rohitg00/ai-engineering-from-scratch · error · RuntimeError

illegal transition {self.state.value} -> {target.value}

Error message

illegal transition {self.state.value} -> {target.value}

What it means

Error "illegal transition {self.state.value} -> {target.value}" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/19-capstone-projects/20-agent-harness-loop-contract/code/main.py:195

    def plan(self) -> list[Step]:
        return list(self._plan)

    def _emit(self, etype: str, payload: dict) -> None:
        if etype not in EVENT_TYPES:
            raise ValueError(f"unknown event type: {etype}")
        self._events.append(Event(type=etype, payload=payload, ts=time.time()))

    def _transition(self, target: State) -> None:
        legal: dict[State, set[State]] = {
            State.IDLE: {State.PLANNING},
            State.PLANNING: {State.EXECUTING, State.IDLE, State.DONE},
            State.EXECUTING: {State.AWAITING_TOOL, State.REFLECTING, State.IDLE},
            State.AWAITING_TOOL: {State.REFLECTING, State.IDLE},
            State.REFLECTING: {State.PLANNING, State.EXECUTING, State.DONE, State.IDLE},
            State.DONE: set(),
        }
        if target not in legal[self.state]:
            raise RuntimeError(f"illegal transition {self.state.value} -> {target.value}")
        self.state = target

    def _check_budget(self) -> PullRequest | None:
        which = self.budget.exceeded()
        if which is None:
            return None
        self._emit("budget.warn", {"limit": which})
        self.hooks.fire("on_budget_exceeded", {"limit": which, "budget": self.budget})
        self._reason = f"budget_exceeded:{which}"
        self._prev_state = self.state
        return self._pause(self._reason)

    def _pause(self, reason: str) -> PullRequest:
        self._emit("session.pause", {"reason": reason})
        self.hooks.fire("on_pause", {"reason": reason})
        self._transition(State.IDLE)
        return PullRequest(reason=reason, state=self.state, payload={"reason": reason})

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/19-capstone-projects/20-agent-harness-loop-contract/code/main.py:195 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rohitg00/ai-engineering-from-scratch@39ea8a1c6d (2026-08-26). Data as JSON: /api/errors/7ed6fa25939ceffc. Report an issue: GitHub.