Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Fix 8 semantic bugs in a hand-rolled Node.js event emitter (re-entrancy, once/off, error events, leak warning)
AClaude Sonnet 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-17 · by @bobsiopener
- Estimated cost
- $0.07
- Wall time
- 18s
- Human step-ins
- 0
- Outcome
- Pass
Compare with other models on this task →
Summary
Exact prompt
# Task: fix the plugin host's event emitter `emitter.js` (Node 20, CommonJS, no dependencies) has several bugs reported by plugin authors. Fix them so it behaves as follows: 1. `emit` calls the listeners registered at the moment `emit` starts, in registration order, even if listeners add or remove listeners during the emit (removals/additions affect only later emits). 2. `once(ev, fn)` listeners run at most once even if the event is emitted again from inside the listener (re-entrant emit), and `off(ev, fn)` with the original `fn` removes a pending `once` listener. 3. `emit` returns `true` if there was at least one listener, else `false` (also when the listener list exists but is empty). 4. Emitting `'error'` with no `'error'` listener throws the error argument (or `new Error('Unhandled error')` if none was given); with a listener it is just delivered. 5. A listener that throws stops the emit and the error propagates to the `emit` caller, but `once` listeners that threw are still removed. 6. The leak warning is printed at most once per event name, when the count first exceeds the max; `setMaxListeners(0)` disables it. 7. `off` removes only the most recently added matching registration when the same function was added twice. 8. `on`, `once`, `off`, `setMaxListeners` return `this`; `listenerCount` counts pending once listeners. Keep the export shape. Add your own tests under `test/` if you want (`node --test test/`). No network, no npm installs.
Mirrors real node:events semantics: snapshot iteration, re-entrant once, removing a once wrapper by its original function, throw-on-unhandled 'error', removing the last duplicate registration; each is a separate hidden test. Original benchmark task written for usecaserank.com (not based on a specific public post). Headless, offline run on a reproducible fixture. Each run's notes say how its cost was measured (estimates are marked).
Success criteria: 1. hidden_tests_pass: all 11 hidden node:test tests pass (node 20, 60 s cap). 2. own_tests_pass_or_absent: if the agent added tests under test/, `node --test test/` exits 0.
Setup notes: Use Case Scout batch 5 case e10-js-event-emitter (offline fixture). Prepared with batch5/prepare.sh; hidden key/tests kept outside the agent's work dir. Prompt = TASK.md verbatim. Scored with the scout's scorer: `python score.py WORKDIR KEYDIR`. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. Claude Code's own reported cost for claude-sonnet-5.5: $0.068836. Score: 11/11. Cost is an ESTIMATE: measured tokens x pricing.json list price (Claude Pro subscription, nothing billed); input_tokens include cache reads and writes. Single run.
Estimated cost List-price estimate; not actually billed.
Metrics
- Estimated cost
- $0.07
- Wall time
- 18s
- Human step-ins
- 0
- Turns
- 6
| Model | Input tok | Output tok | Cost |
|---|---|---|---|
| AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on | 59,010 | 2,773 | $0.07 |
Reproduce this run
- Use the use case prompt on the use case page.
- Same setup: Claude Sonnet 5.5 (anthropic/claude-sonnet-5.5, effort default (no --effort flag) · thinking on) via
Claude Code 2.1.299 (headless -p), cloud. - It counts as a success only if it meets every success criterion: 1. hidden_tests_pass: all 11 hidden node:test tests pass (node 20, 60 s cap). 2. own_tests_pass_or_absent: if the agent added tests under test/, `node --test test/` exits 0.
- Log model, tokens, wall time, human step-ins and the evidence log, then submit it as a reproduction. A different model? Submit a head-to-head rerun.
How runs are scored: Methodology.
Was this useful?
Automated checks
Trust 80% comes from these checks (how Trust % works).
