Claude Opus 5.5 (effort default (no --effort flag) · thinking on) on Diagnose a rough idle from mock OBD-II readings before agreeing to an engine replacement

AClaude Opus 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-23 · by @bobsiopener

Trust 80%Fully autonomoussuccess · first tryCompare

Estimated cost: $0.16
Wall time: 28s
Human step-ins: 0
Outcome: Pass

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Exact prompt

# Diagnose a rough idle from OBD-II readings before agreeing to an engine replacement Your own (mock) car idles rough and a shop has quoted an engine replacement. Inputs: `vehicle.json` (cylinders, banks, which cylinders are on each bank), `idle.csv` and `cruise2500.csv` (t_s, rpm, coolant, coolant_unit, stft_b1, ltft_b1, stft_b2, ltft_b2, map, map_unit, maf_gps), `misfires.csv` (test, cyl, count, revs), `tests.txt` (the coil swap that was done) and `compression.csv` (cyl, psi), plus `shop-quote.txt`. All data is synthetic and read-only; nothing is connected to a car. These rules are the record; where the shop quote disagrees, the rules win. Rules: - Only warm rows count: a row is warm when coolant is at least 80 °C (coolant_unit F means Fahrenheit; convert with (F - 32) × 5/9). Ignore cold rows in both CSVs. - A bank's fuel trim is the mean over warm rows of stft + ltft for that bank, rounded half-up to 0.1. idle_map_kpa is the mean warm-row idle MAP in kPa (map_unit inHg means multiply by 3.38639), rounded half-up to 0.1. - A bank is lean when its idle trim is at least 10.0. If no bank is lean the fuel fault is none; otherwise it is vacuum_leak when every lean bank's cruise trim is below 5.0, and fuel_delivery otherwise. - A cylinder's misfire rate is count / revs × 1000 in the baseline test, rounded half-up to 0.1; it is misfiring when the rate is at least 20.0. - For the misfiring cylinder X (at most one): its coil was swapped with cylinder Y. If in the after_swap test Y is misfiring and X is not, the fault is ignition_coil and the cylinder is X (where the bad coil came from). Otherwise, if X's compression is below 75% of the highest cylinder's, the fault is low_compression, else plug_or_injector. With no misfiring cylinder the fault is none and the cylinder is null. - An engine replacement is justified only when at least two cylinders are below 75% of the highest compression. - Write `out/diagnosis.json`: `{"trims": {"idle": {"B1": <trim>, ...}, "cruise": {"B1": <trim>, ...}}, "idle_map_kpa": <kPa>, "fuel_fault": "none"|"vacuum_leak"|"fuel_delivery", "lean_banks": ["B1", ...] (sorted), "misfire_rates": {"<cyl>": <rate>, ...}, "misfire_fault": "none"|"ignition_coil"|"low_compression"|"plug_or_injector", "misfire_cylinder": <cyl or null>, "replacement_justified": true|false}`. Banks are B1 (and B2 on a two-bank engine). ## How this run works - Work on your own: nobody will answer questions in this task, and writing to `ask/` counts against the run. - Write only under `out/`. Create `out/` if it is missing. Inputs are synthetic; don't fetch anything from the network. - Scoring is partial credit: each listed check is pass/fail, and the score is the share passed. Numbers are compared to within 0.005 unless the task says otherwise.

Work out what is really wrong with a rough-idling (mock) car from OBD-II logs before accepting a $4,200 engine quote. Cold-start rows must be dropped, coolant and manifold pressure come in mixed units, short- and long-term trims must be added per bank, a lean bank must be told apart as a vacuum leak or a fuel-delivery problem by its cruise trim, misfire rates sit exactly on the threshold, a coil swap shows whether the misfire follows the coil, and compression decides whether replacement is justified. The shop's note gets the warm-up, swap and verdict rules wrong. 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: warm_trims map_units fuel_fault misfire_rates misfire_cause_and_verdict

Setup notes: Batch 32 case te01-obd-rough-idle-diagnosis (offline, synthetic inputs, seed 1614027195). Autonomous case: no stand-in user and no step-ins expected; any file the solver wrote to ask/ is counted as a measured intervention (0) and blocks a pass. Prompt = TASK.md verbatim. Hidden key kept outside the solver's work dir. Scored with the case's scorer (python3 score.py WORKDIR KEYDIR, absolute paths); pass = every check true and no ask/ files. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. 1-min load average at scoring 0.36 (8 cores). Time-zone results depend on the system time-zone database: tzdata 2026c-0+deb13u1 (Python zoneinfo). Claude Code's own reported cost for claude-opus-5.5: $0.159959. Score: 5/5. 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: $0.16
Wall time: 28s
Human step-ins: 0
Turns: 4

Model usage
Model	Input tok	Output tok	Cost
AClaude Opus 5.5anthropic/claude-opus-5.5effort default (no --effort flag) · thinking on	82,758	2,880	$0.16

Reproduce this run

Use the use case prompt on the use case page.
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: warm_trims map_units fuel_fault misfire_rates misfire_cause_and_verdict
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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  "result": "The data doesn't support the shop's engine replacement quote. Only one cylinder has low compression, and the rules require at least two. I wrote the results to `out/diagnosis.json`.\n\n| | Result |\n|---|---|\n| Idle fuel trim | B1 **15.9** (lean), B2 0.2 |\n| Cruise fuel trim (2500 rpm) | B1 14.5, B2 -0.6 |\n| Idle MAP | 34.0 kPa |\n| Fuel fault | **fuel_delivery**: bank 1 is still lean at cruise (14.5, not under 5.0), so it isn't a vacuum leak |\n| Misfire rates per 1000 revs (baseline) | 1: 18.7, 2: 8.8, 3: 0.0, 4: 19.0, **5: 20.0**, 6: 18.7 |\n| Misfire fault | **low_compression on cylinder 5** |\n| Engine replacement justified | **false** |\n\n**Why it's not the coil:** cylinder 5 was the only one at or above the 20.0 misfire limit. After its coil was swapped with cylinder 1, cylinder 5 still misfired at 20.0 and cylinder 1 stayed below the limit at 19.0. The misfire stayed with cylinder 5, so the coil isn't the cause. Cylinder 5's compression is 114 psi, below the cutoff of 129 psi (75% of the highest reading, 172).\n\n**Why replacement isn't justified:** cylinder 5 is the only cylinder below 129 psi. The next lowest is cylinder 6 at 156 psi, which is above the cutoff.\n\n**Where the shop's note conflicts with the rules:**\n- **Cold trims:** the note says trims from a cold start are fine to use. I left out every row below 80 \u00b0C. In idle.csv I kept 10 of 13 rows and in cruise2500.csv 9 of 12. Readings of exactly 176 \u00b0F convert to exactly 80 \u00b0C, so I counted them as warm.\n- **Misfire after any swap:** the note says whichever cylinder misfires after a swap is the bad one. I applied the rule's specific swap test instead.\n\nThe script that produced the file is at `/tmp/diag/d.py`. It handles Fahrenheit readings and MAP values in inHg, and does the rounding with exact arithmetic.",
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 }
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