diff --git a/roboco/services/conventions.py b/roboco/services/conventions.py
index 5ce87e60..70253a0f 100644
--- a/roboco/services/conventions.py
+++ b/roboco/services/conventions.py
@@ -393,15 +393,34 @@ class ConventionsService(BaseService):
         mapping: ConventionsStandard,
         status: str,
     ) -> None:
-        self.session.add(
-            ProjectConventionsCacheTable(
-                project_id=project_id,
+        """Persist the effective map for ``(project_id, commit_sha)``.
+
+        Runs the INSERT in a savepoint so a concurrent duplicate (two task
+        creates for the same project/HEAD racing to populate the cache) rolls
+        back ONLY the savepoint on IntegrityError — the loser's insert is
+        dropped, the winner's row satisfies the next ``_cache_get``, and the
+        shared session is not poisoned. A bare ``add`` + ``flush`` here would
+        leave the session in error state and crash the rest of task creation
+        (the task-create transaction rides the same session) — F042.
+        """
+        from sqlalchemy.exc import IntegrityError
+
+        try:
+            async with self.session.begin_nested():
+                self.session.add(
+                    ProjectConventionsCacheTable(
+                        project_id=project_id,
+                        commit_sha=commit_sha,
+                        effective_map=mapping.model_dump(mode="json"),
+                        status=status,
+                    )
+                )
+        except IntegrityError:
+            self.log.debug(
+                "conventions cache row already present (concurrent put)",
+                project_id=str(project_id),
                 commit_sha=commit_sha,
-                effective_map=mapping.model_dump(mode="json"),
-                status=status,
             )
-        )
-        await self.session.flush()
 
     async def _latest_ok_row(
         self, project_id: UUID
diff --git a/tests/unit/services/test_conventions_cache_put.py b/tests/unit/services/test_conventions_cache_put.py
new file mode 100644
index 00000000..edd594d0
--- /dev/null
+++ b/tests/unit/services/test_conventions_cache_put.py
@@ -0,0 +1,111 @@
+"""ConventionsService._cache_put isolates a concurrent-duplicate insert (F042).
+
+Two task creates for the same project/HEAD can race to populate the
+conventions cache; the loser's INSERT fails the partial-unique index with
+IntegrityError. A bare ``session.add`` + ``flush`` poisons the shared
+session — the task-create transaction rides the same session, so every
+subsequent op raises "this session is in error state" and task creation
+crashes. The fix runs the insert in a savepoint and rolls back ONLY the
+savepoint on conflict, leaving the outer transaction usable (the winner's
+row satisfies the next ``_cache_get``).
+
+These tests exercise the contract with a fake session: a duplicate must not
+raise out of ``_cache_put``, must not call a full ``session.rollback()``
+(which would undo the outer task-create transaction), and must not poison
+the session — while the happy path still adds + commits the savepoint.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+from uuid import uuid4
+
+import pytest
+from roboco.foundation.policy.conventions import ConventionsStandard
+from roboco.services.conventions import ConventionsService
+from sqlalchemy.exc import IntegrityError
+
+
+class _FakeNested:
+    """A stand-in for the savepoint returned by ``session.begin_nested()``.
+
+    On release (``__aexit__``) it flushes the queued insert: a concurrent
+    duplicate raises IntegrityError and the SAVEPOINT is rolled back — the
+    outer session is NOT poisoned (only the savepoint failed). The happy path
+    releases the savepoint cleanly.
+    """
+
+    def __init__(self, session: _FakeSession) -> None:
+        self._session = session
+
+    async def __aenter__(self) -> None:
+        self._session.savepoint_started += 1
+
+    async def __aexit__(self, exc_type: Any, exc: Any, tb: Any) -> None:
+        if self._session._duplicate:
+            self._session.savepoint_rolled_back = True
+            raise IntegrityError(
+                "INSERT ... conventions_cache", {}, Exception("unique")
+            )
+        self._session.savepoint_committed = True
+
+
+class _FakeSession:
+    def __init__(self, *, duplicate: bool) -> None:
+        self._duplicate = duplicate
+        self.added: list[Any] = []
+        self.poisoned = False
+        self.full_rollback_called = False
+        self.savepoint_started = 0
+        self.savepoint_committed = False
+        self.savepoint_rolled_back = False
+
+    def add(self, obj: Any) -> None:
+        self.added.append(obj)
+
+    async def flush(self) -> None:
+        # Old-code path: a bare flush on a duplicate poisons the shared session.
+        if self._duplicate:
+            self.poisoned = True
+            raise IntegrityError(
+                "INSERT ... conventions_cache", {}, Exception("unique")
+            )
+
+    def begin_nested(self) -> _FakeNested:
+        return _FakeNested(self)
+
+    async def rollback(self) -> None:
+        self.full_rollback_called = True
+
+
+def _mapping() -> ConventionsStandard:
+    return ConventionsStandard(modules=[], rules={}, waivers=[])
+
+
+@pytest.mark.asyncio
+async def test_cache_put_tolerates_concurrent_duplicate_without_poisoning() -> None:
+    # F042: the loser of a concurrent cache-populate race must not crash the
+    # shared task-create session. The duplicate IntegrityError is contained to
+    # a savepoint; _cache_put returns cleanly, the session is not poisoned, and
+    # no full rollback undoes the outer task-create transaction.
+    session = _FakeSession(duplicate=True)
+    svc = ConventionsService(session=session)  # type: ignore[arg-type]
+
+    await svc._cache_put(uuid4(), "deadbeef", _mapping(), "ok")
+
+    assert not session.poisoned
+    assert not session.full_rollback_called
+    assert session.savepoint_rolled_back is True
+
+
+@pytest.mark.asyncio
+async def test_cache_put_happy_path_adds_and_releases_savepoint() -> None:
+    session = _FakeSession(duplicate=False)
+    svc = ConventionsService(session=session)  # type: ignore[arg-type]
+
+    await svc._cache_put(uuid4(), "deadbeef", _mapping(), "ok")
+
+    assert len(session.added) == 1
+    assert session.savepoint_committed is True
+    assert not session.poisoned
+    assert not session.full_rollback_called