langchain-ai/langgraph · error · InvalidUpdateError

PASSTHROUGH value must be replaced

Error message

PASSTHROUGH value must be replaced

What it means

Error "PASSTHROUGH value must be replaced" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/pregel/_write.py:119

            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
        )

    @staticmethod
    def get_static_writes(

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/pregel/_write.py:119 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/fa213a0d7c691885. Report an issue: GitHub.