diff --git a/src/cleanup_service.py b/src/cleanup_service.py
index ec1503d9c..b19e8c5c7 100644
--- a/src/cleanup_service.py
+++ b/src/cleanup_service.py
@@ -37,6 +37,41 @@ def _apply_owner_filter(query, DbSession, owner: Optional[str]):
     return query.filter(DbSession.owner == owner)
 
 
+def _message_count_subquery(db, DbSession, DbChatMessage):
+    """Count durable rows instead of trusting the derived session counter."""
+    from sqlalchemy import func
+    return db.query(func.count(DbChatMessage.id)).filter(
+        DbChatMessage.session_id == DbSession.id
+    ).correlate(DbSession).scalar_subquery()
+
+
+def _recent_session_ids(db, DbSession, owner: Optional[str]) -> set:
+    """Fetch the bounded set of recently created session IDs to preserve."""
+    recent_q = db.query(DbSession.id).order_by(DbSession.created_at.desc())
+    recent_q = _apply_owner_filter(recent_q, DbSession, owner)
+    return {row[0] for row in recent_q.limit(CleanupConfig.PRESERVE_RECENT_COUNT).all()}
+
+
+def _supports_delete_returning(db) -> bool:
+    """Check if the current database backend and runtime version support DELETE ... RETURNING."""
+    dialect = getattr(db.get_bind(), "dialect", None)
+    if dialect is None:
+        return False
+    if dialect.name == "postgresql":
+        return True
+    if dialect.name == "sqlite":
+        # SQLite added RETURNING support in 3.35.0 (March 2021).
+        version = getattr(dialect, "server_version_info", None)
+        if version is not None:
+            return version >= (3, 35, 0)
+        try:
+            import sqlite3
+            return sqlite3.sqlite_version_info >= (3, 35, 0)
+        except Exception:
+            return False
+    return getattr(dialect, "delete_returning", False)
+
+
 async def archive_inactive_sessions(session_manager, owner: Optional[str] = None) -> int:
     """
     Archive sessions that haven't been accessed in the configured number of days.
@@ -96,29 +131,26 @@ async def cleanup_old_sessions(session_manager, owner: Optional[str] = None) ->
     from src.database import SessionLocal, Session as DbSession, ChatMessage as DbChatMessage
     db = SessionLocal()
     try:
-        recent_q = db.query(DbSession).order_by(DbSession.created_at.desc())
-        recent_q = _apply_owner_filter(recent_q, DbSession, owner)
-        all_sessions = recent_q.all()
-        recent_session_ids = {session.id for session in all_sessions[:CleanupConfig.PRESERVE_RECENT_COUNT]}
+        recent_session_ids = _recent_session_ids(db, DbSession, owner)
 
         base_query = db.query(DbSession).filter(
             DbSession.archived == True,
             DbSession.last_accessed < cutoff_date,
             DbSession.is_important == False,
-            DbSession.message_count < CleanupConfig.MIN_MESSAGES_TO_KEEP
         )
         base_query = _apply_owner_filter(base_query, DbSession, owner)
-
-        candidate_sessions = base_query.all()
+        actual_count = _message_count_subquery(db, DbSession, DbChatMessage)
+        candidate_sessions = base_query.add_columns(actual_count).all()
         sessions_to_delete = []
+        message_counts = {}
         preserved_count = 0
 
-        for session in candidate_sessions:
+        for session, message_count in candidate_sessions:
             if session.id in recent_session_ids:
                 preserved_count += 1
                 continue
 
-            if session.message_count >= CleanupConfig.MIN_MESSAGES_TO_KEEP:
+            if message_count >= CleanupConfig.MIN_MESSAGES_TO_KEEP:
                 preserved_count += 1
                 continue
 
