diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2ea97c6..04cbdb5 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/api/routes.py b/api/routes.py
index bfbce17..c6785a2 100644
--- a/api/routes.py
+++ b/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
diff --git a/tests/test_provider_mismatch.py b/tests/test_provider_mismatch.py
index 255a8a2..9377178 100644
--- a/tests/test_provider_mismatch.py
+++ b/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."
+    )