@@ -126,11 +126,29 @@ edit (two registered `description` strings) and was ruled a hold, as 1.0.4's was
   ObjectManager, already no-opped and returned the row, so the two api-contract lanes
   disagreed on this input.
 
-  **The cross-port contract for an empty PATCH remains deliberately unsettled** and this does
-  not settle it — `fixtures/api-contract-conformance/scenarios/writeonce-patch-stripped.yaml`
-  records that the C# reference 400s on an empty SET where TypeScript no-ops. That is a
-  ruling to make, not a fix to sneak in under one port, so no conformance fixture pins it
-  here. What is fixed is only this: 500 was defensible under no reading of it.
+  This fix deliberately did not settle the cross-port contract; the entry below now does.
+
+- **An empty PATCH answered five different ways across the ports; it is now one, and gated.**
+  FR-037 PATCH-5 has always said a patch that strips to zero caller assignments reads the
+  current row back — but nothing in the corpus asserted it, and an unasserted corner is where
+  divergence lives. The ports had drifted to: TypeScript 500 on the generated mount and 200 on
+  the reference lane (fixed above), C# and Java 400 `validation` from their base
+  `AuthorApiServer` while `TphReferenceServer`/`JsonbReferenceServer` in the same directory
+  200'd, Kotlin 200, Python 200. Both 400s cited "per FR-035" — a rule that says the opposite.
+  The answer is now **200 with the current row** in all five ports and both lanes, `404` when
+  the row does not exist, pinned by the new
+  `fixtures/api-contract-conformance/scenarios/patch-empty-noop.yaml`.
+
+  The fix in C# and Java is the *removal* of an empty-`SET` guard: the `@autoSet:onUpdate`
+  stamp was already being appended unconditionally right after it, so dropping the guard makes
+  the `SET` non-empty on its own and the ordinary write path answers correctly. That also
+  settles how PATCH-3 and PATCH-5 meet — the server-side stamp IS an assignment, so on an
+  entity carrying one the literal zero-assignment read-back path is never reached, and
+  `patch-empty-noop.yaml` asserts the stamp still bumps. `docs/features/generated-mutations.md`
+  PATCH-5 is corrected accordingly; it previously read "no write is executed", which holds only
+  for an entity with no onUpdate column.
+
+  No adopter action: a request that used to 400 or 500 now succeeds.
 
 - **Java: a field name that is legal metadata could not be a record component.** The JLS
   forbids a record component named after a no-argument `Object` method
 
@@ -34,7 +34,7 @@ regenerate with `ls -d fixtures/<corpus>/*/ | wc -l` for directory-shaped corpor
 | [`fixtures/extract-conformance/`](../fixtures/extract-conformance/) | 33 | ✓ | ✓ | inherits the shared JVM engine | ✓ | ✓ |
 | [`fixtures/output-prompt-conformance/`](../fixtures/output-prompt-conformance/) | 14 | ✓ | ✓ | ✓ | ✓ | ✓ |
 | [`fixtures/persistence-conformance/`](../fixtures/persistence-conformance/) | 33 (27 query + 6 migration) | all 33 | 27 query (migrations TS-only, ADR-0015) | 27 query (via Exposed) | 27 query | 27 query |
-| [`fixtures/api-contract-conformance/`](../fixtures/api-contract-conformance/) | 50 (28 core + 9 tph + 9 m2m + 2 jsonb + 2 write-through) | ✓ (Fastify reference + generated lane) | ✓ (embedded HTTP + JDBC) | ✓ (embedded HTTP + Exposed) | ✓ (HttpListener + Npgsql) | ✓ (FastAPI + pg8000) |
+| [`fixtures/api-contract-conformance/`](../fixtures/api-contract-conformance/) | 51 (29 core + 9 tph + 9 m2m + 2 jsonb + 2 write-through) | ✓ (Fastify reference + generated lane) | ✓ (embedded HTTP + JDBC) | ✓ (embedded HTTP + Exposed) | ✓ (HttpListener + Npgsql) | ✓ (FastAPI + pg8000) |
 | [`fixtures/validation-conformance/`](../fixtures/validation-conformance/) | 16 cases | ✓ | ✓ | ✓ | ✓ | ✓ |
 | [`fixtures/registry-conformance/`](../fixtures/registry-conformance/) | 1 canonical manifest | ✓ (reference emitter) | ✓ | ✓ | ✓ | ✓ |
 | [`fixtures/object-model-conformance/`](../fixtures/object-model-conformance/) | 1 shared metadata fixture (per-port scenarios) | ✓ | ✓ | ✓ | ✓ | ✓ |
@@ -226,9 +226,9 @@ All 31 fixtures → [features/migrations-and-drift.md](features/migrations-and-d
 - `migrations/*` (6) → [features/migrations-and-drift.md](features/migrations-and-drift.md) (schema migration section)
 - `queries/*` (27) → [features/source-kinds.md](features/source-kinds.md) (query semantics against `source.rdb`)
 
