The api_content sidecar ('persist what you send') preserves prompt-cache
stability across turn boundaries by persisting the exact API-bound bytes
(including memory-manager prefetch, plugin injections, and API-only notes)
and substituting them on replay.
When a user turn was already materialized in the database before the
sidecar could be composed (in-place preflight compaction or a close/early
flush racing the prologue on the CLI path), the turn-start crash persist
marker-skips that message. Previously, the backfill was gated strictly on
in-place compaction (_preflight_compressed and _last_compaction_in_place),
so racing CLI flushes left api_content = NULL in SQLite and broke prompt
caching on subsequent turns (#102194).
Positional approaches (such as #102239 and #102286) using LIMIT 1 on the
newest active user row are unsafe: repeated common inputs ('ok', 'yes',
'continue') cause the backfill to match and overwrite the PREVIOUS turn's
row with the new turn's sidecar, corrupting history and breaking cache parity.
Resolve all landing blockers and review feedback from #102411:
1. Bounded state owner (Sahilvishnaliya):
Add SessionDB.set_message_api_content(session_id, row_id, content, api_content)
to SessionMessagesMixin in hermes_state_messages.py instead of growing
hermes_state.py. Update set_latest_user_api_content docstring with durable
warning on the positional hazard.
2. API-only turns & durable content selection (ehz0ah):
When a pre-flushed clean input has an API-only difference (e.g. voice
prefix or model-switch note):
- Retain the differing API-facing bytes as api_content even when no
new memory or plugin context was injected.
- Derive the durable content guard using _override_replaces_content so
the SQL 'content IS ?' guard matches the clean override text stored
in the DB row rather than the restored wire text.
3. Turn prologue gating (_row_id) & fail-closed store duck-typing (ehz0ah):
In agent/turn_context.py::_stamp_api_content_sidecar: check _row_id on
the live user dict (stamped by _insert_message_rows and synced by
sync_flushed_message_markers). If valid (positive int, not bool), address
by exact ID. Do NOT fall back to positional matching when a row ID is
present: if an external or custom wrapper lacks set_message_api_content,
fail closed and skip rather than corrupting a neighbouring row. If absent
but in-place compacted, fall back to positional update. On normal turns,
skip the backfill entirely (single atomic INSERT).
4. Real lifecycle test coverage (salch-cred, ehz0ah):
Comprehensive tests in tests/agent/test_api_content_row_addressed_backfill.py
covering store guards, surrogate scrubbing, gate non-arming, older identical
row protection, real close-flush row_id synchronization, API-only clean
override preservation with exact wire replay, and duck-typed store fail-closed
verification when set_message_api_content is absent.
Fixes#102194.
Closes#102411.
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