@@ -128,20 +160,52 @@ async def cleanup_old_sessions(session_manager, owner: Optional[str] = None) ->
                 continue
 
             sessions_to_delete.append(session)
-
-        for session in sessions_to_delete:
-            message_count = db.query(DbChatMessage).filter(
-                DbChatMessage.session_id == session.id
-            ).count()
-            space_freed += message_count * CleanupConfig.ESTIMATED_MESSAGE_SIZE_BYTES
+            message_counts[session.id] = message_count
 
         session_ids = [session.id for session in sessions_to_delete]
         if session_ids:
-            db.query(DbSession).filter(DbSession.id.in_(session_ids)).delete(synchronize_session=False)
-            deleted_count = len(session_ids)
+            # Recheck the real count in the DELETE statement itself. A message
+            # arriving after candidate selection must retain the protection.
+            # Using RETURNING accurately identifies rows deleted by this cleanup,
+            # avoiding misattributing space freed if another transaction independently
+            # deleted a candidate row.
+            from sqlalchemy import delete
+            stmt = delete(DbSession).filter(
+                DbSession.id.in_(session_ids),
+                DbSession.archived == True,
+                DbSession.last_accessed < cutoff_date,
+                DbSession.is_important == False,
+                actual_count < CleanupConfig.MIN_MESSAGES_TO_KEEP,
+            )
+            stmt = _apply_owner_filter(stmt, DbSession, owner)
+            if _supports_delete_returning(db):
+                deleted_rows = db.execute(stmt.returning(DbSession.id)).scalars().all()
+                deleted_ids = set(deleted_rows)
+                deleted_count = len(deleted_ids)
+                surviving_ids = {row[0] for row in db.query(DbSession.id).filter(
+                    DbSession.id.in_(session_ids)
+                ).all()}
+                unaccounted_ids = set(session_ids) - surviving_ids - deleted_ids
+                space_freed = sum(message_counts[sid] for sid in deleted_ids) * CleanupConfig.ESTIMATED_MESSAGE_SIZE_BYTES
+            else:
+                # Portable fallback for legacy SQLite (< 3.35.0) which lacks RETURNING syntax.
+                res = db.execute(stmt)
+                deleted_count = res.rowcount or 0
+                surviving_ids = {row[0] for row in db.query(DbSession.id).filter(
+                    DbSession.id.in_(session_ids)
+                ).all()}
+                deleted_ids = set(session_ids) - surviving_ids
+                unaccounted_ids = set()
+                if deleted_count == 0:
+                    space_freed = 0
+                elif len(deleted_ids) <= deleted_count:
+                    space_freed = sum(message_counts[sid] for sid in deleted_ids) * CleanupConfig.ESTIMATED_MESSAGE_SIZE_BYTES
+                else:
+                    space_freed = sum(message_counts[sid] for sid in sorted(deleted_ids)[:deleted_count]) * CleanupConfig.ESTIMATED_MESSAGE_SIZE_BYTES
             db.commit()
 
-            for session_id in session_ids:
+            cache_evict_ids = deleted_ids | unaccounted_ids
+            for session_id in cache_evict_ids:
                 if session_id in session_manager.sessions:
                     del session_manager.sessions[session_id]
 
@@ -176,7 +240,7 @@ async def get_cleanup_preview(owner: Optional[str] = None) -> Dict[str, Any]:
     estimated_space_freed = 0
     preserved_sessions = []
 
-    from src.database import SessionLocal, Session as DbSession
+    from src.database import SessionLocal, Session as DbSession, ChatMessage as DbChatMessage
     db = SessionLocal()
     try:
         archive_q = db.query(DbSession).filter(
@@ -184,49 +248,46 @@ async def get_cleanup_preview(owner: Optional[str] = None) -> Dict[str, Any]:
             DbSession.archived == False
         )
         archive_q = _apply_owner_filter(archive_q, DbSession, owner)
-        archive_candidates = archive_q.all()
+        actual_count = _message_count_subquery(db, DbSession, DbChatMessage)
+        archive_candidates = archive_q.add_columns(actual_count).all()
 
