langchain-ai/langgraph · error · ValueError

tee() requires n >= 1

Error message

tee() requires n >= 1

What it means

Error "tee() requires n >= 1" thrown in langchain-ai/langgraph.

Source

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

        Each branch has its own buffer; items pulled from the
        underlying cursor are copied into every branch. Branches are
        naturally bounded by caller pace since the sync pump is
        caller-driven.

        Args:
            n: Number of branches to create. Must be >= 1.

        Returns:
            A tuple of `n` iterators over the same underlying stream.

        Raises:
            TypeError: If the channel is unbound or bound to async mode.
            RuntimeError: If the channel already has a subscriber.
            ValueError: If `n` < 1.
        """
        if n < 1:
            raise ValueError("tee() requires n >= 1")
        source = self.__iter__()
        buffers: list[deque[T]] = [deque() for _ in range(n)]
        exhausted = [False]

        def branch(i: int) -> Iterator[T]:
            buf = buffers[i]
            while True:
                if buf:
                    yield buf.popleft()
                elif exhausted[0]:
                    return
                else:
                    try:
                        item = next(source)
                    except StopIteration:
                        exhausted[0] = True
                        return
                    for b in buffers:

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/stream/stream_channel.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/bf79cb591bf9da3c. Report an issue: GitHub.