langchain-ai/langgraph · error · TypeError

StreamChannel has not been bound yet. Register the…

Error message

StreamChannel {name!r} has not been bound yet. Register the transformer with a StreamMux first.

What it means

Raised by RunStream.interleave() when a requested name maps to an extension that is not a StreamChannel or whose binding state is unknown (never registered with a StreamMux). The channel cannot be consumed because no transformer has produced events for it. Register the transformer with a StreamMux before interleaving.

Solutions

1. Register the owning transformer with the StreamMux before consuming channels
2. Obtain channels from a running run's extensions rather than creating them standalone

When it happens

Trigger: Thrown at libs/langgraph/langgraph/stream/run_stream.py:265 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/8c970c4840ca7430. Report an issue: GitHub.

Appendix: source

Thrown at libs/langgraph/langgraph/stream/run_stream.py:265

                if name == "messages":
                    print("msg:", item)
                else:
                    print("val:", item)
            ```
        """
        from langgraph.stream.stream_channel import StreamChannel

        channels: dict[str, StreamChannel[Any]] = {}
        try:
            for name in names:
                ch = self.extensions[name]
                if not isinstance(ch, StreamChannel):
                    raise TypeError(
                        f"interleave() requires StreamChannel projections, "
                        f"got {type(ch).__name__} for {name!r}"
                    )
                if ch._is_async is None:
                    raise TypeError(
                        f"StreamChannel {name!r} has not been bound yet. "
                        "Register the transformer with a StreamMux first."
                    )
                if ch._is_async:
                    raise TypeError(
                        f"StreamChannel {name!r} is bound to async mode — "
                        "sync interleave() cannot consume async channels."
                    )
                if ch._subscribed:
                    raise RuntimeError(
                        f"StreamChannel {name!r} already has a subscriber; "
                        "use .tee(n) for fan-out."
                    )
                ch._subscribed = True
                channels[name] = ch

            done: set[str] = set()

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