error message

libs/langgraph/langgraph/pregel/_loop.py

@@ -637,6 +637,14 @@ class PregelLoop:
 
         # map command to writes
         if isinstance(self.input, Command):
+            if self.input.update:
+                raise ValueError(
+                    "Command(update=...) is not allowed as graph input. "
+                    "To modify state before resuming, use graph.update_state(...) "
+                    "to fork the graph state, then resume with Command(resume=...). "
+                    "Note: Command(update=...) can still be used as a return "
+                    "value from a node to update graph state during execution."
+                )
             if (resume := self.input.resume) is not None:
                 if not self.checkpointer:
                     raise RuntimeError(

libs/langgraph/tests/test_pregel.py

@@ -5850,6 +5850,31 @@ def test_task_before_interrupt_resume(
     assert result == {"answers": ["answer1", "answer2"]}
 
 
+def test_command_with_update_input_raises(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Test that Command(update=...) raises ValueError when used as graph input."""
+
+    @entrypoint(checkpointer=sync_checkpointer)
+    def workflow(inputs: str) -> str:
+        answer = interrupt("question")
+        return answer
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # First invocation triggers interrupt
+    result = workflow.invoke("start", config=config)
+    assert "__interrupt__" in result
+
+    # Command with update (and resume) should raise
+    with pytest.raises(ValueError, match="Command\\(update="):
+        workflow.invoke(Command(resume="answer", update={"foo": "bar"}), config=config)
+
+    # Command with update only (no resume) should also raise
+    with pytest.raises(ValueError, match="Command\\(update="):
+        workflow.invoke(Command(update={"foo": "bar"}), config=config)
+
+
 def test_multiple_tasks_before_interrupt_resume(
     sync_checkpointer: BaseCheckpointSaver,
 ) -> None: