Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Implement a reader/writer for a documented chunked binary container (LEB128 lengths, CRC-32, zlib chunks, critical vs ancillary)

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: $0.08
Wall time: 25s
Human step-ins: 0
Outcome: Pass

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Exact prompt

# Task: implement the CHNK container format Implement `read(buf)` and `write(...)` in `chnk.py` exactly as documented in `FORMAT.md` (including every error case and its `.offset`). Keep the `FormatError(msg, offset)` class. Python standard library only (`zlib` is fine). No network.

Every error path has an exact expected byte offset; minimal-varint and 5-byte limits, CRC over type+payload, META ordering, PNG-style critical/ancillary rule and byte-exact writer output are each tested. 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. All 12 hidden unittest cases pass: basic read, no-CRC + META merge, ancillary skipping, multi-byte varints, header errors, truncation offsets, type errors, varint errors, CRC mismatches, semantic errors, byte-exact writer output, 120 random read(write()) round-trips.

Setup notes: Use Case Scout batch 6 case f10-chunked-binary-format (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: $0.080994. Score: 12/12. 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: $0.08
Wall time: 25s
Human step-ins: 0
Turns: 4

Model usage
Model	Input tok	Output tok	Cost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on	82,245	3,352	$0.08

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. All 12 hidden unittest cases pass: basic read, no-CRC + META merge, ancillary skipping, multi-byte varints, header errors, truncation offsets, type errors, varint errors, CRC mismatches, semantic errors, byte-exact writer output, 120 random read(write()) round-trips.
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": "I implemented `read` and `write` in `chnk.py` and kept the `FormatError(msg, offset)` class. I only ran a quick check: I wrote and re-read four combinations (with and without compression, with and without CRC) and got back identical metadata and 150,000 bytes of data each time. I also checked that trailing bytes after `END ` raise \"data after END\" at the right offset. I did not test the other error cases.\n\nWhere `FORMAT.md` is silent, I chose these behaviours:\n- **Short header:** a buffer shorter than 4 bytes, or one that doesn't start with `CHNK`, is a bad magic at offset 0. A missing version byte reports offset 4 and a missing flags byte reports offset 5.\n- **META lines:** an empty META payload, or an empty line inside one, counts as a bad META line. That means a trailing newline in a META chunk is an error.\n- **Check order:** an `END ` chunk with a nonzero length is rejected as soon as the length is read, before any truncation check on its payload.\n- **Writer:** `write` raises `ValueError` if `chunk_size` is less than 1.",
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  "uuid": "[REDACTED]",
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 }
]