-### `fixtures/api-contract-conformance/` (50)
+### `fixtures/api-contract-conformance/` (51)
 
-All 50 scenarios → [features/api-contract.md](features/api-contract.md) (cross-port
+All 51 scenarios → [features/api-contract.md](features/api-contract.md) (cross-port
 REST API URL grammar + JSON wire format). Verifies every backend's emitted CRUD
 routes answer identically over HTTP — list / get / create / patch+put / delete,
 plus pagination (`limit`/`offset`), sort (`sort=field:dir`), the `withCount=1`
@@ -238,10 +238,11 @@ status codes.
 The corpus also covers the 9 cross-port filter operators (`eq`, `ne`, `gt`,
 `gte`, `lt`, `lte`, `in`, `like`, `isNull`) plus the implicit-AND combinator
 and 2 error shapes (`invalid_filter_field` / `invalid_filter_op`) under the
-URL grammar `?filter[<field>][<op>]=<value>` (FR-009). On top of the 28 core
+URL grammar `?filter[<field>][<op>]=<value>` (FR-009). On top of the 29 core
 scenarios the corpus carries four sub-corpora — `tph/` (9, single-table
-inheritance), `m2m/` (8 — 3 plain plus 5 gating TPH x M:N together, the
-combination each corpus alone could not reach), `jsonb/` (2, typed value-object columns) and
+inheritance), `m2m/` (9 — 3 plain, 5 gating TPH x M:N together, the combination
+each corpus alone could not reach, and 1 pinning the collection-URL spelling),
+`jsonb/` (2, typed value-object columns) and
 `write-through/` (2, table-write + view-read entities). All 5 ports — TS, Java,
 Kotlin, C#, Python — run it in BOTH lanes: a hand-rolled reference server and
 the port's own GENERATED API artifact booted over HTTP.
 
@@ -49,8 +49,15 @@ but the semantics are one contract — call them **PATCH-1..7** (FR-035):
   entity carrying one.
 - **PATCH-4 (validation).** Assigned values run the same per-field validation and
   the same write codec as `create`.
-- **PATCH-5 (empty patch).** Zero assignments is a no-op — the current row is read
-  back and returned, never an empty-`SET` SQL error.
+- **PATCH-5 (empty patch).** A body that strips to zero *caller* assignments answers
+  `200` with the current row — never a `400`, and never an empty-`SET` SQL error.
+  Note how this meets PATCH-3: the server-side `@autoSet:onUpdate` stamp is itself an
+  assignment, so on an entity carrying one the `SET` is never actually empty and an
+  ordinary write runs (bumping that column, as it does for any other patch). The
+  literal zero-assignment read-back path is reachable only for an entity with **no**
+  onUpdate column. Both routes answer identically, which is what makes the contract
+  stable; what a port must not do is let the caller's empty set decide the status.
+  Gated cross-port by `fixtures/api-contract-conformance/scenarios/patch-empty-noop.yaml`.
 - **PATCH-6 (result).** The patch returns the full updated row (`RETURNING`, or a
   same-transaction re-read); a missing row is the port's not-found idiom
   (`null` / `Optional.empty` / `404`).
 
