false, _seen_psk = false, _seen_wifi = false;
+    bool _seen_mqtt = false, _seen_rate = false, _seen_hashtag = false, _seen_region = false;
+  protected:
+    void structure() override {
+      defStrict("enabled", _enabled, _seen_enabled);
+      defStrict("psk_hex", _prefs->alert_psk_hex, sizeof(_prefs->alert_psk_hex), _seen_psk);
+      defStrict("wifi_minutes", _wifi_minutes, _seen_wifi);
+      defStrict("mqtt_minutes", _mqtt_minutes, _seen_mqtt);
+      defStrict("rate_limit_min", _rate_limit_min, _seen_rate);
+      defStrict("hashtag", _prefs->alert_hashtag, sizeof(_prefs->alert_hashtag), _seen_hashtag);
+      defStrict("region", _prefs->alert_region, sizeof(_prefs->alert_region), _seen_region);
+    }
+  public:
+    explicit AlertPrefs(MQTTPrefs* prefs)
+        : _prefs(prefs), _enabled(prefs->alert_enabled),
+          _wifi_minutes(prefs->alert_wifi_minutes), _mqtt_minutes(prefs->alert_mqtt_minutes),
+          _rate_limit_min(prefs->alert_min_interval_min) {}
+    void apply(bool* repaired) {
+      if (_enabled < 0 || _enabled > 1) { _enabled = 0; *repaired = true; }
+      if (_wifi_minutes < 0 || _wifi_minutes > 1440) { _wifi_minutes = 30; *repaired = true; }
+      if (_mqtt_minutes < 0 || _mqtt_minutes > 10080) { _mqtt_minutes = 240; *repaired = true; }
+      if (_rate_limit_min < 60 || _rate_limit_min > 10080) { _rate_limit_min = 60; *repaired = true; }
+      if (_prefs->alert_psk_hex[0] != '\0') {
+        const size_t len = strlen(_prefs->alert_psk_hex);
+        bool valid_hex = len == 32;
+        for (size_t i = 0; valid_hex && i < len; ++i) {
+          const char c = _prefs->alert_psk_hex[i];
+          valid_hex = (c >= '0' && c <= '9') || (c >= 'A' && c <= 'F') ||
+                      (c >= 'a' && c <= 'f');
+        }
+        if (!valid_hex) {
+          _prefs->alert_psk_hex[0] = '\0';
+          _prefs->alert_hashtag[0] = '\0';
+          *repaired = true;
+        }
+      }
+      _prefs->alert_enabled = static_cast<uint8_t>(_enabled);
+      _prefs->alert_wifi_minutes = static_cast<uint16_t>(_wifi_minutes);
+      _prefs->alert_mqtt_minutes = static_cast<uint16_t>(_mqtt_minutes);
+      _prefs->alert_min_interval_min = static_cast<uint16_t>(_rate_limit_min);
+    }
+  };
+
+  MQTTPrefs* _prefs;
+  int32_t _version = MQTT_PREFS_JSON_FORMAT_VERSION;
+  bool _seen_version = false;
+  WifiPrefs _wifi;
+  TimePrefs _time;
+  MqttPrefs _mqtt;
+  SnmpPrefs _snmp;
+  RadioPrefs _radio;
+  AlertPrefs _alert;
+
+protected:
+  void structure() override {
+    defStrict("version", _version, _seen_version);  // must stay first; see above
+    def("wifi", _wifi);
+    def("time", _time);
+    def("mqtt", _mqtt);
+    def("snmp", _snmp);
+    def("radio", _radio);
+    def("alert", _alert);
+  }
+
+public:
+  explicit MQTTPrefsSerializer(MQTTPrefs* prefs, const MQTTPrefs* repair_defaults = nullptr)
+      : _prefs(prefs), _wifi(prefs),
+        _time(prefs, repair_defaults ? repair_defaults : prefs),
+        _mqtt(prefs, repair_defaults ? repair_defaults : prefs), _snmp(prefs),
+        _radio(prefs), _alert(prefs) {}
+
+  bool hasSupportedVersion() const {
+    return _seen_version && _version == MQTT_PREFS_JSON_FORMAT_VERSION;
+  }
+  bool hasFutureVersion() const {
+    return _seen_version && _version > MQTT_PREFS_JSON_FORMAT_VERSION;
+  }
+
+  bool normalize(bool* repaired) {
+    if (repaired == nullptr) return false;
+    *repaired = false;
+    _wifi.apply(repaired);
+    _time.apply(repaired);
+    _mqtt.apply(repaired);
+    _snmp.apply(repaired);
+    _radio.apply(repaired);
+    _alert.apply(repaired);
+    return true;
+  }
+
+  bool apply(bool* repaired) {
+    return hasSupportedVersion() && normalize(repaired);
+  }
+};
+
+#endif  // WITH_MQTT_BRIDGE
diff --git a/src/helpers/MQTTPrefsStorage.h b/src/helpers/MQTTPrefsStorage.h
index e3c6a098be..0bc0867ee2 100644
--- a/src/helpers/MQTTPrefsStorage.h
+++ b/src/helpers/MQTTPrefsStorage.h
@@ -62,10 +62,9 @@ struct PreWifiPowerOldMQTTPrefs {
   char mqtt_email[64];
 };
 
-// MQTT preferences stored separately from NodePrefs to avoid upstream layout
-// conflicts. The full layout is the frozen v1 payload baseline. The prefix
-// before observer settings is also an explicitly supported v1 payload: it was
-// used before the observer fields were appended.
+// Runtime MQTT preferences, stored separately from NodePrefs to avoid upstream
+// layout conflicts. JSON persistence is field-based; this type is deliberately
+// free to evolve independently of the frozen binary migration layouts below.
 struct MQTTPrefs {
   char mqtt_origin[32];
   char mqtt_iata[8];
@@ -110,17 +109,57 @@ struct MQTTPrefs {
   char alert_region[31];
 
   // Neighbors publishing (PSRAM boards only). Appended at the end of the
-  // observer tail so a shorter (pre-neighbors) /mqtt_prefs payload from earlier
-  // firmware still loads with these defaulting off/24h; keeps the format at
-  // VERSION 1. Field order and sizes are kept byte-identical to the flex
-  // neighbors build so a /mqtt_prefs written by either firmware is
-  // interchangeable (see the offsetof static_asserts below).
+  // observer tail in the former binary format. Binary compatibility is now
+  // represented only by LegacyV1MQTTPrefs below.
   uint8_t mqtt_neighbors_enabled;
   uint32_t mqtt_neighbors_interval;
 
   // Per-slot payload-type allow masks. Bit N controls MeshCore packet type N
-  // for both packets and raw MQTT topics. Appended so older v1 payloads load
-  // with the default all-types masks intact.
+  // for both packets and raw MQTT topics.
+  uint16_t mqtt_slot_packet_filter[MQTT_PREFS_SLOT_COUNT];
+};
+
+// Frozen payload written by the version-1 binary format. Keep this distinct
+// from the runtime MQTTPrefs type: JSON persistence must not make the runtime
+// object's padding or member order an on-flash ABI again. Legacy decoding reads
+// into this type and field-copies into a defaulted runtime object.
+struct LegacyV1MQTTPrefs {
+  char mqtt_origin[32];
+  char mqtt_iata[8];
+  uint8_t mqtt_status_enabled;
+  uint8_t mqtt_packets_enabled;
+  uint8_t mqtt_raw_enabled;
+  uint8_t mqtt_tx_enabled;
+  uint32_t mqtt_status_interval;
+  char wifi_ssid[32];
+  char wifi_password[64];
+  uint8_t wifi_power_save;
+  char timezone_string[32];
+  int8_t timezone_offset;
+  char mqtt_slot_preset[MQTT_PREFS_SLOT_COUNT][24];
+  char mqtt_slot_host[MQTT_PREFS_SLOT_COUNT][64];
+  uint16_t mqtt_slot_port[MQTT_PREFS_SLOT_COUNT];
+  char mqtt_slot_username[MQTT_PREFS_SLOT_COUNT][32];
+  char mqtt_slot_password[MQTT_PREFS_SLOT_COUNT][64];
+  char mqtt_owner_public_key[65];
+  char mqtt_email[64];
+  char mqtt_slot_token[MQTT_PREFS_SLOT_COUNT][48];
+  char mqtt_slot_topic[MQTT_PREFS_SLOT_COUNT][96];
+  char mqtt_slot_audience[MQTT_PREFS_SLOT_COUNT][64];
+  uint8_t mqtt_rx_enabled;
+  char mqtt_ntp_server[64];
+  uint8_t snmp_enabled;
+  char snmp_community[24];
+  uint8_t radio_watchdog_minutes;
+  uint8_t alert_enabled;
+  char alert_psk_hex[33];
+  uint16_t alert_wifi_minutes;
+  uint16_t alert_mqtt_minutes;
+  uint16_t alert_min_interval_min;
+  char alert_hashtag[24];
+  char alert_region[31];
+  uint8_t mqtt_neighbors_enabled;
+  uint32_t mqtt_neighbors_interval;
   uint16_t mqtt_slot_packet_filter[MQTT_PREFS_SLOT_COUNT];
 };
 
@@ -272,20 +311,20 @@ static const size_t LEGACY6_AUDIENCE_RX_SIZE = 2840;
 
 // Frozen on-flash layouts; every firmware and native fixture build checks them.
 static_assert(sizeof(MQTTPrefsHeader) == 8, "versioned /mqtt_prefs header must stay 8 bytes");
-static_assert(offsetof(MQTTPrefs, snmp_enabled) == MQTT_PREFS_V1_PRE_OBSERVER_PAYLOAD_SIZE,
+static_assert(offsetof(LegacyV1MQTTPrefs, snmp_enabled) == MQTT_PREFS_V1_PRE_OBSERVER_PAYLOAD_SIZE,
               "v1 pre-observer /mqtt_prefs boundary changed");
-static_assert(sizeof(MQTTPrefs) == MQTT_PREFS_V1_FULL_PAYLOAD_SIZE,
+static_assert(sizeof(LegacyV1MQTTPrefs) == MQTT_PREFS_V1_FULL_PAYLOAD_SIZE,
               "v1 /mqtt_prefs payload layout changed");
 // Lock the neighbors tail to the flex neighbors build's layout so a /mqtt_prefs
 // written by either firmware is byte-for-byte interchangeable. The enable flag
 // lands in the old struct's zeroed trailing padding (offset 2857), and the
 // interval begins exactly at the pre-neighbors payload size (2860) so a
 // pre-neighbors read stops right before it and the interval keeps its default.
-static_assert(offsetof(MQTTPrefs, mqtt_neighbors_enabled) == 2857,
+static_assert(offsetof(LegacyV1MQTTPrefs, mqtt_neighbors_enabled) == 2857,
               "neighbors enable flag must sit at the flex-compatible offset");
-static_assert(offsetof(MQTTPrefs, mqtt_neighbors_interval) == MQTT_PREFS_V1_PRE_NEIGHBORS_PAYLOAD_SIZE,
+static_assert(offsetof(LegacyV1MQTTPrefs, mqtt_neighbors_interval) == MQTT_PREFS_V1_PRE_NEIGHBORS_PAYLOAD_SIZE,
               "neighbors interval offset must equal the pre-neighbors payload size");
-static_assert(offsetof(MQTTPrefs, mqtt_slot_packet_filter) == MQTT_PREFS_V1_PRE_FILTER_PAYLOAD_SIZE,
+static_assert(offsetof(LegacyV1MQTTPrefs, mqtt_slot_packet_filter) == MQTT_PREFS_V1_PRE_FILTER_PAYLOAD_SIZE,
               "packet filters must begin at the pre-filter payload boundary");
 static_assert(sizeof(OldMQTTPrefs) == 472, "frozen pre-slot /mqtt_prefs layout changed");
 static_assert(sizeof(PreWifiPowerOldMQTTPrefs) == 472, "frozen pre-WiFi-power /mqtt_prefs layout changed");
diff --git a/src/helpers/bridges/MQTTBridge.h b/src/helpers/bridges/MQTTBridge.h
index bda752a6fe..0ad539e936 100644
--- a/src/helpers/bridges/MQTTBridge.h
+++ b/src/helpers/bridges/MQTTBridge.h
@@ -529,7 +529,7 @@ class MQTTBridge : public BridgeBase {
   LifecycleOps _lifecycle_ops;
   MQTTLifecycle::Coordinator _lifecycle;
 
