langchain-ai/langgraph · error · ValueError

atee() requires n >= 1

Error message

atee() requires n >= 1

What it means

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

Source

Thrown at libs/langgraph/langgraph/stream/stream_channel.py:309

        Caller-driven fan-out: each branch's `__anext__` either pops
        from its own buffer or, under a shared `asyncio.Lock`, pulls
        one item from the underlying cursor and distributes it to
        every branch's buffer.

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

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

        Raises:
            TypeError: If the channel is unbound or bound to sync mode.
            RuntimeError: If the channel already has a subscriber.
            ValueError: If `n` < 1.
        """
        if n < 1:
            raise ValueError("atee() requires n >= 1")
        source = self.__aiter__()
        buffers: list[deque[T]] = [deque() for _ in range(n)]
        exhausted = [False]
        error: list[BaseException | None] = [None]
        lock = asyncio.Lock()

        async def branch(i: int) -> AsyncIterator[T]:
            buf = buffers[i]
            while True:
                if buf:
                    yield buf.popleft()
                    continue
                if exhausted[0]:
                    if error[0] is not None:
                        raise error[0]
                    return
                async with lock:
                    if buf or exhausted[0]:

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/stream/stream_channel.py:309 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/620bc4ea6f2ca8f4. Report an issue: GitHub.