diff --git a/api/providers/trace/trace-langsmith/src/dify_trace_langsmith/langsmith_trace.py b/api/providers/trace/trace-langsmith/src/dify_trace_langsmith/langsmith_trace.py
index 145bd70dbc..045ec44e4e 100644
--- a/api/providers/trace/trace-langsmith/src/dify_trace_langsmith/langsmith_trace.py
+++ b/api/providers/trace/trace-langsmith/src/dify_trace_langsmith/langsmith_trace.py
@@ -64,7 +64,9 @@ class LangSmithDataTrace(BaseTraceInstance):
             self.generate_name_trace(trace_info)
 
     def workflow_trace(self, trace_info: WorkflowTraceInfo):
-        trace_id = trace_info.trace_id or trace_info.message_id or trace_info.workflow_run_id
+        # trace_id must equal the root run's run_id (LangSmith protocol); external trace_id
+        # cannot be used here as it would cause HTTP 400.
+        trace_id = trace_info.message_id or trace_info.workflow_run_id
         if trace_info.start_time is None:
             trace_info.start_time = datetime.now()
         message_dotted_order = (
@@ -77,6 +79,8 @@ class LangSmithDataTrace(BaseTraceInstance):
         )
         metadata = trace_info.metadata
         metadata["workflow_app_log_id"] = trace_info.workflow_app_log_id
+        if trace_info.trace_id:
+            metadata["external_trace_id"] = trace_info.trace_id
 
         if trace_info.message_id:
             message_run = LangSmithRunModel(
diff --git a/api/providers/trace/trace-langsmith/tests/unit_tests/langsmith_trace/test_langsmith_trace.py b/api/providers/trace/trace-langsmith/tests/unit_tests/langsmith_trace/test_langsmith_trace.py
index ee59acb17e..edc4aafd87 100644
--- a/api/providers/trace/trace-langsmith/tests/unit_tests/langsmith_trace/test_langsmith_trace.py
+++ b/api/providers/trace/trace-langsmith/tests/unit_tests/langsmith_trace/test_langsmith_trace.py
@@ -208,13 +208,17 @@ def test_workflow_trace(trace_instance, monkeypatch: pytest.MonkeyPatch):
 
     assert call_args[0].id == "msg-1"
     assert call_args[0].name == TraceTaskName.MESSAGE_TRACE
+    # trace_id must equal root run's id (message_id), not the external trace_id "trace-1"
+    assert call_args[0].trace_id == "msg-1"
 
     assert call_args[1].id == "run-1"
     assert call_args[1].name == TraceTaskName.WORKFLOW_TRACE
     assert call_args[1].parent_run_id == "msg-1"
+    assert call_args[1].trace_id == "msg-1"
 
     assert call_args[2].id == "node-llm"
     assert call_args[2].run_type == LangSmithRunType.llm
+    assert call_args[2].trace_id == "msg-1"
 
     assert call_args[3].id == "node-other"
     assert call_args[3].run_type == LangSmithRunType.tool
@@ -604,3 +608,83 @@ def test_get_project_url_error(trace_instance):
     trace_instance.langsmith_client.get_run_url.side_effect = Exception("error")
     with pytest.raises(ValueError, match="LangSmith get run url failed: error"):
         trace_instance.get_project_url()
+
+
+def _make_workflow_trace_info(
+    *, message_id: str | None, workflow_run_id: str, trace_id: str | None
+) -> WorkflowTraceInfo:
+    workflow_data = MagicMock()
+    workflow_data.created_at = _dt()
+    workflow_data.finished_at = _dt() + timedelta(seconds=1)
+    return WorkflowTraceInfo(
+        tenant_id="tenant-1",
+        workflow_id="wf-1",
+        workflow_run_id=workflow_run_id,
+        workflow_run_inputs={},
+        workflow_run_outputs={},
+        workflow_run_status="succeeded",
+        workflow_run_version="1.0",
+        workflow_run_elapsed_time=1.0,
+        total_tokens=0,
+        file_list=[],
+        query="q",
+        message_id=message_id,
+        conversation_id="conv-1" if message_id else None,
+        start_time=_dt(),
+        end_time=_dt() + timedelta(seconds=1),
+        trace_id=trace_id,
+        metadata={"app_id": "app-1"},
+        workflow_app_log_id=None,
+        error=None,
+        workflow_data=workflow_data,
+    )
+
+
+def _patch_workflow_trace_deps(monkeypatch, trace_instance):
+    monkeypatch.setattr("dify_trace_langsmith.langsmith_trace.sessionmaker", lambda bind: lambda: MagicMock())
+    monkeypatch.setattr("dify_trace_langsmith.langsmith_trace.db", MagicMock(engine="engine"))
+    repo = MagicMock()
+    repo.get_by_workflow_execution.return_value = []
+    factory = MagicMock()
+    factory.create_workflow_node_execution_repository.return_value = repo
+    monkeypatch.setattr("dify_trace_langsmith.langsmith_trace.DifyCoreRepositoryFactory", factory)
+    monkeypatch.setattr(trace_instance, "get_service_account_with_tenant", lambda app_id: MagicMock())
+    trace_instance.add_run = MagicMock()
+
+
+def test_workflow_trace_id_uses_message_id_not_external(trace_instance, monkeypatch):
+    """Chatflow with external trace_id: LangSmith trace_id must be message_id, not external."""
+    trace_info = _make_workflow_trace_info(
+        message_id="msg-abc",
+        workflow_run_id="run-xyz",
+        trace_id="external-999",
+    )
+    _patch_workflow_trace_deps(monkeypatch, trace_instance)
+
+    trace_instance.workflow_trace(trace_info)
+
+    calls = [c[0][0] for c in trace_instance.add_run.call_args_list]
+    # message run (root) and workflow run (child) must both use message_id as trace_id
+    assert calls[0].id == "msg-abc"
+    assert calls[0].trace_id == "msg-abc"
+    assert calls[1].id == "run-xyz"
+    assert calls[1].trace_id == "msg-abc"
+    # external_trace_id preserved in metadata
+    assert trace_info.metadata.get("external_trace_id") == "external-999"
+
+
+def test_workflow_trace_id_pure_workflow_uses_run_id(trace_instance, monkeypatch):
+    """Pure workflow (no message_id) with external trace_id: trace_id must be workflow_run_id."""
+    trace_info = _make_workflow_trace_info(
+        message_id=None,
+        workflow_run_id="run-xyz",
+        trace_id="external-999",
+    )
+    _patch_workflow_trace_deps(monkeypatch, trace_instance)
+
+    trace_instance.workflow_trace(trace_info)
+
+    calls = [c[0][0] for c in trace_instance.add_run.call_args_list]
+    # workflow run is the root; trace_id must equal its run_id
+    assert calls[0].id == "run-xyz"
+    assert calls[0].trace_id == "run-xyz"