langchain-ai/langgraph · error · RuntimeError

StreamChannel {name!r} already has a subscriber; use .tee(n)

Error message

StreamChannel {name!r} already has a subscriber; use .tee(n) for fan-out.

What it means

Error "StreamChannel {name!r} already has a subscriber; use .tee(n) for fan-out." thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/stream/run_stream.py:275

            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()

            while len(done) < len(channels):
                best: tuple[int, str] | None = None
                for name, ch in channels.items():
                    if name in done:
                        continue
                    if ch._closed and not ch._items:
                        if ch._error is not None:
                            raise ch._error
                        done.add(name)
                        continue

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/stream/run_stream.py:275 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/11ff9d8f8e413b87. Report an issue: GitHub.