-  // Observer config (MQTT/WiFi/timezone/SNMP/alert), persisted to /mqtt_prefs.
+  // Observer config (MQTT/WiFi/timezone/SNMP/alert), persisted to /mqtt.json.
   // _prefs (held by BridgeBase) still provides upstream fields (freq/sf/node_name…).
   MQTTPrefs* _obs = nullptr;
 
diff --git a/test/README.md b/test/README.md
index 711e648406..801825105d 100644
--- a/test/README.md
+++ b/test/README.md
@@ -33,7 +33,8 @@ does not reflect the GoogleTest count — run the built binary directly
 | `test_mqtt_packet_filter` | `src/helpers/MQTTPacketFilter.h` | per-slot 0-15 allowlist parsing/formatting, numeric and named spellings; exact bounds; membership; candidate/eligible split and retry-completion policy; pre-queue union gate; default-mask detection |
 | `test_mqtt_runtime_buffer_lifecycle` | `src/helpers/MQTTRuntimeBufferLifecycle.h` | idempotent allocation/release; partial-allocation degradation; retry of only missing buffers |
 | `test_mqtt_prefs_codec` | `src/helpers/MQTTPrefsStorage.h`, `src/helpers/MQTTPrefsCodec.h` | binary pre-slot/3-slot/6-slot migration fixtures; v1 header integrity; downgrade preservation; shortest-payload write policy (default filters stay downgrade-readable) |
-| `test_mqtt_prefs_atomic_store` | `src/helpers/MQTTPrefsAtomicStore.h` | transactional MQTT writes and legacy `/node_prefs` handoff; exact short-write detection; begin/finish/rename failure cleanup; original-file preservation |
+| `test_mqtt_prefs_serializer` | `src/helpers/MQTTPrefsSerializer.h`, `src/helpers/ConfigSerializer.*` | semantic nested `/mqtt.json` round trips; numeric slot keys; required/future version handling; strict length/overflow/duplicate rejection; safe semantic repair; scratch-before-live loading |
+| `test_mqtt_prefs_atomic_store` | `src/helpers/MQTTPrefsAtomicStore.h`, `src/helpers/MQTTPrefsRecovery.h` | production JSON begin/write/checksum-finish/schema-verify/commit orchestration; first-migration and rename-boundary recovery; legacy `/node_prefs` handoff; failure cleanup and original-file preservation |
 | `test_mqtt_payload_builder` | `src/helpers/MQTTPayloadBuilder.cpp` | status/packet/raw JSON contracts; optional fields; escaping; RX metrics and path; score handling; exact buffer bounds; maximum representative payloads |
 | `test_utils` | `src/Utils.cpp` | `Utils::toHex` (upstream) |
 
diff --git a/test/test_config_serializer/test_config_serializer.cpp b/test/test_config_serializer/test_config_serializer.cpp
index 27c3c8119e..ae63f68248 100644
--- a/test/test_config_serializer/test_config_serializer.cpp
+++ b/test/test_config_serializer/test_config_serializer.cpp
@@ -214,6 +214,118 @@ TEST(NodePrefs, FemGainSettingsRoundTrip) {
     EXPECT_EQ(1, loaded.radio_fem_txgain);
 }
 
+TEST(ConfigSerializer, LoadSerial_KeyDigitsAfterFirstCharacter) {
+    MockInputStream s("{age:1,flags:2,name:\"ok\",slot1:7}");
+    TestStruct data;
+    data.age = data.flags = 0;
+    strcpy(data.name, "before");
+    EXPECT_TRUE(data.loadSerial(s));
+    EXPECT_EQ(1, data.age);
+    EXPECT_EQ(2, data.flags);
+    EXPECT_STREQ("ok", data.name);
+}
+
+TEST(ConfigSerializer, LoadSerial_RejectsLeadingDigitKey) {
+    MockInputStream s("{1slot:7}");
+    TestStruct data;
+    EXPECT_FALSE(data.loadSerial(s));
+}
+
+// ── /prefs.json compatibility under the strict shape checks ─────────────────
+//
+// The scalar-vs-object rejection added for /mqtt.json also runs for the plain
+// def() overloads that NodePrefs uses, and loadPrefsInt() applies /prefs.json
+// straight onto the live object without consulting the return value. These
+// pin what that combination does to files already on deployed devices.
+
+// A settings file in the exact shape the firmware writes, transcribed rather
+// than produced by saveSerial() so a change to the writer cannot quietly move
+// the fixture with it.
+static const char* DEPLOYED_PREFS_JSON =
+    "{name:\"Repeater-1\",pass:\"hunter2\",guest:\"\",owner:\"ops@example.com\","
+    "adv_int:4,f_adv_int:12,lat:47.612345,lon:-122.334567,disc_mod:1719791234,"
+    "radio:{freq:910.5250,bw:250.0000,sf:10,cr:5,cad:0,int_thr:0,rxgain:1,"
+    "fem_rxgain:0,fem_txgain:1,tx:22,af:1.0000,rxdelay:1000.0000,"
+    "f_txdelay:0.5000,d_txdelay:0.2000,agc_int:0,hash_mode:0,multi_ack:0},"
+    "bridge:{en:0,delay:500,src:1,baud:115200,ch:1,secret:\"\"},"
+    "gps:{en:0,int:60,adv_loc:0},"
+    "repeat:{disable:0,f_max:64,f_max_uns:32,f_max_adv:16,loop:1},"
+    "room:{rd_only:0},power:{adc_mult:1.0000,pwr_sav_en:0}}";
+
+TEST(NodePrefs, DeployedPrefsJsonStillLoadsCompletely) {
+    MockInputStream s(DEPLOYED_PREFS_JSON);
+    NodePrefs prefs;
+
+    ASSERT_TRUE(prefs.loadSerial(s));
+    EXPECT_STREQ("Repeater-1", prefs.node_name);
+    EXPECT_STREQ("ops@example.com", prefs.owner_info);
+    EXPECT_EQ(4, prefs.advert_interval);
+    EXPECT_DOUBLE_EQ(47.612345, prefs.node_lat);
+    EXPECT_DOUBLE_EQ(-122.334567, prefs.node_lon);
+    EXPECT_EQ(1719791234u, prefs.discovery_mod_timestamp);
+    EXPECT_FLOAT_EQ(910.525f, prefs.freq);
+    EXPECT_EQ(10, prefs.sf);
+    EXPECT_EQ(22, prefs.tx_power_dbm);
+    EXPECT_EQ(1, prefs.radio_fem_txgain);
+    EXPECT_EQ(1, prefs.bridge_pkt_src);
+    EXPECT_EQ(115200u, prefs.bridge_baud);
+    EXPECT_EQ(60u, prefs.gps_interval);
+    EXPECT_EQ(64, prefs.flood_max);
+    EXPECT_EQ(1, prefs.loop_detect);
+}
+
+TEST(NodePrefs, UnknownNestedObjectCannotBeMistakenForARootScalar) {
+    // Depth is what keeps an unknown group's inner keys from matching a root
+    // field of the same name, so an unrecognized section must stay ignorable.
+    MockInputStream s("{name:\"Repeater-1\",mqtt:{name:\"other\",owner:\"nobody\"},adv_int:4}");
+    NodePrefs prefs;
+    strcpy(prefs.owner_info, "ops@example.com");
+
+    EXPECT_TRUE(prefs.loadSerial(s));
+    EXPECT_STREQ("Repeater-1", prefs.node_name);
+    EXPECT_STREQ("ops@example.com", prefs.owner_info);
+    EXPECT_EQ(4, prefs.advert_interval);
+}
+
+TEST(NodePrefs, TornPrefsJsonAppliesOnlyTheFieldsBeforeTheTear) {
+    // A power cut during the old non-transactional /prefs.json write leaves a
+    // file like this. loadPrefsInt() ignores the failed return and keeps the
+    // partial result, which is the pre-existing behavior; the strict checks
+    // must not turn it into something worse than a partial load.
+    MockInputStream s("{name:\"Repeater-1\",adv_int:4,radio:{freq:910.5250,sf:1");
+    NodePrefs prefs;
+    prefs.sf = 9;
+
+    EXPECT_FALSE(prefs.loadSerial(s));
+    EXPECT_STREQ("Repeater-1", prefs.node_name);
+    EXPECT_EQ(4, prefs.advert_interval);
+    EXPECT_FLOAT_EQ(910.525f, prefs.freq);
+    EXPECT_EQ(9, prefs.sf);  // the torn value never completed a token
+}
+
+TEST(NodePrefs, ShapeMismatchIsRejectedAndStopsFurtherApplication) {
+    // Hand-edited or corrupted files are the regression surface for the strict
+    // checks: a known scalar holding an object now fails the load and stops
+    // parsing, so nothing after the mismatch reaches the live object.
+    MockInputStream scalar_as_object("{name:{x:1},adv_int:4}");
+    NodePrefs prefs;
+    prefs.advert_interval = 7;
+    EXPECT_FALSE(prefs.loadSerial(scalar_as_object));
+    EXPECT_EQ(7, prefs.advert_interval);
+
+    // The reverse mismatch is rejected too: a known group given a scalar.
+    MockInputStream object_as_scalar("{name:\"Repeater-1\",radio:1}");
+    NodePrefs scalar_group;
+    EXPECT_FALSE(scalar_group.loadSerial(object_as_scalar));
+
+    // And a known scalar nested one level deeper than the schema places it.
+    MockInputStream over_nested("{radio:{sf:{value:10}}}");
+    NodePrefs nested;
+    nested.sf = 9;
+    EXPECT_FALSE(nested.loadSerial(over_nested));
+    EXPECT_EQ(9, nested.sf);
+}
+
 