-        for session in archive_candidates:
+        for session, message_count in archive_candidates:
             sessions_to_archive.append({
                 "id": session.id,
                 "name": session.name,
                 "last_accessed": session.last_accessed.isoformat() if session.last_accessed else "Unknown",
-                "message_count": session.message_count
+                "message_count": message_count
             })
 
-        recent_q = db.query(DbSession).order_by(DbSession.created_at.desc())
-        recent_q = _apply_owner_filter(recent_q, DbSession, owner)
-        all_sessions = recent_q.all()
-        recent_session_ids = {session.id for session in all_sessions[:CleanupConfig.PRESERVE_RECENT_COUNT]}
+        recent_session_ids = _recent_session_ids(db, DbSession, owner)
 
         base_query = db.query(DbSession).filter(
             DbSession.archived == True,
             DbSession.last_accessed < cutoff_delete,
             DbSession.is_important == False,
-            DbSession.message_count < CleanupConfig.MIN_MESSAGES_TO_KEEP
         )
         base_query = _apply_owner_filter(base_query, DbSession, owner)
 
-        candidate_sessions = base_query.all()
+        candidate_sessions = base_query.add_columns(actual_count).all()
 
-        for session in candidate_sessions:
+        for session, message_count in candidate_sessions:
             if session.id in recent_session_ids:
                 preserved_sessions.append({
                     "id": session.id,
                     "name": session.name,
                     "reason": f"part of last {CleanupConfig.PRESERVE_RECENT_COUNT} sessions",
                     "last_accessed": session.last_accessed.isoformat() if session.last_accessed else "Unknown",
-                    "message_count": session.message_count
+                    "message_count": message_count
                 })
                 continue
 
-            if session.message_count >= CleanupConfig.MIN_MESSAGES_TO_KEEP:
+            if message_count >= CleanupConfig.MIN_MESSAGES_TO_KEEP:
                 preserved_sessions.append({
                     "id": session.id,
                     "name": session.name,
                     "reason": f"has {CleanupConfig.MIN_MESSAGES_TO_KEEP}+ messages",
                     "last_accessed": session.last_accessed.isoformat() if session.last_accessed else "Unknown",
-                    "message_count": session.message_count
+                    "message_count": message_count
                 })
                 continue
 
@@ -238,18 +299,18 @@ async def get_cleanup_preview(owner: Optional[str] = None) -> Dict[str, Any]:
                     "name": session.name,
                     "reason": f"contains keyword: {matching_keywords[0]}",
                     "last_accessed": session.last_accessed.isoformat() if session.last_accessed else "Unknown",
-                    "message_count": session.message_count
+                    "message_count": message_count
                 })
                 continue
 
-            session_space = session.message_count * CleanupConfig.ESTIMATED_MESSAGE_SIZE_BYTES
+            session_space = message_count * CleanupConfig.ESTIMATED_MESSAGE_SIZE_BYTES
             estimated_space_freed += session_space
 
             sessions_to_delete.append({
                 "id": session.id,
                 "name": session.name,
                 "last_accessed": session.last_accessed.isoformat() if session.last_accessed else "Unknown",
-                "message_count": session.message_count,
+                "message_count": message_count,
                 "estimated_size_kb": round(session_space / 1024, 2)
             })
 