@@ -30,6 +30,7 @@ fixtures/api-contract-conformance/
     ├── create-constraint-violation-400.yaml # FR-036 POST field-constraint → 400 {error:"validation"}
     ├── update-patch-and-put.yaml
     ├── update-constraint-violation-400.yaml # FR-036 PATCH present-value constraint → 400
+    ├── patch-empty-noop.yaml       # FR-037 PATCH-5 a patch stripping to zero assignments is a no-op read-back
     ├── delete-204-and-404.yaml
     ├── invalid-sort-400.yaml
     ├── filter-eq.yaml             # FR-009 filter operators
 
@@ -0,0 +1,61 @@
+name: patch-empty-noop
+description: >
+  FR-037 PATCH-5 — a PATCH whose body strips to ZERO CALLER assignments answers
+  200 with the current row. Never a 400, and never an empty-`SET` SQL error.
+
+  This is a documented contract (`docs/features/generated-mutations.md` PATCH-5:
+  "Zero assignments → no write is executed; the current row is read back and
+  returned"), but until 2026-09-20 nothing in this corpus asserted it, and the
+  ports had drifted to FIVE different answers for one input: TS 500'd on the
+  generated mount and 200'd on the reference lane, C# and Java 400'd from
+  `AuthorApiServer` while their own Tph/Jsonb reference servers 200'd, Kotlin
+  200'd but wrote, and Python 200'd. An unasserted corner is where divergence
+  lives; this scenario closes it.
+
+  The probe names ONLY `issuedCurrency`, which is `@mutability: "writeOnce"` and
+  therefore STRIPPED on this path (PATCH-3). Stripping the sole key is what
+  produces the empty assignment set, so this exercises the real mechanism rather
+  than a synthetic empty body — which also keeps the request non-empty as JSON,
+  so no runner has to decide whether to send `{}` at all. It is exactly the arm
+  `writeonce-patch-stripped.yaml` avoids by pairing the frozen key with a
+  settable sibling; that file's r3 comment recorded this case as deliberately
+  unsettled, and now points here instead.
+requests:
+  - id: r1
+    method: PATCH
+    path: /api/authors/1
+    body:
+      issuedCurrency: "EUR"
+    expect:
+      status: 200
+      body:
+        row:
+          # The row comes back whole and UNCHANGED — a read, answered at a write verb.
+          id: 1
+          name: "Ada Lovelace"
+          bio: "First computer programmer"
+          # The writeOnce column keeps its created value; the presented "EUR" was stripped.
+          issuedCurrency: "USD"
+        # PATCH-3 and PATCH-5 meet here, and the order matters. The onUpdate
+        # @autoSet column is stamped server-side "regardless of the caller", and
+        # that stamp IS an assignment — so on an entity carrying one, the SET is
+        # never actually empty and a real write runs. PATCH-5's literal
+        # zero-assignment read-back path is reachable only for an entity with NO
+        # onUpdate column. Both readings answer 200 with the current row, which is
+        # why the contract is stable either way; what a port must not do is let the
+        # CALLER's empty set decide, and 400 or 500 before the stamp is applied.
+        # Asserted field-vs-field (format- and timing-agnostic: the seed's 2000
+        # sentinel vs a 2026 now()), same idiom as autoset-patch.yaml.
+        fieldsNotEqual: [autoCreatedAt, autoUpdatedAt]
+  - id: r2
+    method: PATCH
+    path: /api/authors/999
+    body:
+      issuedCurrency: "EUR"
+    expect:
+      # The arm a short-circuit misses. A no-op patch still has to find the row, or
+      # the fast path becomes a hole that reports success for any id — and on a TPH
+      # route, a hole through which one subtype answers for another's row.
+      status: 404
+      body:
+        error: not_found
@@ -53,11 +53,12 @@ requests:
     path: /api/authors/1
     body:
       # A readWrite field rides along DELIBERATELY. Sending the null ALONE would
-      # also probe the empty-PATCH contract, which this corpus has never settled
-      # cross-port (TS no-ops and returns the row; the C# reference 400s on an
-      # empty SET) — and settling that is not this scenario's business. With a
-      # settable sibling present, the request is unambiguously non-empty and the
-      # only thing under test is the writeOnce column's response to a null.
+      # also probe the empty-PATCH contract, and settling that is not this
+      # scenario's business. With a settable sibling present, the request is
+      # unambiguously non-empty and the only thing under test is the writeOnce
+      # column's response to a null. (The empty-PATCH contract was unsettled
+      # cross-port when this was written; it is FR-037 PATCH-5 — a no-op
+      # read-back — and is now pinned by `patch-empty-noop.yaml`.)
       bio: "cleared-currency probe"
       issuedCurrency: null
     expect:
 