 // ── main ───────────────────────────────────────────────────────
 
diff --git a/test/test_mqtt_prefs_atomic_store/test_mqtt_prefs_atomic_store.cpp b/test/test_mqtt_prefs_atomic_store/test_mqtt_prefs_atomic_store.cpp
index 67880a8244..1979483965 100644
--- a/test/test_mqtt_prefs_atomic_store/test_mqtt_prefs_atomic_store.cpp
+++ b/test/test_mqtt_prefs_atomic_store/test_mqtt_prefs_atomic_store.cpp
@@ -20,6 +20,7 @@ enum class FailurePoint {
   PayloadWrite,
   ImageWrite,
   Finish,
+  Verify,
   Commit,
 };
 
@@ -44,7 +45,8 @@ class InMemoryStore {
     ++write_calls;
     if (!_open) return 0;
     const bool should_fail = (write_calls == 1 && _failure == FailurePoint::HeaderWrite) ||
-        (write_calls == 2 && _failure == FailurePoint::PayloadWrite);
+        (write_calls == 2 && _failure == FailurePoint::PayloadWrite) ||
+        _failure == FailurePoint::ImageWrite;
     const size_t written = should_fail && size > 0 ? size - 1 : size;
     _staging.insert(_staging.end(), bytes, bytes + written);
     return written;
@@ -113,6 +115,186 @@ AtomicStore::Result runWithObserverTail(InMemoryStore* store) {
   return AtomicStore::write(*store, header, sizeof(header), payload, sizeof(payload));
 }
 
+// Models MQTTPrefsJsonFileStore under the SPIFFS rule it was written for:
+// rename() refuses an existing destination, so publishing must move the old
+// primary to .bak before the verified temp can take its name. Both halves of
+// that publish, and the rollback that undoes a half-done one, are injectable.
+class InMemoryJsonStore {
+public:
+  struct Options {
+    FailurePoint failure = FailurePoint::None;
+    bool rollback_rename_fails = false;
+    bool rollback_remove_fails = false;
+    bool has_primary = true;
+    bool discard_remove_fails = false;
+    bool abort_remove_fails = false;
+  };
+
+  explicit InMemoryJsonStore(FailurePoint failure) : _opts{failure, false, false, true} {
+    _files["/mqtt.json"] = oldImage();
+  }
+  explicit InMemoryJsonStore(Options opts) : _opts(opts) {
+    if (_opts.has_primary) _files["/mqtt.json"] = oldImage();
+  }
+
+  bool begin() {
+    ++begin_calls;
+    // Production refuses to open a new transaction while recovery still owns
+    // an artifact, so a rollback that left one behind blocks the retry.
+    if (has("/mqtt.json.tmp") || has("/mqtt.json.bak")) return false;
+    _staging.clear();
+    _open = _opts.failure != FailurePoint::Begin;
+    return _open;
+  }
+
+  size_t write(const uint8_t* bytes, size_t size) {
+    ++write_calls;
+    if (!_open) return 0;
+    const size_t written =
+        _opts.failure == FailurePoint::ImageWrite && size > 0 ? size - 1 : size;
+    _staging.insert(_staging.end(), bytes, bytes + written);
+    return written;
+  }
+
+  bool finish() {
+    ++finish_calls;
+    _open = false;
+    // Production writes straight to /mqtt.json.tmp, so the bytes are on flash
+    // before finish() reads them back. A finish() failure can therefore be a
+    // failed read-back of an otherwise complete image, not a torn one.
+    _files["/mqtt.json.tmp"] = _staging;
+    if (_opts.failure == FailurePoint::Finish) return false;
+    _finished = true;
+    return true;
+  }
+
+  bool verify() {
+    ++verify_calls;
+    return _opts.failure != FailurePoint::Verify;
+  }
+
+  bool commit() {
+    ++commit_calls;
+    if (!_finished) return false;
+    if (has("/mqtt.json.bak")) return false;
+    if (has("/mqtt.json") && !rename("/mqtt.json", "/mqtt.json.bak")) return false;
+    // Fail the publish rename, the boundary that leaves the old primary parked
+    // in .bak with the verified new image still sitting in the temp.
+    if (_opts.failure == FailurePoint::Commit) return false;
+    if (!rename("/mqtt.json.tmp", "/mqtt.json")) return false;
+    _files.erase("/mqtt.json.bak");
+    _finished = false;
+    return true;
+  }
+
+  bool rollbackFailedCommit() {
+    ++rollback_calls;
+    if (!has("/mqtt.json") && has("/mqtt.json.bak")) {
+      if (_opts.rollback_rename_fails || !rename("/mqtt.json.bak", "/mqtt.json")) {
+        return false;
+      }
+    }
+    if (has("/mqtt.json.tmp")) {
+      if (_opts.rollback_remove_fails) return has("/mqtt.json");
+      _files.erase("/mqtt.json.tmp");
+    }
+    return true;
+  }
+
+  // Both cleanups report the same disposition as production: false means a temp
+  // survived with no primary to outrank it, so boot recovery may still promote
+  // the image the caller is about to report as not saved.
+  bool discardFinishedTemp() {
+    ++discard_calls;
+    const bool removed = !_opts.discard_remove_fails;
+    if (removed) _files.erase("/mqtt.json.tmp");
+    _finished = false;
+    return removed || has("/mqtt.json");
+  }
+
+  bool abort() {
+    ++abort_calls;
+    _open = false;
+    _staging.clear();
+    bool removed = true;
+    if (!_finished && has("/mqtt.json.tmp")) {
+      removed = !_opts.abort_remove_fails;
+      if (removed) _files.erase("/mqtt.json.tmp");
+    }
+    _finished = false;
+    return removed || has("/mqtt.json");
+  }
+
+  // Apply the boot-recovery policy to whatever the transaction left behind and
+  // report the image the node would come up on. Every file present in these
+  // scenarios is a complete image, so each maps to Usable.
+  std::vector<uint8_t> imageAfterReboot() {
+    const auto state = [this](const char* path) {
+      return has(path) ? Recovery::FileState::Usable : Recovery::FileState::Missing;
+    };
+    switch (Recovery::select(state("/mqtt.json"), state("/mqtt.json.tmp"),
+                             state("/mqtt.json.bak"))) {
+      case Recovery::Action::PromoteTemp:
+        rename("/mqtt.json.tmp", "/mqtt.json");
+        break;
+      case Recovery::Action::PromoteBackup:
+        rename("/mqtt.json.bak", "/mqtt.json");
+        break;
+      case Recovery::Action::DiscardTemp:
+        _files.erase("/mqtt.json.tmp");
+        break;
+      default:
+        break;
+    }
+    return has("/mqtt.json") ? _files.at("/mqtt.json") : std::vector<uint8_t>();
+  }
+
+  bool has(const char* path) const { return _files.count(path) != 0; }
+  bool canStartSave() const { return !has("/mqtt.json.tmp") && !has("/mqtt.json.bak"); }
+  const std::vector<uint8_t>& source() const { return _files.at("/mqtt.json"); }
+  static std::vector<uint8_t> oldImage() {
+    const char* text = "{version:1,wifi:{ssid:\"old\"}}";
+    return std::vector<uint8_t>(text, text + strlen(text));
+  }
+  static std::vector<uint8_t> newImage() {
+    const char* text = "{version:1,wifi:{ssid:\"mesh\"}}";
+    return std::vector<uint8_t>(text, text + strlen(text));
+  }
+
+  int begin_calls = 0;
+  int write_calls = 0;
+  int finish_calls = 0;
+  int commit_calls = 0;
+  int abort_calls = 0;
+  int verify_calls = 0;
+  int discard_calls = 0;
+  int rollback_calls = 0;
+
+private:
+  bool rename(const char* from, const char* to) {
+    if (_files.count(from) == 0 || _files.count(to) != 0) return false;
+    _files[to] = _files[from];
+    _files.erase(from);
+    return true;
+  }
+
+  Options _opts;
+  bool _open = false;
+  bool _finished = false;
+  std::vector<uint8_t> _staging;
+  std::map<std::string, std::vector<uint8_t>> _files;
+};
+
+AtomicStore::VerifiedImageResult runVerifiedJson(InMemoryJsonStore* store) {
+  const std::vector<uint8_t> json = InMemoryJsonStore::newImage();
+  return AtomicStore::writeVerifiedImage(
+      *store,
+      [store, &json]() {
+        return store->write(json.data(), json.size()) == json.size();
+      },
+      [store]() { return store->verify(); });
+}
+
 class LegacyComPrefs {
 public:
   LegacyComPrefs() : bytes({'l', 'e', 'g', 'a', 'c', 'y', '-', 'c', 'o', 'm'}) {}
@@ -258,7 +440,8 @@ class SpiffsMqttTransaction {
   }
 
   // Inject ordinary operation failures (as distinct from a power cut). A
-  // failed temp rename leaves both the verified temp and old backup intact.
+  // failed temp rename leaves both the verified temp and old backup intact,
+  // which is the state rollbackFailedPublish() then undoes.
   bool publish(bool fail_backup_rename, bool fail_temp_rename, bool fail_cleanup) {
     writeVerifiedTemp();
     if (fail_backup_rename || !rename("/mqtt_prefs", "/mqtt_prefs.bak")) return false;
@@ -267,6 +450,15 @@ class SpiffsMqttTransaction {
     return true;  // backup cleanup is intentionally non-fatal after publish
   }
 
+  bool rollbackFailedPublish() {
+    if (!has("/mqtt_prefs") && has("/mqtt_prefs.bak") &&
+        !rename("/mqtt_prefs.bak", "/mqtt_prefs")) {
+      return false;
+    }
+    _files.erase("/mqtt_prefs.tmp");
+    return true;
+  }
+
   void recover(Recovery::FileState primary = Recovery::FileState::Usable,
                Recovery::FileState temp = Recovery::FileState::Usable,
                Recovery::FileState backup = Recovery::FileState::Usable) {
@@ -286,9 +478,18 @@ class SpiffsMqttTransaction {
       }
       return;
     }
+    if (action == Recovery::Action::DiscardTemp) {
+      _files.erase("/mqtt_prefs.tmp");
+      return;
+    }
+    if (action == Recovery::Action::UseBackupHeld ||
+        action == Recovery::Action::RunDefaultsHeld) {
+      return;  // production renames nothing, so the transaction state survives
+    }
     if (action == Recovery::Action::PromoteBackup) {
       rename("/mqtt_prefs.bak", "/mqtt_prefs");
-      if (backup == Recovery::FileState::Usable && temp != Recovery::FileState::Missing) {
+      if (backup == Recovery::FileState::Usable && !Recovery::uncertain(temp) &&
+          temp != Recovery::FileState::Missing) {
         _files.erase("/mqtt_prefs.tmp");
       }
       return;
@@ -298,12 +499,13 @@ class SpiffsMqttTransaction {
     // incomplete transaction artifact. It only preserves artifacts when the
     // primary itself is opaque.
     if (had_primary && primary == Recovery::FileState::Usable) {
-      _files.erase("/mqtt_prefs.tmp");
-      _files.erase("/mqtt_prefs.bak");
+      if (!Recovery::uncertain(temp)) _files.erase("/mqtt_prefs.tmp");
+      if (!Recovery::uncertain(backup)) _files.erase("/mqtt_prefs.bak");
     }
   }
 
   bool has(const char* path) const { return _files.count(path) != 0; }
+  void removePrimary() { _files.erase("/mqtt_prefs"); }
   bool canStartSave() const { return !has("/mqtt_prefs.tmp") && !has("/mqtt_prefs.bak"); }
   const std::vector<uint8_t>& primary() const { return _files.at("/mqtt_prefs"); }
   static std::vector<uint8_t> oldImage() { return {'o', 'l', 'd'}; }
@@ -337,6 +539,144 @@ TEST(MQTTPrefsAtomicStore, CommitPublishesExactHeaderThenPayload) {
   EXPECT_EQ(0, store.abort_calls);
 }
 
