fix: null/empty session model must not trigger index rebuild — v0.50.101 (#753)

## Summary

Follow-up to #751/#752. Code review identified a case where `_normalize_session_model_in_place` could call `session.save()` (which triggers a full session index rebuild) for sessions with `model: null` or missing model field.

Root cause: `_resolve_compatible_session_model(None)` returns `(default_model, True)` when a default exists — which was interpreted as "changed, needs save." But there's nothing to correct for a session with no model; the default is just a fallback for display purposes, not a cross-provider correction worth persisting.

Fix: capture `original_model` before calling `_resolve_compatible_session_model`. Only call `session.save()` if `original_model` was non-empty and actually changed.

Adds a test asserting `save_calls == []` when `session.model is None`.

No behavior change for sessions with a real model (the primary use case of #751 is unaffected).

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.101] — 2026-04-20
+
+### Fixed
+- **Session model normalization: null/empty model no longer triggers index rebuild** — sessions with no stored model (`model: null` or missing) now return the provider default without writing to disk. Previously a spurious `session.save()` (and full session index rebuild) could fire for any such session. (#751 follow-up)
+
 ## [v0.50.100] — 2026-04-20
 
 ### Fixed

api/routes.py

@@ -227,8 +227,12 @@ def _resolve_compatible_session_model(model_id: str | None) -> tuple[str, bool]:
 
 
 def _normalize_session_model_in_place(session) -> str:
-    effective_model, changed = _resolve_compatible_session_model(getattr(session, "model", None))
-    if changed and effective_model and getattr(session, "model", None) != effective_model:
+    original_model = getattr(session, "model", None) or ""
+    effective_model, changed = _resolve_compatible_session_model(original_model or None)
+    # Only persist the correction if the session had an explicit model that needed changing.
+    # Sessions with no model stored (empty/None) get the effective default returned without
+    # a disk write — no need to rebuild the index for a fill-in-blank operation.
+    if changed and effective_model and original_model and original_model != effective_model:
         session.model = effective_model
         session.save(touch_updated_at=False)
     return effective_model

tests/test_provider_mismatch.py

@@ -459,3 +459,36 @@ def test_unknown_prefix_model_passes_through_unchanged(monkeypatch):
         assert effective == custom_model, (
             f"Expected '{custom_model}', got '{effective}'"
         )
+
+
+def test_empty_model_session_does_not_trigger_save(monkeypatch):
+    """Sessions with no model stored must not trigger session.save() — index rebuild is expensive."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "openai-codex",
+            "default_model": "gpt-5.4-mini",
+        },
+    )
+
+    save_calls = []
+
+    class DummySession:
+        def __init__(self):
+            self.model = None  # no model stored
+
+        def save(self, touch_updated_at=True):
+            save_calls.append(touch_updated_at)
+
+    session = DummySession()
+    effective = routes._normalize_session_model_in_place(session)
+
+    # Must return the default, but must NOT write to disk
+    assert effective == "gpt-5.4-mini"
+    assert save_calls == [], (
+        "_normalize_session_model_in_place must not call session.save() when "
+        "the session has no stored model — no correction needed, just a fallback."
+    )