Per-session /model overrides (_session_model_overrides) were in-memory only,
so a gateway restart silently reverted every session to the global default
model. Persist the non-secret parts (model/provider/base_url ONLY — never
api_key) into the session entry in sessions.json and lazily rehydrate them
on first use after a restart, re-resolving credentials through the normal
runtime provider resolution.
- gateway/session.py: SessionEntry.model_override field with
sanitize_model_override() (allowlist: model/provider/base_url) applied on
both serialization and deserialization; SessionStore.set_model_override /
get_model_override accessors. reset_session() already creates a fresh entry,
so /new keeps its clear-on-reset semantics — a restart cannot resurrect an
override the user reset away.
- gateway/slash_commands.py: write-through at both /model set sites (text
command + picker) after storing the in-memory override.
- gateway/run.py: _rehydrate_session_model_override() called from
_resolve_session_agent_runtime(); in-memory state always wins, credentials
are re-resolved per provider (credential-less fallback on failure). Session
expiry finalization also drops the persisted override.
- tests/gateway/test_session_model_override_persistence.py: restart
round-trip, /new clearing, api_key-never-serialized (including tampered
sessions.json), rehydration + live-state precedence + credential-failure
degradation.
Salvaged from #3659 by @Git-on-my-level, narrowed to the restart-persistence
gap confirmed in triage.