diff --git a/tests/test_cleanup_transcript_preservation.py b/tests/test_cleanup_transcript_preservation.py
new file mode 100644
index 000000000..e2c27277e
--- /dev/null
+++ b/tests/test_cleanup_transcript_preservation.py
@@ -0,0 +1,377 @@
+"""Cleanup must use durable transcript rows for its deletion safeguards."""
+from datetime import datetime, timedelta, timezone
+from types import SimpleNamespace
+
+import pytest
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from sqlalchemy import create_engine, event, insert
+from sqlalchemy.orm import sessionmaker
+
+import core.database as database
+import src.database as legacy_database
+import routes.cleanup.cleanup_routes as routes
+
+
+@pytest.fixture
+def cleanup_api(tmp_path, monkeypatch):
+    engine = create_engine(
+        f"sqlite:///{(tmp_path / 'cleanup.db').as_posix()}",
+        connect_args={"check_same_thread": False},
+    )
+    database.Base.metadata.create_all(engine)
+    factory = sessionmaker(bind=engine, autoflush=False)
+    monkeypatch.setattr(legacy_database, "SessionLocal", factory)
+    monkeypatch.setattr(routes, "get_current_user", lambda request: "alice")
+    manager = SimpleNamespace(sessions={})
+    app = FastAPI()
+    app.include_router(routes.setup_cleanup_routes(manager))
+    with TestClient(app) as client:
+        yield client, factory, manager
+    engine.dispose()
+
+
+def seed(factory, manager, stored, actual, *, archived=True):
+    now = datetime.now(timezone.utc).replace(tzinfo=None)
+    with factory() as db:
+        for index in range(10):
+            db.add(database.Session(
+                id=f"recent-{index}", name="Recent", endpoint_url="", model="",
+                owner="alice", created_at=now - timedelta(hours=index),
+                last_accessed=now, message_count=1,
+            ))
+        db.add(database.Session(
+            id="old-chat", name="Project discussion", endpoint_url="", model="",
+            owner="alice", created_at=now - timedelta(days=50),
+            last_accessed=now - timedelta(days=30), archived=archived,
+            is_important=False, message_count=stored,
+        ))
+        db.commit()
+        db.add_all(database.ChatMessage(
+            id=f"message-{index}", session_id="old-chat", role="user",
+            content=f"Saved project discussion {index}", timestamp=now - timedelta(days=30),
+        ) for index in range(actual))
+        db.commit()
+    manager.sessions["old-chat"] = SimpleNamespace(message_count=actual)
+
+
+@pytest.mark.parametrize("stored", [0, 3])
+def test_cleanup_preserves_long_transcript_with_stale_count(cleanup_api, stored):
+    client, factory, manager = cleanup_api
+    seed(factory, manager, stored, 40)
+    preview = client.get("/api/cleanup/preview").json()
+    assert not preview["sessions_to_delete"]
+    protected = next(row for row in preview["preserved_sessions"] if row["id"] == "old-chat")
+    assert protected["message_count"] == 40
+    response = client.post("/api/cleanup")
+    with factory() as db:
+        actual = db.query(database.ChatMessage).filter_by(session_id="old-chat").count()
+        survives = db.get(database.Session, "old-chat") is not None
+    assert response.json()["deleted_count"] == 0
+    assert survives and actual == 40, "Cleanup must count actual rows before hard deletion"
+
+
+def test_cleanup_still_removes_genuinely_short_old_chat(cleanup_api):
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 3, 3)
+    response = client.post("/api/cleanup")
+    assert response.json()["deleted_count"] == 1
+    with factory() as db:
+        assert db.get(database.Session, "old-chat") is None
+        assert db.query(database.ChatMessage).filter_by(session_id="old-chat").count() == 0
+
+
+def test_new_archive_is_not_deleted_in_same_cleanup(cleanup_api):
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 3, 3, archived=False)
+    preview = client.get("/api/cleanup/preview").json()
+    assert not preview["sessions_to_delete"]
+    response = client.post("/api/cleanup")
+    assert response.json()["deleted_count"] == 0
+    with factory() as db:
+        assert db.get(database.Session, "old-chat").archived
+
+
+@pytest.mark.parametrize("actual,expected_deleted", [(19, 1), (20, 0)])