@@ -617,15 +617,17 @@ private async Task UpdateAuthorAsync(HttpListenerContext ctx, string idStr)
         // 400'd, matching every other excluded key on this path. Do NOT add a branch for
         // it: that would leave the mode unenforced. The omission also covers the FR-035
         // present-null arm for free, since clearing a column is itself a write.
-        if (sets.Count == 0)
-        {
-            await SendJsonAsync(ctx, 400, new Dictionary<string, object?> { ["error"] = "validation" });
-            return;
-        }
+        // FR-037 PATCH-5 (patch-empty-noop): NO empty-SET guard here. A patch that strips to
+        // zero CALLER assignments is a no-op answering 200 with the current row — never a 400.
+        // This used to 400 "per FR-035", citing a rule that says the opposite, and it disagreed
+        // with TphReferenceServer/JsonbReferenceServer in this very directory.
+
         // Issue #203 / ADR-0045 — bump the onUpdate @autoSet column (autoUpdatedAt) to now();
         // autoCreatedAt (onCreate) is NEVER written on update (the write-once created-at contract),
-        // so after a PATCH the two diverge. Appended AFTER the empty-body guard so a no-op PATCH
-        // still 400s per FR-035. Unspecified kind to match the plain-TIMESTAMP column.
+        // so after a PATCH the two diverge. Appended UNCONDITIONALLY, which is also what keeps the
+        // SET non-empty when the caller assigned nothing — so the empty-SET SQL error PATCH-5
+        // warns about cannot arise on an entity carrying an onUpdate column.
+        // Unspecified kind to match the plain-TIMESTAMP column.
         sets.Add("\"autoUpdatedAt\" = @p" + vals.Count);
         vals.Add(DateTime.SpecifyKind(DateTimeOffset.UtcNow.UtcDateTime, DateTimeKind.Unspecified));
         int updated;
 
@@ -517,12 +517,16 @@ private void updateAuthor(HttpExchange exchange, String idStr) throws IOExceptio
         // rejected, matching every other excluded key on this path. Do NOT add a branch:
         // that would leave the mode unenforced. The omission covers the FR-035
         // present-null arm too, since clearing a column is itself a write.
-        if (setClauses.isEmpty()) { sendJson(exchange, 400, Map.of("error", "validation")); return; }
+        // FR-037 PATCH-5 (patch-empty-noop): NO empty-SET guard here. A patch that strips to
+        // zero CALLER assignments is a no-op answering 200 with the current row — never a 400.
+        // This used to 400 "per FR-035", citing a rule that says the opposite, and it disagreed
+        // with TphReferenceServer/JsonbReferenceServer in this very package.
 
         // Issue #203 / ADR-0045: honor @autoSet onUpdate — bump autoUpdatedAt to now() on EVERY
         // update, and NEVER write autoCreatedAt (created-at is immutable; rewriting it is the
-        // lost-update bug the gate catches). Appended after the caller-field empty-check so the
-        // empty-body → 400 semantics stay keyed on the caller's fields only.
+        // lost-update bug the gate catches). Appended UNCONDITIONALLY, which is also what keeps
+        // the SET non-empty when the caller assigned nothing — so the empty-SET SQL error
+        // PATCH-5 warns about cannot arise on an entity carrying an onUpdate column.
         setClauses.add("\"autoUpdatedAt\" = ?");
         values.add(OffsetDateTime.now(ZoneOffset.UTC));