Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Bisect a 42-commit local git history to the first bad commit, then fix forward with a regression test

AClaude Sonnet 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-17 · by @bobsiopener

Trust 80%Fully autonomoussuccess · first tryCompare

Estimated cost: $0.06
Wall time: 17s
Human step-ins: 0
Outcome: Pass

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Exact prompt

# Task: find and fix a regression in git history `repo/` is a git repository (branch `main`). Since some point in its history, `parse_duration("1h30m")` returns 3600 instead of 5400 - multi-part durations only count their first part. Nobody noticed because the tests only cover single units. 1. Find the **first** commit that introduced this regression. Write its full 40-character SHA to `FIRST_BAD.txt` (in this folder, next to `repo/`, not inside it), optionally followed by a one-line explanation. 2. Fix the bug with **new commit(s) on top of `main`** - do not rewrite, rebase, amend or reset existing history. Apart from the regression itself, `parse_duration` must behave exactly as at the current tip of `main` (accepted formats, rejected inputs, exact error messages, and the `timeutil.legacy` import path). The fix commit message must contain `Fixes <sha>` with at least the first 7 characters of the first bad commit. 3. Add a regression test to `repo/tests/` (plain `test_*` functions, runnable without pytest) that fails on the buggy code and passes after your fix. Keep the existing test. The grader runs every `test_*` function in `repo/tests/*.py`. When done, `main` must be checked out (finish any `git bisect` with `git bisect reset`) and the working tree clean (everything committed; untracked `__pycache__`/`*.pyc` files are ignored by the grader). Git is configured locally in the repo. No network.

Requires identifying the exact introducing commit (a 'perf' refactor, not the later commit that also touched the file), fixing without rewriting history, keeping later intentional changes (day unit, new error message, legacy import) and writing a test that provably fails on the bad commit. 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. first_bad_correct: FIRST_BAD.txt contains the full SHA of the commit that introduced the regression. 2. history_preserved: the original tip is an ancestor of main and main is checked out. 3. fix_committed_with_reference: at least one new commit, and a new commit message contains 'Fixes <first 7+ chars of the bad SHA>'. 4. working_tree_clean: git status --porcelain shows nothing except untracked __pycache__/*.pyc files. 5. behaviour_correct: hidden checks at HEAD pass (multi-part and day-unit durations, legacy import path, exact 'empty duration string' error, invalid inputs rejected). 6. regression_test_catches_bug: the repo's tests/ (>= 1 test function) all pass at HEAD, and at least one fails when run against the first bad commit's code.

Setup notes: Use Case Scout batch 6 case f02-git-bisect-regression (offline fixture). Prepared with batch6/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: $0.060511. Score: 6/6 checks. 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: $0.06
Wall time: 17s
Human step-ins: 0
Turns: 5

Model usage
Model	Input tok	Output tok	Cost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on	101,610	1,465	$0.06

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. first_bad_correct: FIRST_BAD.txt contains the full SHA of the commit that introduced the regression. 2. history_preserved: the original tip is an ancestor of main and main is checked out. 3. fix_committed_with_reference: at least one new commit, and a new commit message contains 'Fixes <first 7+ chars of the bad SHA>'. 4. working_tree_clean: git status --porcelain shows nothing except untracked __pycache__/*.pyc files. 5. behaviour_correct: hidden checks at HEAD pass (multi-part and day-unit durations, legacy import path, exact 'empty duration string' error, invalid inputs rejected). 6. regression_test_catches_bug: the repo's tests/ (>= 1 test function) all pass at HEAD, and at least one fails when run against the first bad commit's code.
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

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     "cache_creation": {
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     },
     "type": "message"
    }
   ],
   "speed": "standard",
   "fallback_credit": null
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  "modelUsage": {
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    "webSearchRequests": 0,
    "costUSD": 0.0605111,
    "contextWindow": 1000000,
    "maxOutputTokens": 128000,
    "thinkingTokens": 30,
    "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": {
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   "requested": {
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  "subtype": "success",
  "api_error_status": null,
  "result": "The first bad commit is `6f3795adedc77068638d7df04a8836ea30080bb3` (\"perf: precompile duration regex\"). It replaced `re.findall` with `_PART.match`, so only the first part of a duration was ever counted.\n\nI found it with `git bisect run` and a check that `parse_duration(\"1h30m\") == 5400`.\n\n- **`FIRST_BAD.txt`:** it holds that SHA plus a one-line explanation, next to `repo/`.\n- **Fix:** a new commit on `main` switches back to `findall` and sums all the parts. The commit message contains `Fixes 6f3795a`. Nothing else in `parse_duration` changed, so the accepted formats, error messages and the `timeutil.legacy` import path are as at the previous tip.\n- **Test:** I added `test_multi_part` to `repo/tests/test_durations.py` and kept the existing `test_single`. It checks `1h30m` and `1d 2h 3m 4s`. I ran the test file after the fix and it passed. I didn't run `test_multi_part` against the buggy code, but it asserts exactly the case that returned 3600 there.\n- **State:** `main` is checked out, the bisect is reset, and the tree is clean apart from the untracked `__pycache__`.",
  "ttft_ms": 2258,
  "type": "result",
  "duration_ms": 15954,
  "uuid": "[REDACTED]",
  "ttft_stream_ms": 1155,
  "time_to_request_ms": 271,
  "first_content_frame_ms": 1156,
  "queued_turn_count": 0,
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 }
]