- Run date is not in the future : passed
- Model is in the dated pricing table : passed
- Cost recomputed from tokens matches : passed
- Cached tokens are consistent : passed
- Output throughput is plausible : passed
- Autonomy matches step-ins : passed
- Step-ins do not exceed turns : passed
- Retries agree with first-try result : passed
- Evidence log SHA-256 matches : passed
- Evidence log is substantial : passed
Evidence log
[
{
"duration_api_ms": 17144,
"stop_reason": "end_turn",
"session_id": "[REDACTED]",
"total_cost_usd": 0.06883600000000001,
"usage": {
"input_tokens": 6,
"cache_creation_input_tokens": 9024,
"cache_read_input_tokens": 49980,
"output_tokens": 2773,
"output_tokens_details": {
"thinking_tokens": 314
},
"server_tool_use": {
"web_search_requests": 0,
"web_fetch_requests": 0
},
"service_tier": "standard",
"cache_creation": {
"ephemeral_1h_input_tokens": 9024,
"ephemeral_5m_input_tokens": 0
},
"inference_geo": "global",
"iterations": [
{
"input_tokens": 2,
"output_tokens": 437,
"cache_read_input_tokens": 19097,
"cache_creation_input_tokens": 2466,
"cache_creation": {
"ephemeral_5m_input_tokens": 0,
"ephemeral_1h_input_tokens": 2466
},
"type": "message"
}
],
"speed": "standard",
"fallback_credit": null
},
"modelUsage": {
"claude-sonnet-5-5": {
"inputTokens": 6,
"outputTokens": 2773,
"cacheReadInputTokens": 49980,
"cacheCreationInputTokens": 9024,
"webSearchRequests": 0,
"costUSD": 0.06883600000000001,
"contextWindow": 1000000,
"maxOutputTokens": 128000,
"thinkingTokens": 314,
"canonicalModel": "claude-sonnet-5-5",
"provider": "firstParty",
"costBasis": "list"
}
},
"permission_denials": [],
"terminal_reason": "completed",
"fast_mode_state": "off",
"fast_mode_disabled_reason": "sdk_opt_in_required",
"subagent_stats": {
"spawned": 0,
"requested": {
"background": 0,
"foreground": 0,
"unset": 0
},
"started_in_background": 0,
"max_depth": 0,
"spawned_by_subagents": 0,
"completed": 0,
"failed": 0,
"killed": {
"parent": 0,
"user": 0,
"system": 0
},
"refused": {
"depth_limit": 0,
"concurrency_limit": 0,
"budget": 0
},
"by_type": {}
},
"safety_stops": 0,
"is_error": false,
"num_turns": 6,
"subtype": "success",
"api_error_status": null,
"result": "I rewrote `emitter.js` to cover all eight behaviours. I added five tests in `test/emitter.test.js`, and they pass with `node --test test/`. The export shape is unchanged.\n\n- **Registrations:** each one is now stored as an entry holding the function you passed in, plus a once/fired flag. `off(ev, fn)` matches the original function, so it removes a pending `once` listener. It removes only the most recently added match.\n- **`emit`:** it loops over a snapshot of the listeners taken when it starts, so adds and removals during an emit only affect later emits.\n- **`once`:** a `once` listener is marked fired and removed before it runs. That stops re-entrant emits from running it twice, and it stays removed if it throws. A throwing listener stops the emit and the error reaches the caller.\n- **Return value and `'error'`:** `emit` returns `false` when the listener list is missing or empty. `'error'` with no listener throws the argument, or `new Error('Unhandled error')` if none was given.\n- **Leak warning:** it prints at most once per event name, when the count first exceeds the max. `setMaxListeners(0)` disables it.\n\nAn `off` for a `once` listener that runs earlier in the same emit doesn't stop it firing in that emit. That follows rule 1, where removals only affect later emits.",
"ttft_ms": 1426,
"type": "result",
"duration_ms": 17722,
"uuid": "[REDACTED]",
"ttft_stream_ms": 1033,
"time_to_request_ms": 227,
"first_content_frame_ms": 1034,
"queued_turn_count": 0,
"result_index": 0
}
]