+TEST(MQTTPrefsAtomicStore, ProductionJsonPolicyCoversEveryVerificationBoundary) {
+  const struct {
+    FailurePoint point;
+    AtomicStore::VerifiedImageResult expected;
+    int writes;
+    int finishes;
+    int verifies;
+    int commits;
+    int aborts;
+    int discards;
+    int rollbacks;
+  } cases[] = {
+      {FailurePoint::None, AtomicStore::VerifiedImageResult::Committed,
+       1, 1, 1, 1, 0, 0, 0},
+      {FailurePoint::Begin, AtomicStore::VerifiedImageResult::BeginFailed,
+       0, 0, 0, 0, 1, 0, 0},
+      {FailurePoint::ImageWrite, AtomicStore::VerifiedImageResult::WriteFailed,
+       1, 0, 0, 0, 1, 0, 0},
+      {FailurePoint::Finish, AtomicStore::VerifiedImageResult::FinishFailed,
+       1, 1, 0, 0, 1, 0, 0},
+      {FailurePoint::Verify, AtomicStore::VerifiedImageResult::VerifyFailed,
+       1, 1, 1, 0, 0, 1, 0},
+      {FailurePoint::Commit, AtomicStore::VerifiedImageResult::CommitFailed,
+       1, 1, 1, 1, 1, 0, 1},
+  };
+
+  for (const auto& test_case : cases) {
+    InMemoryJsonStore store(test_case.point);
+    EXPECT_EQ(test_case.expected, runVerifiedJson(&store));
+    EXPECT_EQ(test_case.writes, store.write_calls);
+    EXPECT_EQ(test_case.finishes, store.finish_calls);
+    EXPECT_EQ(test_case.verifies, store.verify_calls);
+    EXPECT_EQ(test_case.commits, store.commit_calls);
+    EXPECT_EQ(test_case.aborts, store.abort_calls);
+    EXPECT_EQ(test_case.discards, store.discard_calls);
+    EXPECT_EQ(test_case.rollbacks, store.rollback_calls);
+    // Every failure leaves the previous image published and no artifact behind,
+    // so the next save can start immediately.
+    if (test_case.point != FailurePoint::None) {
+      EXPECT_EQ(InMemoryJsonStore::oldImage(), store.source());
+      EXPECT_TRUE(store.canStartSave());
+    }
+  }
+}
+
+TEST(MQTTPrefsAtomicStore, FailedPublishIsUndoneSoTheRefusedValueCannotReturnAtBoot) {
+  InMemoryJsonStore store(FailurePoint::Commit);
+
+  // The publish moved the old primary to .bak before failing to rename the
+  // verified temp into place. Reporting the change as rolled back is only
+  // truthful because that half-done transaction is undone here.
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::CommitFailed, runVerifiedJson(&store));
+  EXPECT_EQ(InMemoryJsonStore::oldImage(), store.source());
+  EXPECT_FALSE(store.has("/mqtt.json.tmp"));
+  EXPECT_FALSE(store.has("/mqtt.json.bak"));
+  EXPECT_EQ(InMemoryJsonStore::oldImage(), store.imageAfterReboot());
+}
+
+TEST(MQTTPrefsAtomicStore, UnrestorablePublishFailureIsReportedAsIndeterminate) {
+  // Rollback cannot republish the backup: the primary name stays empty and the
+  // verified temp still wins recovery, so the refused value does come back at
+  // the next boot. The caller must say so rather than claim a rollback.
+  InMemoryJsonStore backup_stuck(InMemoryJsonStore::Options{
+      FailurePoint::Commit, /*rollback_rename_fails=*/true, false, true});
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::CommitIndeterminate,
+            runVerifiedJson(&backup_stuck));
+  EXPECT_TRUE(backup_stuck.has("/mqtt.json.tmp"));
+  EXPECT_TRUE(backup_stuck.has("/mqtt.json.bak"));
+  EXPECT_FALSE(backup_stuck.canStartSave());  // retries fail until this resolves
+  EXPECT_EQ(InMemoryJsonStore::newImage(), backup_stuck.imageAfterReboot());
+
+  // Same verdict for the first-ever save, where there is no backup to restore
+  // and the undeletable temp is the only image the next boot can find.
+  InMemoryJsonStore first_save(InMemoryJsonStore::Options{
+      FailurePoint::Commit, false, /*rollback_remove_fails=*/true,
+      /*has_primary=*/false});
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::CommitIndeterminate,
+            runVerifiedJson(&first_save));
+  EXPECT_FALSE(first_save.has("/mqtt.json"));
+  EXPECT_EQ(InMemoryJsonStore::newImage(), first_save.imageAfterReboot());
+}
+
+TEST(MQTTPrefsAtomicStore, RejectedTempThatCannotBeDeletedIsReportedAsIndeterminate) {
+  // The temp is a complete, byte-verified, perfectly valid image: only the
+  // scratch allocation that reparses it failed. With no primary on a fresh
+  // install, a temp that cleanup cannot delete is exactly what boot recovery
+  // promotes, so this must not be answered as a rollback.
+  InMemoryJsonStore first_save(InMemoryJsonStore::Options{
+      FailurePoint::Verify, false, false, /*has_primary=*/false,
+      /*discard_remove_fails=*/true});
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::CleanupIndeterminate,
+            runVerifiedJson(&first_save));
+  EXPECT_TRUE(AtomicStore::saveOutcomeUnresolved(
+      AtomicStore::VerifiedImageResult::CleanupIndeterminate));
+  EXPECT_TRUE(first_save.has("/mqtt.json.tmp"));
+  EXPECT_FALSE(first_save.canStartSave());  // retries fail until this resolves
+  EXPECT_EQ(InMemoryJsonStore::newImage(), first_save.imageAfterReboot());
+
+  // With a primary published, the same stuck temp cannot win recovery, so the
+  // change really is gone and the ordinary rejection verdict still holds.
+  InMemoryJsonStore with_primary(InMemoryJsonStore::Options{
+      FailurePoint::Verify, false, false, /*has_primary=*/true,
+      /*discard_remove_fails=*/true});
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::VerifyFailed,
+            runVerifiedJson(&with_primary));
+  EXPECT_FALSE(AtomicStore::saveOutcomeUnresolved(
+      AtomicStore::VerifiedImageResult::VerifyFailed));
+  EXPECT_EQ(InMemoryJsonStore::oldImage(), with_primary.imageAfterReboot());
+}
+
+TEST(MQTTPrefsAtomicStore, UnverifiedTempThatCannotBeDeletedIsReportedAsIndeterminate) {
+  // finish() failing does not prove the bytes are torn — a complete image whose
+  // read-back failed still parses at the next boot. If abort() cannot remove it
+  // and no primary outranks it, the outcome is unresolved for the same reason.
+  InMemoryJsonStore store(InMemoryJsonStore::Options{
+      FailurePoint::Finish, false, false, /*has_primary=*/false,
+      /*discard_remove_fails=*/false, /*abort_remove_fails=*/true});
+
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::CleanupIndeterminate,
+            runVerifiedJson(&store));
+  EXPECT_TRUE(store.has("/mqtt.json.tmp"));
+  EXPECT_FALSE(store.canStartSave());
+  EXPECT_EQ(InMemoryJsonStore::newImage(), store.imageAfterReboot());
+}
+
+TEST(MQTTPrefsAtomicStore, FirstSavePublishFailureLeavesNoImageToPromote) {
+  InMemoryJsonStore store(InMemoryJsonStore::Options{
+      FailurePoint::Commit, false, false, /*has_primary=*/false});
+
+  // No prior /mqtt.json exists, so rollback only has to discard the temp. The
+  // next boot re-runs migration from the legacy source instead of adopting the
+  // value the CLI just refused.
+  ASSERT_EQ(AtomicStore::VerifiedImageResult::CommitFailed, runVerifiedJson(&store));
+  EXPECT_FALSE(store.has("/mqtt.json.tmp"));
+  EXPECT_TRUE(store.imageAfterReboot().empty());
+  EXPECT_TRUE(store.canStartSave());
+}
+
 TEST(MQTTPrefsAtomicStore, AnyFailureAbortsAndPreservesExistingSource) {
   const std::vector<uint8_t> source = {'o', 'l', 'd', '-', 'p', 'r', 'e', 'f', 's'};
   const struct {
@@ -546,6 +886,23 @@ TEST(MQTTPrefsAtomicStore, PowerCutDuringTempWriteKeepsPrimaryAndAllowsNextSave)
   EXPECT_TRUE(store.canStartSave());
 }
 
