langchain-ai/langgraph · error · RuntimeError

StreamMux._make_child requires the mux to be constructed wit

Error message

StreamMux._make_child requires the mux to be constructed with `factories=`; pre-built transformers can't be cloned to a new scope.

What it means

Error "StreamMux._make_child requires the mux to be constructed with `factories=`; pre-built transformers can't be cloned to a new scope." thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/stream/_mux.py:210

    def _make_child(self, scope: tuple[str, ...]) -> StreamMux:
        """Build a mini-mux with the same factories scoped to `scope`.

        Used by `SubgraphTransformer` to attach a fresh transformer
        pipeline to each discovered subgraph handle. The child mux
        inherits the current pump bindings (so cursors on its
        projection logs drive the root pump), carries the same factory
        list forward to any grandchild subgraphs, and does not assign
        `seq` numbers so forwarded events can be shared without
        mutating their envelope.

        Raises:
            RuntimeError: If the mux was not constructed with
                `factories=`. Mini-muxes require factories so each scope
                gets its own fresh transformer instances.
        """
        if self._factories is None:
            raise RuntimeError(
                "StreamMux._make_child requires the mux to be constructed "
                "with `factories=`; pre-built transformers can't be "
                "cloned to a new scope."
            )
        child = StreamMux(
            factories=self._factories,
            is_async=self.is_async,
            scope=scope,
            _assign_seq=False,
        )
        if self._pump_fn is not None:
            child.bind_pump(self._pump_fn)
        if self._apump_fn is not None:
            child.bind_apump(self._apump_fn)
        return child

    def _register(self, transformer: StreamTransformer) -> None:
        """Register a single transformer.

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/stream/_mux.py:210 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/fc41eeb9f4972491. Report an issue: GitHub.