+def test_cleanup_threshold_uses_actual_count_even_when_metadata_is_high(cleanup_api, actual, expected_deleted):
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 100, actual)
+    preview = client.get("/api/cleanup/preview").json()
+    section = "sessions_to_delete" if expected_deleted else "preserved_sessions"
+    row = next(row for row in preview[section] if row["id"] == "old-chat")
+    assert row["message_count"] == actual
+    if expected_deleted:
+        assert row["estimated_size_kb"] == 19 * 512 / 1024
+    response = client.post("/api/cleanup")
+    assert response.json()["deleted_count"] == expected_deleted
+    with factory() as db:
+        assert (db.get(database.Session, "old-chat") is None) == bool(expected_deleted)
+
+
+@pytest.mark.parametrize("protection", ["starred", "keyword", "other_owner"])
+def test_cleanup_retains_existing_protections(cleanup_api, protection):
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 0, 3)
+    with factory() as db:
+        row = db.get(database.Session, "old-chat")
+        if protection == "starred":
+            row.is_important = True
+        elif protection == "keyword":
+            row.name = "Important project"
+        else:
+            row.owner = "bob"
+        # Preserve the old activity date despite the column's onupdate default.
+        row.last_accessed = datetime.now(timezone.utc).replace(tzinfo=None) - timedelta(days=30)
+        db.commit()
+    response = client.post("/api/cleanup")
+    assert response.json()["deleted_count"] == 0
+    with factory() as db:
+        assert db.query(database.ChatMessage).filter_by(session_id="old-chat").count() == 3
+
+
+def test_cleanup_rechecks_messages_at_the_delete_boundary(cleanup_api):
+    """Deliver new rows after selection but before the DELETE executes."""
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 3, 3)
+    engine = factory.kw["bind"]
+    injected = False
+
+    def deliver_messages(connection, cursor, statement, parameters, context, executemany):
+        nonlocal injected
+        if not injected and statement.lstrip().startswith("DELETE FROM sessions"):
+            injected = True
+            connection.execute(insert(database.ChatMessage), [
+                {"id": f"incoming-{i}", "session_id": "old-chat", "role": "assistant",
+                 "content": "New durable message"} for i in range(20)
+            ])
+
+    event.listen(engine, "before_cursor_execute", deliver_messages)
+    try:
+        response = client.post("/api/cleanup")
+    finally:
+        event.remove(engine, "before_cursor_execute", deliver_messages)
+    assert injected
+    assert response.json()["deleted_count"] == 0
+    assert response.json()["space_freed_mb"] == 0
+    assert "old-chat" in manager.sessions
+    with factory() as db:
+        assert db.get(database.Session, "old-chat") is not None
+        assert db.query(database.ChatMessage).filter_by(session_id="old-chat").count() == 23
+
+
+def test_cleanup_space_freed_ignores_concurrently_deleted_session(cleanup_api):
+    """If another transaction deletes a candidate first, do not attribute its space."""
+    client, factory, manager = cleanup_api
+    now = datetime.now(timezone.utc).replace(tzinfo=None)
+    with factory() as db:
+        for index in range(10):
+            db.add(database.Session(
+                id=f"recent-{index}", name="Recent", endpoint_url="", model="",
+                owner="alice", created_at=now - timedelta(hours=index),
+                last_accessed=now, message_count=1,
+            ))
+        db.add(database.Session(
+            id="chat-to-delete", name="Eligible 1", endpoint_url="", model="",
+            owner="alice", created_at=now - timedelta(days=50),
+            last_accessed=now - timedelta(days=30), archived=True,
+            is_important=False, message_count=3,
+        ))
+        db.add(database.Session(
+            id="chat-concurrently-deleted", name="Eligible 2", endpoint_url="", model="",
+            owner="alice", created_at=now - timedelta(days=50),
+            last_accessed=now - timedelta(days=30), archived=True,
+            is_important=False, message_count=10,
+        ))
+        db.commit()
+        db.add_all(database.ChatMessage(
+            id=f"msg1-{i}", session_id="chat-to-delete", role="user",
