langchain-ai/langgraph · error · ValueError

Auto-generated error handler node

Error message

Auto-generated error handler node `{handler_node_name}` already exists.

What it means

Raised when adding an error handler node whose auto-generated internal node name already exists in the graph. The generated name (derived for the error_handler feature) collides with an existing node, so registration is refused to avoid silently overwriting a node. Rename the conflicting node or adjust the handler setup.

Solutions

1. Rename the conflicting user node so the generated handler name is free
2. Configure a custom handler node name if the API allows it

When it happens

Trigger: Thrown at libs/langgraph/langgraph/graph/state.py:870 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of langchain-ai/langgraph@38031739e5 (2026-08-26). Data as JSON: /api/errors/44b865d43cbba158. Report an issue: GitHub.

Appendix: source

Thrown at libs/langgraph/langgraph/graph/state.py:870

                        and (rargs := get_args(rtn))
                        and get_origin(rargs[0]) is Literal
                        and (vals := get_args(rargs[0]))
                    ):
                        ends = vals
        except (NameError, TypeError, StopIteration):
            pass

        if destinations is not None:
            ends = destinations

        resolved_input_schema: type[Any] = (
            input_schema or inferred_input_schema or self.state_schema
        )
        handler_node_name: str | None = None
        if error_handler is not None:
            handler_node_name = f"__error_handler__{node}"
            if handler_node_name in self.nodes:
                raise ValueError(
                    f"Auto-generated error handler node `{handler_node_name}` already exists."
                )
            self.nodes[handler_node_name] = StateNodeSpec[Any, ContextT](
                coerce_to_runnable(error_handler, name=handler_node_name, trace=False),  # type: ignore[arg-type]
                metadata=None,
                input_schema=resolved_input_schema,
                retry_policy=None,
                cache_policy=None,
                is_error_handler=True,
            )

        if input_schema is not None:
            self.nodes[node] = StateNodeSpec[NodeInputT, ContextT](
                coerce_to_runnable(action, name=node, trace=False),  # type: ignore[arg-type]
                metadata,
                input_schema=input_schema,
                retry_policy=retry_policy,
                cache_policy=cache_policy,

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