+TEST(MQTTPrefsAtomicStore, TornFirstMigrationTempIsDiscardedSoLegacyCanRetry) {
+  SpiffsMqttTransaction store;
+  store.removePrimary();
+  store.cutDuringTempWrite();
+
+  EXPECT_EQ(Recovery::Action::DiscardTemp,
+            Recovery::select(Recovery::FileState::Missing,
+                             Recovery::FileState::Preserve,
+                             Recovery::FileState::Missing));
+  store.recover(Recovery::FileState::Missing,
+                Recovery::FileState::Preserve,
+                Recovery::FileState::Missing);
+  EXPECT_FALSE(store.has("/mqtt_prefs"));
+  EXPECT_FALSE(store.has("/mqtt_prefs.tmp"));
+  EXPECT_TRUE(store.canStartSave());
+}
+
 TEST(MQTTPrefsAtomicStore, RecoveredUsablePrimaryClearsOpaqueTransactionArtifacts) {
   {
     SpiffsMqttTransaction store;
@@ -584,8 +941,11 @@ TEST(MQTTPrefsAtomicStore, SpiffsRenameAndCleanupFailuresRemainRecoverable) {
     EXPECT_FALSE(store.has("/mqtt_prefs"));
     EXPECT_TRUE(store.has("/mqtt_prefs.tmp"));
     EXPECT_TRUE(store.has("/mqtt_prefs.bak"));
+    // Unlike a power cut at this boundary, a returned failure is answered to
+    // the caller, so the transaction is undone before recovery ever sees it.
+    EXPECT_TRUE(store.rollbackFailedPublish());
     store.recover();
-    EXPECT_EQ(SpiffsMqttTransaction::newImage(), store.primary());
+    EXPECT_EQ(SpiffsMqttTransaction::oldImage(), store.primary());
     EXPECT_FALSE(store.has("/mqtt_prefs.tmp"));
     EXPECT_FALSE(store.has("/mqtt_prefs.bak"));
   }
@@ -605,8 +965,14 @@ TEST(MQTTPrefsAtomicStore, RecoveryNeverOverwritesOpaqueNewerLayout) {
   EXPECT_EQ(Recovery::Action::KeepPrimary,
             Recovery::select(Recovery::FileState::Preserve, Recovery::FileState::Usable,
                              Recovery::FileState::Usable));
-  // If there is no primary, a usable backup wins over an opaque temp. Once
-  // promoted, production treats the backup as authoritative and clears temp.
+  // A syntactically valid future temp may already have passed verification and
+  // reached the rename phase. It wins over the stale supported backup and is
+  // promoted into the authoritative name, where older firmware will hold it.
+  EXPECT_EQ(Recovery::Action::PromoteTemp,
+            Recovery::select(Recovery::FileState::Missing, Recovery::FileState::FutureUsable,
+                             Recovery::FileState::Usable));
+  // A corrupt or incomplete temp is different: the supported backup remains
+  // the last known committed image.
   EXPECT_EQ(Recovery::Action::PromoteBackup,
             Recovery::select(Recovery::FileState::Missing, Recovery::FileState::Preserve,
                              Recovery::FileState::Usable));
@@ -617,6 +983,145 @@ TEST(MQTTPrefsAtomicStore, RecoveryNeverOverwritesOpaqueNewerLayout) {
                              Recovery::FileState::Preserve));
 }
 
+TEST(MQTTPrefsAtomicStore, AmbiguousFutureOrOomTempIsNeverDeleted) {
+  for (const Recovery::FileState uncertain : {
+           Recovery::FileState::FutureClaimed,
+           Recovery::FileState::Indeterminate}) {
+    SpiffsMqttTransaction store;
+    store.cutAt(SpiffsMqttTransaction::Boundary::AfterBackupRename);
+    store.recover(Recovery::FileState::Missing, uncertain,
+                  Recovery::FileState::Usable);
+
+    // The supported backup runs this boot from its own name. Nothing is
+    // renamed, so the empty primary name still records that the candidate had
+    // reached the publish phase. Its presence also blocks a new transaction.
+    EXPECT_FALSE(store.has("/mqtt_prefs"));
+    EXPECT_TRUE(store.has("/mqtt_prefs.tmp"));
+    EXPECT_TRUE(store.has("/mqtt_prefs.bak"));
+    EXPECT_FALSE(store.canStartSave());
+  }
+}
+
+TEST(MQTTPrefsAtomicStore, PreservedCandidateIsPromotedByTheBootThatCanReadIt) {
+  // The whole point of keeping an uncertain temp is that a later boot can act
+  // on it. Publishing the backup on the first boot would defeat that: the
+  // candidate would then look like a stale artifact next to a usable primary,
+  // and get deleted exactly when it finally became readable.
+  const struct {
+    Recovery::FileState first_boot;
+    Recovery::FileState second_boot;
+    // A promoted current-format image ends the transaction, so its backup goes
+    // too. A future-format one keeps the last readable image and stays held.
+    bool clears_backup;
+  } cases[] = {
+      // Classification scratch could not be allocated, then heap recovered.
+      {Recovery::FileState::Indeterminate, Recovery::FileState::Usable, true},
+      // Downgraded firmware could not parse it, then the node rolled forward.
+      {Recovery::FileState::FutureClaimed, Recovery::FileState::Usable, true},
+      // Still a newer schema on the second boot, but now classifiable.
+      {Recovery::FileState::FutureClaimed, Recovery::FileState::FutureUsable, false},
+  };
+
+  for (const auto& test_case : cases) {
+    SpiffsMqttTransaction store;
+    store.cutAt(SpiffsMqttTransaction::Boundary::AfterBackupRename);
+
+    store.recover(Recovery::FileState::Missing, test_case.first_boot,
+                  Recovery::FileState::Usable);
+    ASSERT_TRUE(store.has("/mqtt_prefs.tmp"));
+    ASSERT_FALSE(store.has("/mqtt_prefs"));
+
+    store.recover(Recovery::FileState::Missing, test_case.second_boot,
+                  Recovery::FileState::Usable);
+    EXPECT_EQ(SpiffsMqttTransaction::newImage(), store.primary());
+    EXPECT_FALSE(store.has("/mqtt_prefs.tmp"));
+    EXPECT_EQ(test_case.clears_backup, !store.has("/mqtt_prefs.bak"));
+    EXPECT_EQ(test_case.clears_backup, store.canStartSave());
+  }
+}
+
+TEST(MQTTPrefsAtomicStore, CandidateThatProvesInvalidYieldsToTheHeldBackup) {
+  SpiffsMqttTransaction store;
+  store.cutAt(SpiffsMqttTransaction::Boundary::AfterBackupRename);
+  store.recover(Recovery::FileState::Missing, Recovery::FileState::Indeterminate,
+                Recovery::FileState::Usable);
+
+  // The second boot can classify it and finds it definitively corrupt, so the
+  // last committed image is published and the candidate is dropped. The node
+  // leaves the held state without ever having discarded an unread candidate.
+  store.recover(Recovery::FileState::Missing, Recovery::FileState::Preserve,
+                Recovery::FileState::Usable);
+  EXPECT_EQ(SpiffsMqttTransaction::oldImage(), store.primary());
+  EXPECT_FALSE(store.has("/mqtt_prefs.tmp"));
+  EXPECT_TRUE(store.canStartSave());
+}
+
+TEST(MQTTPrefsAtomicStore, UncertainTempWithNoUsableBackupStillOwnsThePrimaryName) {
+  // With no image that can run this boot, the candidate is the only thing left
+  // to protect, so it takes the authoritative name and CommonCLI holds it.
+  EXPECT_EQ(Recovery::Action::PromoteTemp,
+            Recovery::select(Recovery::FileState::Missing,
+                             Recovery::FileState::Indeterminate,
+                             Recovery::FileState::Missing));
+  EXPECT_EQ(Recovery::Action::PromoteTemp,
+            Recovery::select(Recovery::FileState::Missing,
+                             Recovery::FileState::FutureClaimed,
+                             Recovery::FileState::Preserve));
+}
+
+TEST(MQTTPrefsAtomicStore, UncertainTempDoesNotSpendAnOpaqueBackup) {
+  // Newer firmware was replacing a future-version primary when power was lost.
+  // This boot can prove the backup is a syntactically valid future image but
+  // cannot run it, and cannot classify the candidate at all.
+  EXPECT_EQ(Recovery::Action::RunDefaultsHeld,
+            Recovery::select(Recovery::FileState::Missing,
+                             Recovery::FileState::FutureClaimed,
+                             Recovery::FileState::FutureUsable));
+  EXPECT_EQ(Recovery::Action::RunDefaultsHeld,
+            Recovery::select(Recovery::FileState::Missing,
+                             Recovery::FileState::Indeterminate,
+                             Recovery::FileState::Indeterminate));
+  // A backup that is definitively invalid has ceased to be a fallback, so the
+  // candidate may take the authoritative name.
+  EXPECT_EQ(Recovery::Action::PromoteTemp,
+            Recovery::select(Recovery::FileState::Missing,
+                             Recovery::FileState::Indeterminate,
+                             Recovery::FileState::Preserve));
+}
+
+TEST(MQTTPrefsAtomicStore, HeldOpaqueBackupSurvivesACandidateThatProvesInvalid) {
+  SpiffsMqttTransaction store;
+  store.cutAt(SpiffsMqttTransaction::Boundary::AfterBackupRename);
+
+  // Nothing is renamed, so the empty primary name still records the interrupted
+  // commit and the previous committed image keeps its backup name.
+  store.recover(Recovery::FileState::Missing, Recovery::FileState::FutureClaimed,
+                Recovery::FileState::FutureUsable);
+  EXPECT_FALSE(store.has("/mqtt_prefs"));
+  EXPECT_TRUE(store.has("/mqtt_prefs.tmp"));
+  EXPECT_TRUE(store.has("/mqtt_prefs.bak"));
+  EXPECT_FALSE(store.canStartSave());
+
+  // A later boot understands both and finds the candidate corrupt. Because the
+  // first boot did not spend the primary name on it, the last committed image
+  // is still there to publish instead of being stranded behind a bad primary.
+  store.recover(Recovery::FileState::Missing, Recovery::FileState::Preserve,
+                Recovery::FileState::Usable);
+  EXPECT_EQ(SpiffsMqttTransaction::oldImage(), store.primary());
+  EXPECT_FALSE(store.has("/mqtt_prefs.tmp"));
+  EXPECT_TRUE(store.canStartSave());
+}
+
+TEST(MQTTPrefsAtomicStore, UsablePrimaryDoesNotCleanIndeterminateArtifact) {
+  SpiffsMqttTransaction store;
+  store.cutDuringTempWrite();
+  store.recover(Recovery::FileState::Usable,
+                Recovery::FileState::Indeterminate);
+  EXPECT_EQ(SpiffsMqttTransaction::oldImage(), store.primary());
+  EXPECT_TRUE(store.has("/mqtt_prefs.tmp"));
+  EXPECT_FALSE(store.canStartSave());
+}
+
 int main(int argc, char** argv) {
   ::testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();
diff --git a/test/test_mqtt_prefs_codec/test_mqtt_prefs_codec.cpp b/test/test_mqtt_prefs_codec/test_mqtt_prefs_codec.cpp
index 7bf49746df..73ec5ec84c 100644
--- a/test/test_mqtt_prefs_codec/test_mqtt_prefs_codec.cpp
+++ b/test/test_mqtt_prefs_codec/test_mqtt_prefs_codec.cpp
@@ -28,6 +28,7 @@ MQTTPrefs defaults() {
   prefs.alert_wifi_minutes = 30;
   prefs.alert_mqtt_minutes = 240;
   prefs.alert_min_interval_min = 60;
+  prefs.mqtt_neighbors_interval = MQTT_NEIGHBORS_DEFAULT_INTERVAL_MS;
   return prefs;
 }
 
@@ -59,6 +60,21 @@ Codec::DecodePlan classify(const std::vector<uint8_t>& bytes) {
   return Codec::classify(bytes.data(), prefix_size, bytes.size());
 }
 
+void writeV1Payload(std::vector<uint8_t>* bytes, const MQTTPrefs& prefs, size_t payload_len) {
+  LegacyV1MQTTPrefs frozen;
+  Codec::freezeV1(prefs, &frozen);
+  ASSERT_GE(bytes->size(), sizeof(MQTTPrefsHeader) + payload_len);
+  memcpy(bytes->data() + sizeof(MQTTPrefsHeader), &frozen, payload_len);
+}
+
+MQTTPrefs decodeV1(const std::vector<uint8_t>& bytes, const Codec::DecodePlan& plan) {
+  LegacyV1MQTTPrefs frozen = {};
+  memcpy(&frozen, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  MQTTPrefs loaded = defaults();
+  Codec::migrateV1(frozen, plan.payload_len, &loaded);
+  return loaded;
+}
+
 void fillHighEntropy(std::vector<uint8_t>* bytes) {
   uint32_t state = 0x89abcdef;
   for (size_t i = 0; i < bytes->size(); ++i) {
@@ -244,8 +260,7 @@ TEST(MQTTPrefsCodec, CurrentVersionedPayloadRoundTripsExactly) {
   ASSERT_EQ(Codec::Source::Current, plan.source);
   ASSERT_FALSE(plan.preserve_file);
   ASSERT_TRUE(plan.observer_fields_present);
-  MQTTPrefs loaded = defaults();
-  memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  MQTTPrefs loaded = decodeV1(bytes, plan);
   EXPECT_EQ(0, memcmp(&source, &loaded, sizeof(source)));
 }
 
@@ -277,8 +292,7 @@ TEST(MQTTPrefsCodec, DefaultFiltersKeepTheDowngradeReadablePayloadLength) {
   ASSERT_EQ(Codec::kV1PreFilterPayloadSize, plan.payload_len);
   ASSERT_FALSE(plan.preserve_file);
 
-  MQTTPrefs loaded = defaults();
-  memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  MQTTPrefs loaded = decodeV1(bytes, plan);
   EXPECT_EQ(0, memcmp(&source, &loaded, sizeof(source)));
 }
 
@@ -299,8 +313,7 @@ TEST(MQTTPrefsCodec, AnyNonDefaultFilterOptsIntoTheLongerPayload) {
       const Codec::DecodePlan plan = classify(bytes);
       ASSERT_EQ(Codec::kV1BaselinePayloadSize, plan.payload_len);
 
-      MQTTPrefs loaded = defaults();
-      memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+      MQTTPrefs loaded = decodeV1(bytes, plan);
       EXPECT_EQ(mask, loaded.mqtt_slot_packet_filter[slot]) << slot;
       EXPECT_EQ(0, memcmp(&source, &loaded, sizeof(source)));
     }
@@ -338,7 +351,7 @@ TEST(MQTTPrefsCodec, PreFilterV1PayloadDefaultsEverySlotToAllTypes) {
   std::vector<uint8_t> bytes(sizeof(MQTTPrefsHeader) + Codec::kV1PreFilterPayloadSize, 0);
   writeHeader(&bytes, MQTT_PREFS_VERSION,
               static_cast<uint16_t>(Codec::kV1PreFilterPayloadSize));
-  memcpy(bytes.data() + sizeof(MQTTPrefsHeader), &source, Codec::kV1PreFilterPayloadSize);
+  writeV1Payload(&bytes, source, Codec::kV1PreFilterPayloadSize);
 
   const Codec::DecodePlan plan = classify(bytes);
   ASSERT_EQ(Codec::Source::Current, plan.source);
@@ -346,8 +359,7 @@ TEST(MQTTPrefsCodec, PreFilterV1PayloadDefaultsEverySlotToAllTypes) {
   ASSERT_TRUE(plan.observer_fields_present);
   ASSERT_FALSE(plan.preserve_file);
 
-  MQTTPrefs loaded = defaults();
-  memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  MQTTPrefs loaded = decodeV1(bytes, plan);
   EXPECT_STREQ("pre-filter-node", loaded.mqtt_origin);
   EXPECT_EQ(1u, loaded.mqtt_neighbors_enabled);
   EXPECT_EQ(MQTT_NEIGHBORS_MAX_INTERVAL_MS, loaded.mqtt_neighbors_interval);
@@ -368,7 +380,7 @@ TEST(MQTTPrefsCodec, CompatibleShortV1PayloadPreservesDefaultsBeyondObserverBoun
   std::vector<uint8_t> bytes(sizeof(MQTTPrefsHeader) + Codec::kV1PreObserverPayloadSize, 0);
   writeHeader(&bytes, MQTT_PREFS_VERSION,
               static_cast<uint16_t>(Codec::kV1PreObserverPayloadSize));
-  memcpy(bytes.data() + sizeof(MQTTPrefsHeader), &source, Codec::kV1PreObserverPayloadSize);
+  writeV1Payload(&bytes, source, Codec::kV1PreObserverPayloadSize);
 
   const Codec::DecodePlan plan = classify(bytes);
   ASSERT_EQ(Codec::Source::Current, plan.source);
@@ -376,8 +388,7 @@ TEST(MQTTPrefsCodec, CompatibleShortV1PayloadPreservesDefaultsBeyondObserverBoun
   ASSERT_FALSE(plan.preserve_file);
   ASSERT_FALSE(plan.observer_fields_present);
 
-  MQTTPrefs loaded = defaults();
-  memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  MQTTPrefs loaded = decodeV1(bytes, plan);
   EXPECT_STREQ("short-v1-node", loaded.mqtt_origin);
   EXPECT_STREQ("ntp.short.example", loaded.mqtt_ntp_server);
   EXPECT_EQ(0, loaded.snmp_enabled);
@@ -400,7 +411,7 @@ TEST(MQTTPrefsCodec, PreNeighborsV1PayloadLoadsObserverFieldsAndDefaultsNeighbor
   std::vector<uint8_t> bytes(sizeof(MQTTPrefsHeader) + Codec::kV1PreNeighborsPayloadSize, 0);
   writeHeader(&bytes, MQTT_PREFS_VERSION,
               static_cast<uint16_t>(Codec::kV1PreNeighborsPayloadSize));
-  memcpy(bytes.data() + sizeof(MQTTPrefsHeader), &source, Codec::kV1PreNeighborsPayloadSize);
+  writeV1Payload(&bytes, source, Codec::kV1PreNeighborsPayloadSize);
 
   const Codec::DecodePlan plan = classify(bytes);
   ASSERT_EQ(Codec::Source::Current, plan.source);
@@ -408,10 +419,7 @@ TEST(MQTTPrefsCodec, PreNeighborsV1PayloadLoadsObserverFieldsAndDefaultsNeighbor
   ASSERT_FALSE(plan.preserve_file);
   ASSERT_TRUE(plan.observer_fields_present);
 
-  MQTTPrefs loaded = defaults();
-  loaded.mqtt_neighbors_enabled = 1;                                   // pretend stale
-  loaded.mqtt_neighbors_interval = MQTT_NEIGHBORS_DEFAULT_INTERVAL_MS;  // caller's defaulted tail
-  memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  MQTTPrefs loaded = decodeV1(bytes, plan);
 
   EXPECT_STREQ("pre-neighbors-node", loaded.mqtt_origin);
   EXPECT_STREQ("PNW", loaded.alert_region);
@@ -475,6 +483,34 @@ TEST(MQTTPrefsCodec, CorruptOrShortVersionedInputsArePreserved) {
   EXPECT_TRUE(plan.preserve_file);
 }
 
+TEST(MQTTPrefsCodec, HeaderValidV1StillRequiresBoundedPayloadStrings) {
+  MQTTPrefs source = defaults();
+  LegacyV1MQTTPrefs frozen;
+  Codec::freezeV1(source, &frozen);
+  ASSERT_TRUE(Codec::isPlausibleV1(frozen, Codec::kV1BaselinePayloadSize));
+
+  memset(frozen.mqtt_slot_topic[5], 'x', sizeof(frozen.mqtt_slot_topic[5]));
+  EXPECT_FALSE(Codec::isPlausibleV1(frozen, Codec::kV1BaselinePayloadSize));
+}
+
+TEST(MQTTPrefsCodec, HeaderValidV1AllowsNumericValuesForSerializerRepair) {
+  MQTTPrefs source = defaults();
+  LegacyV1MQTTPrefs frozen;
+  Codec::freezeV1(source, &frozen);
+
+  // A valid v1 header fixes the layout. Numeric bytes cannot trigger an
+  // out-of-bounds read and are normalized after migration, so plausibility
+  // must not reject deployed files merely because these values need repair.
+  frozen.timezone_offset = 99;
+  frozen.mqtt_status_enabled = 0xff;
+  frozen.mqtt_rx_enabled = 0xff;
+  frozen.snmp_enabled = 0xff;
+  frozen.alert_enabled = 0xff;
+  frozen.mqtt_neighbors_enabled = 0xa5;
+  EXPECT_TRUE(Codec::isPlausibleV1(frozen, Codec::kV1BaselinePayloadSize));
+  EXPECT_TRUE(Codec::isPlausibleV1(frozen, Codec::kV1PreNeighborsPayloadSize));
+}
+
 TEST(MQTTPrefsCodec, LegacyPlausibilityRejectsHighEntropyBytesAtEveryWhitelistedSize) {
   // A headerless raw struct has no checksum, so this only reduces false
   // migrations; it cannot prove that a plausible-looking file is authentic.
@@ -531,7 +567,9 @@ TEST(MQTTPrefsCodec, LongerSameVersionPayloadLoadsTheBaselineAndIgnoresTheTail)
   const size_t payload_len = Codec::kV1BaselinePayloadSize + kTail;
   std::vector<uint8_t> bytes(sizeof(MQTTPrefsHeader) + payload_len, 0xA5);
   writeHeader(&bytes, MQTT_PREFS_VERSION, static_cast<uint16_t>(payload_len));
-  memcpy(bytes.data() + sizeof(MQTTPrefsHeader), &source, sizeof(source));
+  LegacyV1MQTTPrefs frozen;
+  Codec::freezeV1(source, &frozen);
+  memcpy(bytes.data() + sizeof(MQTTPrefsHeader), &frozen, sizeof(frozen));
 
   const Codec::DecodePlan plan = classify(bytes);
   ASSERT_EQ(Codec::Source::Current, plan.source);
@@ -542,9 +580,11 @@ TEST(MQTTPrefsCodec, LongerSameVersionPayloadLoadsTheBaselineAndIgnoresTheTail)
 
