Review — end-to-end ✅ (clean approve)

What this ships

Agent's fix for #1169 — race condition where loading any session with a model only present in the live catalog (e.g. Kimi K2 via Nous) silently overwrites the session model on disk with the first static option (often Opus 4.6). 39 +/15 - in static/ui.js only.

This is the agent-prepared version; @bergeouss filed a parallel attempt as PR #1173 with a different shape. This PR is cleaner — see comparison below.

Traced against upstream hermes-agent

Pulled fresh nousresearch/hermes-agent tarball. Pure WebUI-frontend race; _liveModelFetchPending, _addLiveModelsToSelect, _applyModelToDropdown, and syncTopbar are WebUI-internal. No agent-side coupling. The session model that gets persisted via /api/session/update is the same field the agent reads on load — so any correct model write is fine, and the bug fix is about preventing incorrect writes during the race.

End-to-end trace

_liveModelFetchPending flag — Set keyed by provider

static/ui.js:168-172:

const _liveModelFetchPending=new Set();

Set in _fetchLiveModels synchronously before the first await (ui.js:241) and removed in a finally block (ui.js:257-258). Because the synchronous portion runs before the JS event loop yields, by the time populateModelDropdown returns control to its caller and syncTopbar runs, the flag is already set whenever a network fetch is in flight.

Cache-hit path returns early before add() (ui.js:233-237) — by then the dropdown already has the live models synchronously, so syncTopbar will find the model and the flag is unnecessary.

syncTopbar early-return — no UI flicker

static/ui.js:1849-1875:

const liveStillPending=window._activeProvider&&_liveModelFetchPending.has(window._activeProvider);
if(liveStillPending){
  // Live fetch in flight — don't touch sel.value or S.session.model yet.
} else {
  // ... existing reset-and-persist path ...
}

When live fetch is pending, the dropdown is left at its current value (which may be the default/empty option) and S.session.model is unchanged. No visible flicker — the dropdown displays the right value once _addLiveModelsToSelect runs.

This is cleaner than the alternative (set sel.value=first.value while skipping persist) because it avoids briefly showing the wrong model in the chip.

The existing if(!deferModelCorrection) literal is preserved verbatim inside the else branch — keeps the existing test_session_metadata_fast_path regression test intact without modification.

Re-apply on cache+network paths

static/ui.js:222-229 — _addLiveModelsToSelect re-applies S.session.model after adding live models, regardless of how many were added:

if(S.session && S.session.model && sel.id==='modelSelect'){
  const reapplied=_applyModelToDropdown(S.session.model, sel);
  if(reapplied && typeof syncModelChip==='function') syncModelChip();
}

sel.id==='modelSelect' filters to the chat header's primary picker — won't re-target profile-editor pickers or other dropdowns that share the helper. ✅

Static behaviour harness — 8/8 pass

Set declaration: PASS
add() before fetch(): PASS
delete() in finally: PASS
Cache-hit return before add(): PASS
syncTopbar checks .has(): PASS
liveStillPending early return: PASS
reapply S.session.model: PASS
reapply guarded by sel.id===modelSelect: PASS

Comparison vs PR #1173 (the parallel contributor attempt)

Net: this PR is the cleaner fix. Recommend closing #1173 in favour of this one once merged.

Edge-case trace

Tests

• All 5 test_session_metadata_fast_path tests: pass.
• Local full suite: 2586 passed, 47 skipped, 1 PR-unrelated pre-existing failure (test_sprint3.py::test_workspace_add_rejects_system_paths — macOS-only, fails on master too).
• CI on PR: ✅ test (3.11), ✅ test (3.12), ✅ test (3.13).
• Static harness: 8/8 pass.

Minor observations (non-blocking)

• The PR touches ui.js only and doesn't add a regression test for the race itself (just relies on the existing tests not breaking). A behavioural test for the race would be useful but harder — it requires a real DOM and async fetch mock. Acceptable for a focused two-helper fix.
• _addLiveModelsToSelect's re-apply loops through S.session.model even when nothing was added. Cheap (single dropdown lookup), but a if (added > 0) gate would skip the work in the no-models-arrived-yet case. Cosmetic.

Recommendation

Approved. Tight 39-line fix that closes #1169 cleanly. Set + early-return + DRY re-apply are the right primitives. No UI flicker, no incorrect persist, no cross-tool coupling, existing test preserved verbatim. Static harness verifies all 8 invariants. Parked at approval — ready for the release agent's merge/tag pipeline.