Reduce session index churn on chat start

api/models.py

@@ -239,6 +239,14 @@ def _write_session_index(updates=None):
         _write_session_index(updates=None)
 
 
+def prune_session_from_index(session_id: str) -> None:
+    """Remove one session row from the persisted sidebar index if present."""
+    sid = str(session_id or "")
+    if not sid or not SESSION_INDEX_FILE.exists():
+        return
+    _write_session_index(updates=[])
+
+
 def _active_stream_ids():
     with STREAMS_LOCK:
         active_ids = set(STREAMS.keys())

api/routes.py

@@ -2459,6 +2459,7 @@ from api.models import (
     get_state_db_session_messages,
     merge_session_messages_append_only,
     _session_message_merge_key,
+    prune_session_from_index,
     ensure_cron_project,
     is_cron_session,
 )
@@ -5306,7 +5307,7 @@ def handle_post(handler, parsed) -> bool:
             # here makes the active-session external-refresh poll force-reload the
             # current chat every few seconds while the user is typing, and that
             # delayed reload can restore an older draft over newer local input.
-            s.save(touch_updated_at=False)
+            s.save(touch_updated_at=False, skip_index=True)
         return j(handler, {"ok": True, "draft": s.composer_draft})
 
     if parsed.path == "/api/session/update":
@@ -5392,10 +5393,6 @@ def handle_post(handler, parsed) -> bool:
         # Delete from WebUI session store
         with LOCK:
             SESSIONS.pop(sid, None)
-        try:
-            SESSION_INDEX_FILE.unlink(missing_ok=True)
-        except Exception:
-            logger.debug("Failed to unlink session index")
         # Evict cached agent so turn count doesn't leak into a recycled session
         from api.config import _evict_session_agent
         _evict_session_agent(sid)
@@ -5409,6 +5406,10 @@ def handle_post(handler, parsed) -> bool:
             p.with_suffix('.json.bak').unlink(missing_ok=True)
         except Exception:
             logger.debug("Failed to unlink session file %s", p)
+        try:
+            prune_session_from_index(sid)
+        except Exception:
+            logger.debug("Failed to prune deleted session from index: %s", sid, exc_info=True)
         try:
             from api.upload import _session_attachment_dir
 

tests/test_regressions.py

@@ -312,11 +312,8 @@ def test_deleted_session_does_not_appear_in_list(cleanup_test_sessions):
     assert sid not in ids_after,         f"Deleted session {sid} still appears in list -- index not invalidated on delete"
 
 
-def test_server_delete_invalidates_index(cleanup_test_sessions):
-    """R8b: session/delete handler must unlink _index.json.
-    Static check that the fix is in place.
-    Sprint 11: handler moved from server.py to api/routes.py -- check both.
-    """
+def test_server_delete_prunes_session_index(cleanup_test_sessions):
+    """session/delete should prune the deleted row without discarding the index."""
     src = (REPO_ROOT / "server.py").read_text()
     routes_src = (REPO_ROOT / "api" / "routes.py").read_text() if (REPO_ROOT / "api" / "routes.py").exists() else ""
     # Find the delete handler in either file
@@ -327,12 +324,11 @@ def test_server_delete_invalidates_index(cleanup_test_sessions):
             text.find('if parsed.path == "/api/session/delete":'),
         )
         if delete_idx >= 0:
-            # Use 1200 chars to accommodate any validation/guard code added
-            # before the SESSION_INDEX_FILE.unlink() call (e.g. session_id
-            # character checks, path traversal guards).
-            delete_block = text[delete_idx:delete_idx+1200]
-            assert "SESSION_INDEX_FILE" in delete_block, \
-                f"{label} session/delete must invalidate SESSION_INDEX_FILE"
+            delete_block = text[delete_idx:delete_idx+1800]
+            assert "prune_session_from_index" in delete_block, \
+                f"{label} session/delete must prune SESSION_INDEX_FILE"
+            assert "SESSION_INDEX_FILE.unlink" not in delete_block, \
+                f"{label} session/delete must not discard the whole session index"
             return
     assert False, "session/delete handler not found in server.py or api/routes.py"
 

tests/test_session_index.py

@@ -20,7 +20,7 @@ from unittest.mock import patch
 import pytest
 
 import api.models as models
-from api.models import Session, _write_session_index
+from api.models import Session, _write_session_index, prune_session_from_index
 
 
 @pytest.fixture(autouse=True)
@@ -83,6 +83,22 @@ def test_compact_exposes_last_message_at_from_message_timestamp():
     assert compact["last_message_at"] == 200.0
 
 
+def test_prune_session_from_index_removes_only_deleted_row():
+    index_file = models.SESSION_INDEX_FILE
+    s_a = _make_session("sess_a", "A", updated_at=100)
+    s_b = _make_session("sess_b", "B", updated_at=200)
+    s_a.save()
+    s_b.save()
+
+    s_a.path.unlink()
+    prune_session_from_index("sess_a")
+
+    index = _read_index(index_file)
+    ids = [entry["session_id"] for entry in index]
+    assert ids == ["sess_b"]
+    assert index_file.exists()
+
+
 def test_all_sessions_backfills_last_message_at_for_legacy_index_rows():
     index_file = models.SESSION_INDEX_FILE
     s = Session(

tests/test_stage326_composer_draft_validation.py

@@ -100,5 +100,5 @@ def test_draft_save_does_not_touch_session_updated_at():
     src = Path(__file__).parents[1].joinpath("api", "routes.py").read_text(encoding="utf-8")
     persist_idx = src.find("s.composer_draft = draft")
     assert persist_idx != -1, "could not locate composer draft persist site"
-    save_idx = src.find("s.save(touch_updated_at=False)", persist_idx)
-    assert save_idx != -1, "composer draft save must preserve session updated_at"
+    save_idx = src.find("s.save(touch_updated_at=False, skip_index=True)", persist_idx)
+    assert save_idx != -1, "composer draft save must preserve session updated_at and skip index churn"