   // Reading plan.payload_len bytes recovers this build's whole struct exactly,
   // and cannot run past it into the unknown tail.
+  LegacyV1MQTTPrefs loaded_frozen = {};
+  memcpy(&loaded_frozen, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
+  ASSERT_TRUE(Codec::isPlausibleV1(loaded_frozen, plan.payload_len));
   MQTTPrefs loaded = defaults();
-  memcpy(&loaded, bytes.data() + sizeof(MQTTPrefsHeader), plan.payload_len);
-  EXPECT_EQ(0, memcmp(&source, &loaded, sizeof(source)));
+  Codec::migrateV1(loaded_frozen, plan.payload_len, &loaded);
   EXPECT_STREQ("future-node", loaded.mqtt_origin);
   EXPECT_STREQ("field-ssid", loaded.wifi_ssid);
   EXPECT_STREQ("field-secret", loaded.wifi_password);
diff --git a/test/test_mqtt_prefs_serializer/test_mqtt_prefs_serializer.cpp b/test/test_mqtt_prefs_serializer/test_mqtt_prefs_serializer.cpp
new file mode 100644
index 0000000000..ce91b0c1a9
--- /dev/null
+++ b/test/test_mqtt_prefs_serializer/test_mqtt_prefs_serializer.cpp
@@ -0,0 +1,381 @@
+#include <gtest/gtest.h>
+
+#include <string>
+
+#define WITH_MQTT_BRIDGE 1
+#define PROGMEM
+#include "helpers/MQTTPrefsSerializer.h"
+
+class InputStream : public Stream {
+public:
+  explicit InputStream(const std::string& text) : _text(text) {}
+  int available() override { return static_cast<int>(_text.size() - _pos); }
+  int read() override { return _pos < _text.size() ? _text[_pos++] : -1; }
+  int peek() override { return _pos < _text.size() ? _text[_pos] : -1; }
+private:
+  std::string _text;
+  size_t _pos = 0;
+};
+
+class OutputStream : public Stream {
+public:
+  size_t write(uint8_t byte) override { _text.push_back(static_cast<char>(byte)); return 1; }
+  size_t print(int value, int = DEC) override { return appendNumber(value); }
+  size_t print(unsigned int value, int = DEC) override { return appendNumber(value); }
+  size_t print(long value, int = DEC) override { return appendNumber(value); }
+  size_t print(unsigned long value, int = DEC) override { return appendNumber(value); }
+  size_t print(long long value, int = DEC) override { return appendNumber(value); }
+  size_t print(unsigned long long value, int = DEC) override { return appendNumber(value); }
+  int available() override { return 0; }
+  int read() override { return -1; }
+  int peek() override { return -1; }
+  const std::string& text() const { return _text; }
+private:
+  template <typename T> size_t appendNumber(T value) {
+    const std::string number = std::to_string(value);
+    _text += number;
+    return number.size();
+  }
+  std::string _text;
+};
+
+class StickyFailingStream : public Stream {
+public:
+  explicit StickyFailingStream(size_t limit) : _limit(limit) {}
+  size_t write(uint8_t) override {
+    if (_failed || _written >= _limit) {
+      _failed = true;
+      return 0;
+    }
+    ++_written;
+    return 1;
+  }
+  size_t print(int value, int = DEC) override {
+    const std::string number = std::to_string(value);
+    return Print::print(number.c_str());
+  }
+  int available() override { return 0; }
+  int read() override { return -1; }
+  int peek() override { return -1; }
+  bool failed() const { return _failed; }
+private:
+  size_t _limit;
+  size_t _written = 0;
+  bool _failed = false;
+};
+
+static MQTTPrefs defaults() {
+  MQTTPrefs prefs = {};
+  prefs.mqtt_status_enabled = 1;
+  prefs.mqtt_packets_enabled = 1;
+  prefs.mqtt_tx_enabled = 2;
+  prefs.mqtt_rx_enabled = 1;
+  prefs.mqtt_status_interval = 300000;
+  prefs.wifi_power_save = 1;
+  prefs.timezone_offset = -7;
+  prefs.radio_watchdog_minutes = 5;
+  prefs.alert_wifi_minutes = 30;
+  prefs.alert_mqtt_minutes = 240;
+  prefs.alert_min_interval_min = 60;
+  prefs.mqtt_neighbors_interval = MQTT_NEIGHBORS_DEFAULT_INTERVAL_MS;
+  strcpy(prefs.snmp_community, "public");
+  for (int i = 0; i < MQTT_PREFS_SLOT_COUNT; ++i) {
+    strcpy(prefs.mqtt_slot_preset[i], "none");
+    prefs.mqtt_slot_packet_filter[i] = 0xffff;
+  }
+  return prefs;
+}
+
+TEST(MQTTPrefsSerializer, RoundTripsEveryGroupAndNumericSlotKeys) {
+  MQTTPrefs source = defaults();
+  strcpy(source.wifi_ssid, "mesh-net");
+  strcpy(source.wifi_password, "p\\\"ass\nword");
+  strcpy(source.timezone_string, "MST7MDT,M3.2.0");
+  strcpy(source.mqtt_ntp_server, "time.example");
+  strcpy(source.mqtt_origin, "observer-one");
+  strcpy(source.mqtt_iata, "SEA");
+  source.mqtt_neighbors_enabled = 1;
+  source.mqtt_neighbors_interval = MQTT_NEIGHBORS_MAX_INTERVAL_MS;
+  strcpy(source.mqtt_owner_public_key,
+         "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef");
+  strcpy(source.mqtt_email, "owner@example.com");
+  strcpy(source.mqtt_slot_preset[5], "custom");
+  strcpy(source.mqtt_slot_host[5], "broker.example");
+  source.mqtt_slot_port[5] = 65535;
+  strcpy(source.mqtt_slot_username[5], "user-six");
+  strcpy(source.mqtt_slot_password[5], "secret-six");
+  strcpy(source.mqtt_slot_token[5], "token-six");
+  strcpy(source.mqtt_slot_topic[5], "mesh/{iata}/{type}");
+  strcpy(source.mqtt_slot_audience[5], "audience-six");
+  source.mqtt_slot_packet_filter[5] = 0x8001;
+  source.snmp_enabled = 1;
+  source.radio_watchdog_minutes = 120;
+  source.alert_enabled = 1;
+  strcpy(source.alert_psk_hex, "0123456789abcdef0123456789abcdef");
+  strcpy(source.alert_hashtag, "#ops");
+  strcpy(source.alert_region, "PNW");
+
+  OutputStream output;
+  MQTTPrefsSerializer writer(&source);
+  ASSERT_TRUE(writer.saveSerial(output));
+  EXPECT_NE(std::string::npos, output.text().find("slot6:{")) << output.text();
+  EXPECT_NE(std::string::npos, output.text().find("packet_filter:32769")) << output.text();
+
+  MQTTPrefs loaded = defaults();
+  InputStream input(output.text());
+  MQTTPrefsSerializer reader(&loaded);
+  ASSERT_TRUE(reader.loadSerial(input)) << output.text();
+  bool repaired = true;
+  ASSERT_TRUE(reader.apply(&repaired));
+  EXPECT_FALSE(repaired);
+  EXPECT_STREQ(source.wifi_password, loaded.wifi_password);
+  EXPECT_STREQ(source.mqtt_owner_public_key, loaded.mqtt_owner_public_key);
+  EXPECT_STREQ(source.mqtt_slot_host[5], loaded.mqtt_slot_host[5]);
+  EXPECT_EQ(65535, loaded.mqtt_slot_port[5]);
+  EXPECT_EQ(0x8001, loaded.mqtt_slot_packet_filter[5]);
+  EXPECT_EQ(MQTT_NEIGHBORS_MAX_INTERVAL_MS, loaded.mqtt_neighbors_interval);
+  EXPECT_STREQ("PNW", loaded.alert_region);
+}
+
+TEST(MQTTPrefsSerializer, MissingOptionalKeysKeepDefaults) {
+  MQTTPrefs prefs = defaults();
+  InputStream input("{version:1,mqtt:{origin:\"changed\"}}");
+  MQTTPrefsSerializer serializer(&prefs);
+  ASSERT_TRUE(serializer.loadSerial(input));
+  bool repaired = true;
+  ASSERT_TRUE(serializer.apply(&repaired));
+  EXPECT_FALSE(repaired);
+  EXPECT_STREQ("changed", prefs.mqtt_origin);
+  EXPECT_EQ(1, prefs.mqtt_packets_enabled);
+  EXPECT_EQ(300000u, prefs.mqtt_status_interval);
+  EXPECT_EQ(0xffff, prefs.mqtt_slot_packet_filter[5]);
+}
+
+TEST(MQTTPrefsSerializer, RequiresSupportedVersion) {
+  MQTTPrefs missing = defaults();
+  InputStream no_version("{wifi:{ssid:\"x\"}}");
+  MQTTPrefsSerializer missing_serializer(&missing);
+  ASSERT_TRUE(missing_serializer.loadSerial(no_version));
+  bool repaired = false;
+  EXPECT_FALSE(missing_serializer.apply(&repaired));
+
+  MQTTPrefs future = defaults();
+  InputStream future_input("{version:2,wifi:{ssid:\"x\"}}");
+  MQTTPrefsSerializer future_serializer(&future);
+  ASSERT_TRUE(future_serializer.loadSerial(future_input));
+  EXPECT_TRUE(future_serializer.hasFutureVersion());
+  EXPECT_FALSE(future_serializer.apply(&repaired));
+}
+
+TEST(MQTTPrefsSerializer, FutureVersionProbeIgnoresV1FieldTypeChanges) {
+  InputStream probe_input("{version:2,wifi:{power_save:\"max\"}}");
+  MQTTPrefsVersionProbe probe;
+  ASSERT_TRUE(probe.loadSerial(probe_input));
+  EXPECT_TRUE(probe.hasFutureVersion());
+
+  // The same image is not valid under v1, demonstrating why recovery must
+  // probe the version before invoking the current schema.
+  MQTTPrefs prefs = defaults();
+  InputStream v1_input("{version:2,wifi:{power_save:\"max\"}}");
+  MQTTPrefsSerializer v1(&prefs);
+  EXPECT_FALSE(v1.loadSerial(v1_input));
+
+  for (const char* future_text : {
+           "{version:2,this_key_is_too_long:1}",
+           "{version:2,x:[1]}",
+           "{version:2,wifi:{ssid:\"torn\"}"}) {
+    InputStream future_input(future_text);
+    MQTTPrefsVersionProbe future_probe;
+    EXPECT_FALSE(future_probe.loadSerial(future_input)) << future_text;
+    EXPECT_TRUE(future_probe.hasFutureVersion()) << future_text;
+  }
+}
+
+TEST(MQTTPrefsSerializer, VersionIsWrittenAsTheFirstRootProperty) {
+  MQTTPrefs prefs = defaults();
+  MQTTPrefsSerializer writer(&prefs);
+  OutputStream output;
+  ASSERT_TRUE(writer.saveSerial(output));
+  EXPECT_EQ(0u, output.text().find("{version:1,")) << output.text();
+}
+
+TEST(MQTTPrefsSerializer, FutureGrammarAheadOfVersionCannotBeRecognizedAsFuture) {
+  // Why the version-first invariant is part of the format rather than a style
+  // preference. These two files differ only in key order, and only the
+  // compliant one keeps its preservation guarantee on this firmware.
+  InputStream compliant("{version:2,x:[1]}");
+  MQTTPrefsVersionProbe compliant_probe;
+  EXPECT_FALSE(compliant_probe.loadSerial(compliant));
+  EXPECT_TRUE(compliant_probe.hasFutureVersion());
+
+  InputStream violating("{x:[1],version:2}");
+  MQTTPrefsVersionProbe violating_probe;
+  EXPECT_FALSE(violating_probe.loadSerial(violating));
+  EXPECT_FALSE(violating_probe.hasFutureVersion());
+}
+
+TEST(MQTTPrefsSerializer, RejectsDuplicateKnownKey) {
+  MQTTPrefs prefs = defaults();
+  InputStream input("{version:1,mqtt:{origin:\"one\",origin:\"two\"}}");
+  MQTTPrefsSerializer serializer(&prefs);
+  EXPECT_FALSE(serializer.loadSerial(input));
+}
+
+TEST(MQTTPrefsSerializer, RejectsOverlongStringAndIntegerOverflow) {
+  MQTTPrefs prefs = defaults();
+  InputStream long_string(
+      "{version:1,wifi:{ssid:\"12345678901234567890123456789012\"}}");
+  MQTTPrefsSerializer string_serializer(&prefs);
+  EXPECT_FALSE(string_serializer.loadSerial(long_string));
+
+  prefs = defaults();
+  InputStream overflow("{version:1,mqtt:{slot1:{port:999999999999}}}");
+  MQTTPrefsSerializer number_serializer(&prefs);
+  EXPECT_FALSE(number_serializer.loadSerial(overflow));
+
+  prefs = defaults();
+  InputStream quoted_number("{version:\"1\"}");
+  MQTTPrefsSerializer quoted_number_serializer(&prefs);
+  EXPECT_FALSE(quoted_number_serializer.loadSerial(quoted_number));
+
+  prefs = defaults();
+  InputStream bare_string("{version:1,wifi:{ssid:meshnet}}");
+  MQTTPrefsSerializer bare_string_serializer(&prefs);
+  EXPECT_FALSE(bare_string_serializer.loadSerial(bare_string));
+}
+
+TEST(MQTTPrefsSerializer, RejectsScalarObjectShapeMismatches) {
+  MQTTPrefs prefs = defaults();
+  InputStream object_version("{version:{x:1}}");
+  MQTTPrefsSerializer object_version_serializer(&prefs);
+  EXPECT_FALSE(object_version_serializer.loadSerial(object_version));
+
+  prefs = defaults();
+  InputStream object_port("{version:1,mqtt:{slot1:{port:{x:1883}}}}");
+  MQTTPrefsSerializer object_port_serializer(&prefs);
+  EXPECT_FALSE(object_port_serializer.loadSerial(object_port));
+
+  prefs = defaults();
+  InputStream scalar_mqtt("{version:1,mqtt:1}");
+  MQTTPrefsSerializer scalar_mqtt_serializer(&prefs);
+  EXPECT_FALSE(scalar_mqtt_serializer.loadSerial(scalar_mqtt));
+
+  prefs = defaults();
+  InputStream scalar_slot("{version:1,mqtt:{slot1:1}}");
+  MQTTPrefsSerializer scalar_slot_serializer(&prefs);
+  EXPECT_FALSE(scalar_slot_serializer.loadSerial(scalar_slot));
+}
+
+TEST(MQTTPrefsSerializer, RepairsSemanticRanges) {
+  MQTTPrefs prefs = defaults();
+  InputStream input(
+      "{version:1,wifi:{power_save:9},time:{utc_offset:99},"
+      "mqtt:{tx_enabled:7,status:{enabled:3,interval_ms:10},"
+      "neighbors:{enabled:2,interval_ms:100},slot1:{port:-1,packet_filter:-2}},"
+      "radio:{watchdog_min:121},alert:{rate_limit_min:1}}");
+  MQTTPrefsSerializer serializer(&prefs);
+  ASSERT_TRUE(serializer.loadSerial(input));
+  bool repaired = false;
+  ASSERT_TRUE(serializer.apply(&repaired));
+  EXPECT_TRUE(repaired);
+  EXPECT_EQ(1, prefs.wifi_power_save);
+  EXPECT_EQ(-7, prefs.timezone_offset);
+  EXPECT_EQ(2, prefs.mqtt_tx_enabled);
+  EXPECT_EQ(300000u, prefs.mqtt_status_interval);
+  EXPECT_EQ(MQTT_NEIGHBORS_DEFAULT_INTERVAL_MS, prefs.mqtt_neighbors_interval);
+  EXPECT_EQ(0, prefs.mqtt_slot_port[0]);
+  EXPECT_EQ(0xffff, prefs.mqtt_slot_packet_filter[0]);
+  EXPECT_EQ(5, prefs.radio_watchdog_minutes);
+  EXPECT_EQ(60, prefs.alert_min_interval_min);
+}
+
+TEST(MQTTPrefsSerializer, RepairsTextValuesToSafeDefaults) {
+  MQTTPrefs prefs = defaults();
+  InputStream input(
+      "{version:1,time:{ntp_server:\"bad/host\"},mqtt:{iata:\"sea\","
+      "owner:{public_key:\"not-a-key\"},slot1:{preset:\"not-a-preset\"},"
+      "slot2:{preset:\"analyzer-us\"},slot3:{preset:\"analyzer-us\"}},"
+      "alert:{psk_hex:\"not-hex\",hashtag:\"#stale\"}}");
+  MQTTPrefsSerializer serializer(&prefs);
+  ASSERT_TRUE(serializer.loadSerial(input));
+  bool repaired = false;
+  ASSERT_TRUE(serializer.apply(&repaired));
+  EXPECT_TRUE(repaired);
+  EXPECT_STREQ("SEA", prefs.mqtt_iata);
+  EXPECT_STREQ("", prefs.mqtt_ntp_server);
+  EXPECT_STREQ("", prefs.mqtt_owner_public_key);
+  EXPECT_STREQ("none", prefs.mqtt_slot_preset[0]);
+  EXPECT_STREQ("analyzer-us", prefs.mqtt_slot_preset[1]);
+  // Historical firmware allowed duplicate aliases. Preserve them on load;
+  // current setters prevent creating new duplicates without silently changing
+  // a deployed configuration during migration.
+  EXPECT_STREQ("analyzer-us", prefs.mqtt_slot_preset[2]);
+  EXPECT_STREQ("", prefs.alert_psk_hex);
+  EXPECT_STREQ("", prefs.alert_hashtag);
+}
+
+TEST(MQTTPrefsSerializer, LateParseOrVersionFailureCannotMutateLivePrefs) {
+  MQTTPrefs live = defaults();
+  strcpy(live.wifi_ssid, "live-network");
+
+  MQTTPrefs scratch = defaults();
+  InputStream truncated("{wifi:{ssid:\"uncommitted\"},version:1");
+  MQTTPrefsSerializer truncated_serializer(&scratch);
+  EXPECT_FALSE(truncated_serializer.loadSerial(truncated));
+  EXPECT_STREQ("live-network", live.wifi_ssid);
+
+  scratch = defaults();
+  InputStream future("{wifi:{ssid:\"future-network\"},version:2}");
+  MQTTPrefsSerializer future_serializer(&scratch);
+  ASSERT_TRUE(future_serializer.loadSerial(future));
+  EXPECT_TRUE(future_serializer.hasFutureVersion());
+  bool repaired = false;
+  EXPECT_FALSE(future_serializer.apply(&repaired));
+  EXPECT_STREQ("live-network", live.wifi_ssid);
+}
+
+TEST(MQTTPrefsSerializer, StickyShortWriteFailsTheCompleteSave) {
+  MQTTPrefs prefs = defaults();
+  MQTTPrefsSerializer serializer(&prefs);
+  StickyFailingStream output(20);
+  EXPECT_FALSE(serializer.saveSerial(output));
+  EXPECT_TRUE(output.failed());
+}
+
+TEST(MQTTPrefsSerializer, SaveNormalizationIsIdempotentAgainstKnownDefaults) {
+  MQTTPrefs prefs = defaults();
+  prefs.timezone_offset = 99;
+  strcpy(prefs.mqtt_ntp_server, "bad/host");
+  strcpy(prefs.mqtt_iata, "not-iata");
+  strcpy(prefs.mqtt_slot_preset[0], "not-a-preset");
+
+  MQTTPrefs repair_defaults = defaults();
+  repair_defaults.timezone_offset = -7;
+  strcpy(repair_defaults.mqtt_iata, "sea");
+  strcpy(repair_defaults.mqtt_slot_preset[0], "analyzer-us");
+
+  MQTTPrefsSerializer writer(&prefs, &repair_defaults);
+  bool repaired = false;
+  ASSERT_TRUE(writer.normalize(&repaired));
+  EXPECT_TRUE(repaired);
+  EXPECT_EQ(-7, prefs.timezone_offset);
+  EXPECT_STREQ("", prefs.mqtt_ntp_server);
+  EXPECT_STREQ("SEA", prefs.mqtt_iata);
+  EXPECT_STREQ("none", prefs.mqtt_slot_preset[0]);
+
+  OutputStream output;
+  ASSERT_TRUE(writer.saveSerial(output));
+
+  MQTTPrefs loaded = defaults();
+  InputStream input(output.text());
+  MQTTPrefsSerializer reader(&loaded);
+  ASSERT_TRUE(reader.loadSerial(input));
+  repaired = true;
+  ASSERT_TRUE(reader.apply(&repaired));
+  EXPECT_FALSE(repaired) << output.text();
+}
+
+int main(int argc, char** argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}
diff --git a/test/test_observer_validation/test_observer_validation.cpp b/test/test_observer_validation/test_observer_validation.cpp
index 3045caed6a..8e9913ca86 100644
--- a/test/test_observer_validation/test_observer_validation.cpp
+++ b/test/test_observer_validation/test_observer_validation.cpp
@@ -129,6 +129,18 @@ TEST(ValueFits, RejectsNullOrZeroBuffer) {
   EXPECT_FALSE(mqttValueFits("abc", 0));
 }
 
