langchain-ai/langgraph · error · ValueError

Channel '{TASKS}' is reserved and cannot be used in the grap

Error message

Channel '{TASKS}' is reserved and cannot be used in the graph.

What it means

Error "Channel '{TASKS}' is reserved and cannot be used in the graph." thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/pregel/main.py:805

            config_type := deprecated_kwargs.get("config_type", MISSING)
        ) is not MISSING:
            warnings.warn(
                "`config_type` is deprecated and will be removed. Please use `context_schema` instead.",
                category=LangGraphDeprecatedSinceV10,
                stacklevel=2,
            )

            if context_schema is None:
                context_schema = cast(type[ContextT], config_type)

        checkpointer = ensure_valid_checkpointer(checkpointer)

        self.nodes = {
            k: v.build() if isinstance(v, NodeBuilder) else v for k, v in nodes.items()
        }
        self.channels = channels or {}
        if TASKS in self.channels and not isinstance(self.channels[TASKS], Topic):
            raise ValueError(
                f"Channel '{TASKS}' is reserved and cannot be used in the graph."
            )
        else:
            self.channels[TASKS] = Topic(Send, accumulate=False)
        self.stream_mode = stream_mode
        self.stream_eager = stream_eager
        self.output_channels = output_channels
        self.stream_channels = stream_channels
        self.interrupt_after_nodes = interrupt_after_nodes
        self.interrupt_before_nodes = interrupt_before_nodes
        self.input_channels = input_channels
        self.step_timeout = step_timeout
        self.debug = debug if debug is not None else get_debug()
        self.checkpointer = checkpointer
        self.store = store
        self.cache = cache
        self.retry_policy = (
            (retry_policy,) if isinstance(retry_policy, RetryPolicy) else retry_policy

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/pregel/main.py:805 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/8f4fedee96ae64e7. Report an issue: GitHub.