diff --git a/planfile/api/server.py b/planfile/api/server.py
index 3144c45..97564c6 100644
--- a/planfile/api/server.py
+++ b/planfile/api/server.py
@@ -986,6 +986,38 @@ def public_forensic_log_days():
     }
 
 
+#: Sub-models a PATCH may address. A request carries each of them as a complete
+#: object, so they are merged rather than written through.
+_MERGEABLE_TICKET_SECTIONS = ("executor", "execution", "inputs", "outputs")
+
+
+def _merged_ticket_updates(body: "TicketUpdate", current) -> dict:
+    """Apply only the fields a PATCH actually sent.
+
+    FastAPI parses `{"inputs": {"uri_processes": [...]}}` into a complete
+    `TicketInputs`, filling every omitted field with its model default. Writing
+    that straight through replaces the stored section, so one partial update
+    silently erases everything the caller did not mention — `inputs.api_body`,
+    `execution.started_at`, `execution.last_error` and the rest.
+
+    Pydantic records which fields were present at each level, so merge the sent
+    keys onto what is stored. An explicitly sent `null` still clears its field;
+    only omission is now inert.
+    """
+    updates = {k: v for k, v in body.model_dump().items() if v is not None}
+    for section in _MERGEABLE_TICKET_SECTIONS:
+        if section not in body.model_fields_set:
+            continue
+        sent = getattr(body, section)
+        if sent is None:
+            continue
+        supplied = sent.model_dump(mode="json", exclude_unset=True)
+        stored = getattr(current, section, None)
+        base = stored.model_dump(mode="json", exclude_none=True) if stored is not None else {}
+        updates[section] = {**base, **supplied}
+    return updates
+
+
 @app.patch("/tickets/{ticket_id}", tags=["tickets"])
 async def update_ticket(ticket_id: str, body: TicketUpdate):
     pf = get_planfile()
@@ -1001,7 +1033,7 @@ async def update_ticket(ticket_id: str, body: TicketUpdate):
         body.inputs if body.inputs is not None else current.inputs,
         require_for_legacy=False,
     )
-    updates = {k: v for k, v in body.model_dump().items() if v is not None}
+    updates = _merged_ticket_updates(body, current)
     if body.status is not None and str(body.status) != str(current.status.value):
         updates["actor"] = body.actor or "unknown:api"
         updates["reason"] = body.reason or f"status_transition:{current.status.value}->{body.status}"
diff --git a/tests/test_partial_ticket_update.py b/tests/test_partial_ticket_update.py
new file mode 100644
index 0000000..e6802d5
--- /dev/null
+++ b/tests/test_partial_ticket_update.py
@@ -0,0 +1,106 @@
+"""A PATCH must change only the fields it sends.
+
+Observed 2026-09-03 on a running deployment: twelve supervisor plans sat in
+`waiting_input` for up to 152 hours with an automatic recovery that could never
+see them. The recovery keys on `inputs.api_body`; the producer wrote it, and a
+later unrelated PATCH — one that only meant to set `uri_processes` — erased it,
+because FastAPI had filled every omitted field of `TicketInputs` with its model
+default before the store wrote the section through.
+
+The same shape erased `execution.last_error` and `execution.started_at`
+elsewhere. These tests pin the section-merge behaviour that stops it.
+"""
+
+from __future__ import annotations
+
+import pytest
+from fastapi.testclient import TestClient
+
+
+@pytest.fixture()
+def client(tmp_path, monkeypatch):
+    monkeypatch.chdir(tmp_path)
+    from planfile.api import server
+
+    server.get_planfile.cache_clear() if hasattr(server.get_planfile, "cache_clear") else None
+    return TestClient(server.app)
+
+
+def create(client, **overrides):
+    payload = {"name": "Supervisor plan: verify the seam", "priority": "high", **overrides}
+    response = client.post("/tickets", json=payload)
+    assert response.status_code in (200, 201), response.text
+    return response.json()["id"]
+
+
+def get_inputs(client, ticket_id):
+    response = client.get(f"/tickets/{ticket_id}")
+    assert response.status_code == 200, response.text
+    return response.json().get("inputs") or {}
+
+
+def get_execution(client, ticket_id):
+    response = client.get(f"/tickets/{ticket_id}")
+    assert response.status_code == 200, response.text
+    return response.json().get("execution") or {}
+
+
+def patch(client, ticket_id, body):
+    response = client.patch(
+        f"/tickets/{ticket_id}",
+        json={"actor": "test:agent", "reason": "covering the partial-update seam", **body},
+    )
+    assert response.status_code == 200, response.text
+    return response.json()
+
+
+def test_a_partial_inputs_patch_keeps_the_fields_it_did_not_send(client):
+    envelope = {"schema": "subactor.supervisor-ticket-input/v1", "apply": False, "goal": "ship it"}
+    ticket_id = create(client, inputs={"api_body": envelope, "prompt": "original prompt"})
+    assert get_inputs(client, ticket_id)["api_body"] == envelope
+
+    # The exact shape that erased it: a write that only means to set processes.
+    patch(client, ticket_id, {"inputs": {"uri_processes": []}})
+
+    stored = get_inputs(client, ticket_id)
+    assert stored["api_body"] == envelope, "an unmentioned section field must survive"
+    assert stored["prompt"] == "original prompt"
+
+
+def test_a_partial_execution_patch_keeps_the_failure_reason(client):
+    ticket_id = create(client)
+    patch(client, ticket_id, {"execution": {"state": "running", "last_error": "bridge_422:rejected"}})
+    assert get_execution(client, ticket_id)["last_error"] == "bridge_422:rejected"
+
+    # A heartbeat-shaped write that names only the queue.
+    patch(client, ticket_id, {"execution": {"queue": "project-operator-bot"}})
+
+    execution = get_execution(client, ticket_id)
+    assert execution["last_error"] == "bridge_422:rejected", "a heartbeat must not erase the reason"
+    assert execution["queue"] == "project-operator-bot"
+
+
+def test_a_sent_field_still_overwrites(client):
+    ticket_id = create(client, inputs={"prompt": "first"})
+    patch(client, ticket_id, {"inputs": {"prompt": "second"}})
+    assert get_inputs(client, ticket_id)["prompt"] == "second"
+
+
+def test_an_explicit_null_still_clears_a_field(client):
+    ticket_id = create(client, inputs={"prompt": "first"})
+    patch(client, ticket_id, {"inputs": {"prompt": None}})
+    # Omission is inert; sending null is still how a caller clears a field.
+    assert "prompt" not in get_inputs(client, ticket_id)
+
+
+def test_untouched_sections_are_left_alone(client):
+    ticket_id = create(client, inputs={"prompt": "keep me"})
+    patch(client, ticket_id, {"execution": {"queue": "somewhere"}})
+    assert get_inputs(client, ticket_id)["prompt"] == "keep me"
+
+
+def test_a_patch_that_sends_no_section_changes_nothing_in_them(client):
+    envelope = {"schema": "subactor.supervisor-ticket-input/v1", "apply": False}
+    ticket_id = create(client, inputs={"api_body": envelope})
+    patch(client, ticket_id, {"priority": "low"})
+    assert get_inputs(client, ticket_id)["api_body"] == envelope