langchain-ai/langgraph · error · ValueError

RunnableSeq must have at least 2 steps, got {len(steps_flat)

Error message

RunnableSeq must have at least 2 steps, got {len(steps_flat)}

What it means

Error "RunnableSeq must have at least 2 steps, got {len(steps_flat)}" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/_internal/_runnable.py:616

        """Create a new RunnableSeq.

        Args:
            steps: The steps to include in the sequence.
            name: The name of the `Runnable`.

        Raises:
            ValueError: If the sequence has less than 2 steps.
        """
        steps_flat: list[Runnable] = []
        for step in steps:
            if isinstance(step, RunnableSequence):
                steps_flat.extend(step.steps)
            elif isinstance(step, RunnableSeq):
                steps_flat.extend(step.steps)
            else:
                steps_flat.append(coerce_to_runnable(step, name=None, trace=True))
        if len(steps_flat) < 2:
            raise ValueError(
                f"RunnableSeq must have at least 2 steps, got {len(steps_flat)}"
            )
        self.steps = steps_flat
        self.name = name
        self.trace_inputs = trace_inputs
        self.trace_outputs = trace_outputs

    def __or__(
        self,
        other: Any,
    ) -> Runnable:
        if isinstance(other, RunnableSequence):
            return RunnableSeq(
                *self.steps,
                other.first,
                *other.middle,
                other.last,
                name=self.name or other.name,

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/_internal/_runnable.py:616 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/09cb03fd38db84c2. Report an issue: GitHub.