langchain-ai/langgraph · error · InvalidUpdateError

Cannot write to the reserved channel TASKS

Error message

Cannot write to the reserved channel TASKS

What it means

Error "Cannot write to the reserved channel TASKS" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/pregel/_write.py:115

            for write in self.writes
        ]
        self.do_write(
            config,
            writes,
        )
        return input

    @staticmethod
    def do_write(
        config: RunnableConfig,
        writes: Sequence[ChannelWriteEntry | ChannelWriteTupleEntry | Send],
        allow_passthrough: bool = True,
    ) -> None:
        # validate
        for w in writes:
            if isinstance(w, ChannelWriteEntry):
                if w.channel == TASKS:
                    raise InvalidUpdateError(
                        "Cannot write to the reserved channel TASKS"
                    )
                if w.value is PASSTHROUGH and not allow_passthrough:
                    raise InvalidUpdateError("PASSTHROUGH value must be replaced")
            if isinstance(w, ChannelWriteTupleEntry):
                if w.value is PASSTHROUGH and not allow_passthrough:
                    raise InvalidUpdateError("PASSTHROUGH value must be replaced")
        # if we want to persist writes found before hitting a ParentCommand
        # can move this to a finally block
        write: TYPE_SEND = config[CONF][CONFIG_KEY_SEND]
        write(_assemble_writes(writes))

    @staticmethod
    def is_writer(runnable: Runnable) -> bool:
        """Used by PregelNode to distinguish between writers and other runnables."""
        return (
            isinstance(runnable, ChannelWrite)
            or getattr(runnable, "_is_channel_writer", MISSING) is not MISSING

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/pregel/_write.py:115 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/d2ae02935d41a50a. Report an issue: GitHub.