Claude Opus 5.5 (effort default (no --effort flag) · thinking on) on Fix a Node.js circuit breaker state machine (rolling window, half-open concurrency, timeouts, exact transition events)

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  "result": "I rewrote `CircuitBreaker` in `breaker.js` to follow the state machine in the spec. The constructor options and export are unchanged. I also added 10 tests in `test/breaker.test.js`, and all of them pass with `node --test test/`.\n\n**What was wrong in the old version:**\n- Any success reset the failure count to zero, and there was no rolling window.\n- `isFailure` was never checked, so every rejection counted as a failure.\n- A synchronous throw from `fn` turned into a rejection, but only because the old `call` was `async`.\n- It moved to half-open only when `now - openedAt > cooldownMs`; the spec says `>=`. It also only did so inside `call()`, so the `state` getter kept reporting `'open'`, and the open \u2192 half_open change never fired a `stateChange` event.\n- In half-open it didn't limit trial calls, and a single success closed the breaker instead of `successThreshold` successes in a row.\n- `timeoutMs` was ignored.\n- When a failure happened while already open, it emitted a \"transition\" from `open` to `open`.\n\n**How it works now:**\n- Failures are stored as the `now()` time when each call settled. On each new failure, entries with `t <= now - windowMs` are dropped, and the breaker opens when the count reaches `failureThreshold`. Successes don't touch the list, and it is cleared when the breaker closes.\n- Every state change bumps an internal counter. Each call remembers the counter's value when it started, and if a change has happened since, its result is passed to the caller but doesn't affect the state. This is how \"in-flight results don't change state further\" works after a trial fails. It also means a slow call that started while closed can't affect the breaker after it has opened.\n- Both the `state` getter and `call()` check whether the cooldown has passed, so `state` reports `'half_open'` as soon as it has. The open \u2192 half_open event fires once, the first time either one notices.\n- A timed-out call rejects with `ETIMEOUT` and counts as a failure. Whichever happens first (timeout or the call settling) wins, the later one is ignored, and the timer is cleared either way.\n\n**Choices I made where the spec is open:**\n- In half-open, a rejection that `isFailure` says isn't a failure counts as a successful trial, following \"otherwise treated like a success\".\n- A timeout counts as a failure even if `isFailure` would return false for it.\n- If `isFailure` itself throws, the rejection counts as a failure.\n- Calls that are refused with `EOPEN` return a rejected promise rather than throwing synchronously.\n\nThe tests use a fake clock for the open, cooldown, window and half-open logic. For the timeout, they use a real 20 ms timer.",
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