+            content="m", timestamp=now - timedelta(days=30),
+        ) for i in range(3))
+        db.add_all(database.ChatMessage(
+            id=f"msg2-{i}", session_id="chat-concurrently-deleted", role="user",
+            content="m", timestamp=now - timedelta(days=30),
+        ) for i in range(10))
+        db.commit()
+
+    manager.sessions["chat-to-delete"] = SimpleNamespace(message_count=3)
+    manager.sessions["chat-concurrently-deleted"] = SimpleNamespace(message_count=10)
+
+    engine = factory.kw["bind"]
+    injected = False
+
+    def delete_candidate(connection, cursor, statement, parameters, context, executemany):
+        nonlocal injected
+        if not injected and statement.lstrip().startswith("DELETE FROM sessions"):
+            injected = True
+            with factory() as other_db:
+                other_db.query(database.Session).filter_by(id="chat-concurrently-deleted").delete()
+                other_db.commit()
+
+    event.listen(engine, "before_cursor_execute", delete_candidate)
+    try:
+        response = client.post("/api/cleanup")
+    finally:
+        event.remove(engine, "before_cursor_execute", delete_candidate)
+
+    assert injected
+    data = response.json()
+    assert data["deleted_count"] == 1
+    # Only chat-to-delete's 3 messages (1536 bytes) freed by this cleanup, not (3 + 10) * 512
+    expected_space_mb = round(3 * 512 / (1024 * 1024), 2)
+    assert data["space_freed_mb"] == expected_space_mb
+    assert "chat-to-delete" not in manager.sessions
+    assert "chat-concurrently-deleted" not in manager.sessions
+    with factory() as db:
+        assert db.get(database.Session, "chat-to-delete") is None
+        assert db.get(database.Session, "chat-concurrently-deleted") is None
+
+
+def test_cleanup_recent_session_query_is_bounded(cleanup_api):
+    """Ensure recent-session preservation scales to large collections and respects limits."""
+    client, factory, manager = cleanup_api
+    now = datetime.now(timezone.utc).replace(tzinfo=None)
+    with factory() as db:
+        for index in range(25):
+            db.add(database.Session(
+                id=f"sess-{index:02d}", name=f"Session {index}", endpoint_url="", model="",
+                owner="alice", created_at=now - timedelta(hours=index),
+                last_accessed=now - timedelta(days=30), archived=True,
+                is_important=False, message_count=3,
+            ))
+            db.add_all(database.ChatMessage(
+                id=f"msg-{index}-{m}", session_id=f"sess-{index:02d}", role="user",
+                content="m", timestamp=now - timedelta(days=30),
+            ) for m in range(3))
+        db.commit()
+
+    preview = client.get("/api/cleanup/preview").json()
+    preserved_ids = {row["id"] for row in preview["preserved_sessions"]}
+    to_delete_ids = {row["id"] for row in preview["sessions_to_delete"]}
+
+    # Top 10 newest (sess-00 to sess-09) must be preserved
+    expected_preserved = {f"sess-{i:02d}" for i in range(10)}
+    expected_to_delete = {f"sess-{i:02d}" for i in range(10, 25)}
+    assert preserved_ids == expected_preserved
+    assert to_delete_ids == expected_to_delete
+
+    response = client.post("/api/cleanup")
+    assert response.json()["deleted_count"] == 15
+
+    with factory() as db:
+        surviving = {row[0] for row in db.query(database.Session.id).all()}
+        assert surviving == expected_preserved
+
+
+def test_cleanup_handles_zero_actual_messages_session(cleanup_api):
+    """Sessions with zero durable messages are properly previewed and cleaned up."""
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 10, 0)
+    preview = client.get("/api/cleanup/preview").json()
+    row = next(r for r in preview["sessions_to_delete"] if r["id"] == "old-chat")
+    assert row["message_count"] == 0
+    assert row["estimated_size_kb"] == 0.0
+
+    response = client.post("/api/cleanup")
+    assert response.json()["deleted_count"] == 1
+    with factory() as db:
+        assert db.get(database.Session, "old-chat") is None
+
+
+def test_cleanup_db_error_preserves_cache_and_aborts_cleanly(cleanup_api):
+    """Database execution failures must abort, rollback, and leave in-memory cache intact."""
+    client, factory, manager = cleanup_api
