langchain-ai/langgraph · error · ValueError

Cannot subscribe to single channels when other channels are

Error message

Cannot subscribe to single channels when other channels are already subscribed to

What it means

Error "Cannot subscribe to single channels when other channels are already subscribed to" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/pregel/main.py:250

        self._channels = []
        self._triggers = []
        self._tags = []
        self._metadata = {}
        self._writes = []
        self._bound = DEFAULT_BOUND
        self._retry_policy = []
        self._cache_policy = None
        self._timeout = None

    def subscribe_only(
        self,
        channel: str,
    ) -> Self:
        """Subscribe to a single channel."""
        if not self._channels:
            self._channels = channel
        else:
            raise ValueError(
                "Cannot subscribe to single channels when other channels are already subscribed to"
            )

        self._triggers.append(channel)

        return self

    def subscribe_to(
        self,
        *channels: str,
        read: bool = True,
    ) -> Self:
        """Add channels to subscribe to.

        Node will be invoked when any of these channels are updated, with a dict of the
        channel values as input.

        Args:

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/pregel/main.py:250 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/d2b11808bd2210f2. Report an issue: GitHub.