+TEST(AssignedPresetSlot, FindsDuplicatesOutsideTheTargetSlot) {
+  const char presets[4][24] = {
+    "analyzer-us", "none", "analyzer-eu", "analyzer-us"
+  };
+
+  EXPECT_EQ(3, mqttAssignedPresetSlot(presets, "analyzer-us", 0));
+  EXPECT_EQ(0, mqttAssignedPresetSlot(presets, "analyzer-us", 3));
+  EXPECT_EQ(-1, mqttAssignedPresetSlot(presets, "analyzer-eu", 2));
+  EXPECT_EQ(-1, mqttAssignedPresetSlot(presets, "missing", 0));
+  EXPECT_EQ(-1, mqttAssignedPresetSlot(presets, nullptr, 0));
+}
+
 int main(int argc, char** argv) {
   ::testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();
diff --git a/test/test_webconfig_batch/test_webconfig_batch.cpp b/test/test_webconfig_batch/test_webconfig_batch.cpp
index 813412c57c..670c6ead14 100644
--- a/test/test_webconfig_batch/test_webconfig_batch.cpp
+++ b/test/test_webconfig_batch/test_webconfig_batch.cpp
@@ -230,6 +230,7 @@ TEST(WebConfigBatch, CliFailureRepliesAreRecognisedInEveryShapeCommonCLIEmits) {
     "Err - bad params",                          // MyMesh setperm
     "ERR: bad pubkey",                           // neighbor.remove
     "Error: IATA code must be exactly 3 letters",// observer setters
+    "Error: setting not persisted; change rolled back", // observer storage failure
     "(ERR: clock cannot go backwards)",          // clock sync, parenthesised
     "Unknown command",                           // top-level fallthrough
     "unknown config: mqtt.nope",                 // set fallthrough