diff --git a/backend/packages/harness/deerflow/runtime/events/store/db.py b/backend/packages/harness/deerflow/runtime/events/store/db.py
index 9374769f3..b7e54754f 100644
--- a/backend/packages/harness/deerflow/runtime/events/store/db.py
+++ b/backend/packages/harness/deerflow/runtime/events/store/db.py
@@ -11,7 +11,7 @@ import logging
 from datetime import UTC, datetime
 from typing import Any
 
-from sqlalchemy import delete, func, select
+from sqlalchemy import delete, func, select, text
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 
 from deerflow.persistence.models.run_event import RunEventRow
@@ -86,6 +86,28 @@ class DbRunEventStore(RunEventStore):
         user = get_current_user()
         return str(user.id) if user is not None else None
 
+    @staticmethod
+    async def _max_seq_for_thread(session: AsyncSession, thread_id: str) -> int | None:
+        """Return the current max seq while serializing writers per thread.
+
+        PostgreSQL rejects ``SELECT max(...) FOR UPDATE`` because aggregate
+        results are not lockable rows. As a release-safe workaround, take a
+        transaction-level advisory lock keyed by thread_id before reading the
+        aggregate. Other dialects keep the existing row-locking statement.
+        """
+        stmt = select(func.max(RunEventRow.seq)).where(RunEventRow.thread_id == thread_id)
+        bind = session.get_bind()
+        dialect_name = bind.dialect.name if bind is not None else ""
+
+        if dialect_name == "postgresql":
+            await session.execute(
+                text("SELECT pg_advisory_xact_lock(hashtext(CAST(:thread_id AS text))::bigint)"),
+                {"thread_id": thread_id},
+            )
+            return await session.scalar(stmt)
+
+        return await session.scalar(stmt.with_for_update())
+
     async def put(self, *, thread_id, run_id, event_type, category, content="", metadata=None, created_at=None):  # noqa: D401
         """Write a single event — low-frequency path only.
 
@@ -100,10 +122,7 @@ class DbRunEventStore(RunEventStore):
         user_id = self._user_id_from_context()
         async with self._sf() as session:
             async with session.begin():
-                # Use FOR UPDATE to serialize seq assignment within a thread.
-                # NOTE: with_for_update() on aggregates is a no-op on SQLite;
-                # the UNIQUE(thread_id, seq) constraint catches races there.
-                max_seq = await session.scalar(select(func.max(RunEventRow.seq)).where(RunEventRow.thread_id == thread_id).with_for_update())
+                max_seq = await self._max_seq_for_thread(session, thread_id)
                 seq = (max_seq or 0) + 1
                 row = RunEventRow(
                     thread_id=thread_id,
@@ -126,10 +145,8 @@ class DbRunEventStore(RunEventStore):
         async with self._sf() as session:
             async with session.begin():
                 # Get max seq for the thread (assume all events in batch belong to same thread).
-                # NOTE: with_for_update() on aggregates is a no-op on SQLite;
-                # the UNIQUE(thread_id, seq) constraint catches races there.
                 thread_id = events[0]["thread_id"]
-                max_seq = await session.scalar(select(func.max(RunEventRow.seq)).where(RunEventRow.thread_id == thread_id).with_for_update())
+                max_seq = await self._max_seq_for_thread(session, thread_id)
                 seq = max_seq or 0
                 rows = []
                 for e in events:
diff --git a/backend/tests/test_run_event_store.py b/backend/tests/test_run_event_store.py
index d2c78ccf0..17b796af7 100644
--- a/backend/tests/test_run_event_store.py
+++ b/backend/tests/test_run_event_store.py
@@ -268,6 +268,39 @@ class TestEdgeCases:
 class TestDbRunEventStore:
     """Tests for DbRunEventStore with temp SQLite."""
 
+    @pytest.mark.anyio
+    async def test_postgres_max_seq_uses_advisory_lock_without_for_update(self):
+        from sqlalchemy.dialects import postgresql
+
+        from deerflow.runtime.events.store.db import DbRunEventStore
+
+        class FakeSession:
+            def __init__(self):
+                self.dialect = postgresql.dialect()
+                self.execute_calls = []
+                self.scalar_stmt = None
+
+            def get_bind(self):
+                return self
+
+            async def execute(self, stmt, params=None):
+                self.execute_calls.append((stmt, params))
+
+            async def scalar(self, stmt):
+                self.scalar_stmt = stmt
+                return 41
+
+        session = FakeSession()
+
+        max_seq = await DbRunEventStore._max_seq_for_thread(session, "thread-1")
+
+        assert max_seq == 41
+        assert session.execute_calls
+        assert session.execute_calls[0][1] == {"thread_id": "thread-1"}
+        assert "pg_advisory_xact_lock" in str(session.execute_calls[0][0])
+        compiled = str(session.scalar_stmt.compile(dialect=postgresql.dialect()))
+        assert "FOR UPDATE" not in compiled
+
     @pytest.mark.anyio
     async def test_basic_crud(self, tmp_path):
         from deerflow.persistence.engine import close_engine, get_session_factory, init_engine