Codex found a verified lifecycle hole: the bare boolean model_explicitly_picked
was not cleared on /api/session/update (api/routes.py:14438-14460 changes+persists
the model/provider without touching it), so after an update+reload the flag stayed
True and a cold #433 stale leftover (openai/gpt-5.4) was wrongly preserved instead
of stripped to gpt-5.4.
Fix (Codex's prescription): replace the unscoped boolean with a SIGNATURE bound to
the routing context.
- api/models.py: new model_explicit_pick_signature(model, provider) ->
'<model>\x1f<provider>'; Session.model_explicit_pick_signature persisted field
(replaces the boolean; in METADATA_FIELDS, restored via kwargs, survives restart).
- api/routes.py /api/chat/start: on a fresh explicit_model_pick, stamp the signature
of the resolved model+provider.
- api/streaming.py: treat the send as a deliberate pick ONLY when the persisted
signature equals the signature recomputed from the session's CURRENT persisted
model+provider. Any model/provider change (chat-start, session-update,
normalization, provider repair) yields a different signature and auto-invalidates
the stale pick — no per-mutation-site clearing needed.
So: deliberate pick on unchanged context -> preserve (b3nw); any switch away ->
signature mismatch -> unmarked -> legacy strip (#433 stays correct even after a
session-update+cold-restart). Adds signature match/invalidate/persist tests.
75 focused tests pass (resolver + sprint40 + the 2 streaming-mock suites).
53a4b35555