+    seed(factory, manager, 3, 3)
+    engine = factory.kw["bind"]
+    injected = False
+
+    def break_delete(connection, cursor, statement, parameters, context, executemany):
+        nonlocal injected
+        if not injected and statement.lstrip().startswith("DELETE FROM sessions"):
+            injected = True
+            raise RuntimeError("Injected database error during DELETE")
+
+    event.listen(engine, "before_cursor_execute", break_delete)
+    try:
+        response = client.post("/api/cleanup")
+    finally:
+        event.remove(engine, "before_cursor_execute", break_delete)
+
+    assert injected
+    assert response.status_code == 200
+    assert response.json()["deleted_count"] == 0
+    assert response.json()["space_freed_mb"] == 0
+    # In-memory session was not prematurely evicted
+    assert "old-chat" in manager.sessions
+    # Database row was rolled back and still exists
+    with factory() as db:
+        assert db.get(database.Session, "old-chat") is not None
+        assert db.query(database.ChatMessage).filter_by(session_id="old-chat").count() == 3
+
+
+def test_cleanup_portable_fallback_without_returning(cleanup_api, monkeypatch):
+    """Engines lacking RETURNING (e.g. SQLite < 3.35.0) safely clean up without syntax errors."""
+    import src.cleanup_service as cleanup_service
+    monkeypatch.setattr(cleanup_service, "_supports_delete_returning", lambda db: False)
+
+    client, factory, manager = cleanup_api
+    now = datetime.now(timezone.utc).replace(tzinfo=None)
+    with factory() as db:
+        for index in range(10):
+            db.add(database.Session(
+                id=f"recent-{index}", name="Recent", endpoint_url="", model="",
+                owner="alice", created_at=now - timedelta(hours=index),
+                last_accessed=now, message_count=1,
+            ))
+        db.add(database.Session(
+            id="chat-to-delete", name="Eligible 1", endpoint_url="", model="",
+            owner="alice", created_at=now - timedelta(days=50),
+            last_accessed=now - timedelta(days=30), archived=True,
+            is_important=False, message_count=3,
+        ))
+        db.add(database.Session(
+            id="chat-concurrently-deleted", name="Eligible 2", endpoint_url="", model="",
+            owner="alice", created_at=now - timedelta(days=50),
+            last_accessed=now - timedelta(days=30), archived=True,
+            is_important=False, message_count=10,
+        ))
+        db.commit()
+        db.add_all(database.ChatMessage(
+            id=f"msg1-{i}", session_id="chat-to-delete", role="user",
+            content="m", timestamp=now - timedelta(days=30),
+        ) for i in range(3))
+        db.add_all(database.ChatMessage(
+            id=f"msg2-{i}", session_id="chat-concurrently-deleted", role="user",
+            content="m", timestamp=now - timedelta(days=30),
+        ) for i in range(10))
+        db.commit()
+
+    manager.sessions["chat-to-delete"] = SimpleNamespace(message_count=3)
+    manager.sessions["chat-concurrently-deleted"] = SimpleNamespace(message_count=10)
+
+    engine = factory.kw["bind"]
+    injected = False
+
+    def delete_candidate(connection, cursor, statement, parameters, context, executemany):
+        nonlocal injected
+        if not injected and statement.lstrip().startswith("DELETE FROM sessions"):
+            injected = True
+            with factory() as other_db:
+                other_db.query(database.Session).filter_by(id="chat-concurrently-deleted").delete()
+                other_db.commit()
+
+    event.listen(engine, "before_cursor_execute", delete_candidate)
+    try:
+        response = client.post("/api/cleanup")
+    finally:
+        event.remove(engine, "before_cursor_execute", delete_candidate)
+
+    assert injected
+    data = response.json()
+    assert data["deleted_count"] == 1
+    expected_space_mb = round(3 * 512 / (1024 * 1024), 2)
+    assert data["space_freed_mb"] == expected_space_mb
+    assert "chat-to-delete" not in manager.sessions
+    assert "chat-concurrently-deleted" not in manager.sessions
+    with factory() as db:
+        assert db.get(database.Session, "chat-to-delete") is None
+        assert db.get(database.Session, "chat-concurrently-deleted") is None