Claude Opus 5.5 (effort default (no --effort flag) · thinking on) on Model ladder: fix 10 real merged bugs with tests hidden
AClaude Opus 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-16 · by @bobsiopener
- Estimated cost
- $0.20
- Wall time
- 50s
- Human step-ins
- 0
- Outcome
- Pass
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Summary
Exact prompt (this run)
You are working in a checkout of the pillarjs/path-to-regexp repository. A user filed this bug report: --- stringify() merges literal text into a parameter name when an empty text token is in between For custom `TokenData` like `new TokenData([param('foo'), text(''), text('bar')])`, `stringify()` emits `:foobar` instead of `:"foo"bar`. Parsing/compiling the serialized path then has a parameter named `foobar` and no `foo`, so the round trip breaks (compile with `{ foo: 'x' }` fails). The same happens for wildcards (`*foo` followed by an empty text token and then identifier text). Without the empty text token, stringify quotes the name correctly. --- Fix the bug in the library code. Dependencies are already installed (node_modules). Keep the change minimal and in the style of the codebase; you may add or update tests. Make sure the existing test suite still passes. You have no internet access.
Five real bug fixes merged into popular open-source libraries after the models' training cutoff (humanize, marshmallow, tenacity, nanoid, path-to-regexp). Each agent starts from the commit before the fix with only a symptom-style bug report, and the fix PR's own regression tests are applied afterwards to check the result. The size of the agent's source change is compared with the human fix. 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). Upstream fix being reproduced: https://github.com/pillarjs/path-to-regexp/pull/456, https://github.com/pillarjs/path-to-regexp/commit/7dd6f8f7bd63ae8bffca0968144508228dc61729.
Success criteria: Hidden PR tests pass; existing suite stays green; diff under 2x the human diff size.
Setup notes: Real merged bug fix: pillarjs/path-to-regexp https://github.com/pillarjs/path-to-regexp/pull/456 (merged after the models' Jun 2026 cutoff). Agent got a history-free snapshot of the parent commit 8877f41873 with deps installed, and a symptom-only bug report written by the runner from the PR description (no fix hints). The PR's test files were overlaid only after the agent finished. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Diff-size flag (not scored): agent changed 16 source lines vs 27 in the human fix (allowance 54; within allowance). Claude Code's own reported cost for claude-opus-5.5: $0.203351. Score: 2/2. 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
- $0.20
- Wall time
- 50s
- Human step-ins
- 0
- Turns
- 9
| Model | Input tok | Output tok | Cost |
|---|---|---|---|
| AClaude Opus 5.5anthropic/claude-opus-5.5effort default (no --effort flag) · thinking on | 155,357 | 2,681 | $0.20 |
Reproduce this run
- Use the exact prompt shown above for this run (the use case page has the shared prompt).
- Same setup: Claude Opus 5.5 (anthropic/claude-opus-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: Hidden PR tests pass; existing suite stays green; diff under 2x the human diff size.
- 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.
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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": 30603,
"stop_reason": "end_turn",
"session_id": "[REDACTED]",
"total_cost_usd": 0.2033514,
"usage": {
"input_tokens": 14,
"cache_creation_input_tokens": 15206,
"cache_read_input_tokens": 140137,
"output_tokens": 2681,
"output_tokens_details": {
"thinking_tokens": 262
},
"server_tool_use": {
"web_search_requests": 0,
"web_fetch_requests": 0
},
"service_tier": "standard",
"cache_creation": {
"ephemeral_1h_input_tokens": 15206,
"ephemeral_5m_input_tokens": 0
},
"inference_geo": "global",
"iterations": [
{
"input_tokens": 2,
"output_tokens": 747,
"cache_read_input_tokens": 26726,
"cache_creation_input_tokens": 626,
"cache_creation": {
"ephemeral_5m_input_tokens": 0,
"ephemeral_1h_input_tokens": 626
},
"type": "message"
}
],
"speed": "standard",
"fallback_credit": null
},
"modelUsage": {
"claude-opus-5-5": {
"inputTokens": 14,
"outputTokens": 2681,
"cacheReadInputTokens": 140137,
"cacheCreationInputTokens": 15206,
"webSearchRequests": 0,
"costUSD": 0.2033514,
"contextWindow": 1000000,
"maxOutputTokens": 128000,
"thinkingTokens": 262,
"canonicalModel": "claude-opus-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": {
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"concurrency_limit": 0,
"budget": 0
},
"by_type": {}
},
"safety_stops": 0,
"is_error": false,
"num_turns": 9,
"subtype": "success",
"api_error_status": null,
"result": "I fixed the bug: `stringify()` now emits `:\"foo\"bar` for `[param('foo'), text(''), text('bar')]`, and does the same for wildcards. The full `npm test` (lint, tests, size check) passes. One related caveat is under the tests below.\n\n**Cause:** in `src/index.ts`, `stringifyTokens` decided whether to quote a name by looking only at the token right after it. An empty text token prints nothing but hid the `bar` that came next. When `stringifyName` saw `\"\"`, it found nothing that could run into the name, so it left `foo` bare. The output `:foobar` then parses back as one parameter named `foobar`.\n\n**Fix:** I added a small helper, `nextToken`, that skips empty text tokens and returns the first token that actually prints something. The param and wildcard branches now pass that token to `stringifyName` instead of `tokens[index]`. Nothing else changes.\n\n**Tests:** I added two cases to `STRINGIFY_TESTS` in `src/cases.spec.ts`, one for a param and one for a wildcard, each with an empty text token before `stuff`. They expect `/:\"test\"stuff` and `/*\"test\"stuff`.\n\n- All 486 tests pass, including the 2 new ones.\n- Coverage is 100% of functions.\n- The minified, brotli-compressed size is 1.98 kB, just under the 2 kB limit.\n- Running `npx vitest run` on its own reports `src/cases.spec.ts` as a failed suite (\"No test suite found\"). It's a data file with no tests in it, and it reports the same way without my change.\n\n**Not fixed:** I left one similar case alone because it's outside this report. A group that prints nothing (`{}`) between a name and identifier text is still unquoted. Something like `:foo{}bar` should parse back correctly because the braces separate the name from `bar`. A group whose first token is identifier text, as in `:foo{bar}`, is also outside what this fix covers.",
"ttft_ms": 2890,
"type": "result",
"duration_ms": 49227,
"uuid": "[REDACTED]",
"ttft_stream_ms": 1576,
"time_to_request_ms": 266,
"first_content_frame_ms": 1576,
"queued_turn_count": 0,
"result_index": 0
}
]