Claude Opus 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)

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  "result": "I've implemented `read` and `write` in `chnk.py`, kept `FormatError(msg, offset)` unchanged, and used only the standard library (`zlib`). I tested it with a throwaway script (`/tmp/t_chnk.py`, not in the work dir). Write-then-read gives back the same meta and data for every combination of CRC on/off, compression on/off, and empty, one-byte and multi-chunk data. Every error case in `FORMAT.md` raises with the offset it specifies.\n\n**Reader:**\n- **Header errors:** a bad magic, including input shorter than 4 bytes, reports offset 0. A missing or unsupported version reports 4, and missing flags or reserved flag bits set report 5.\n- **Errors inside a chunk** report the offset of that chunk's type field. This covers bad type bytes, a cut-off or invalid length, a cut-off payload or CRC, a CRC mismatch, bad META, META after data, an unknown critical chunk, bad zlib data, and an `END ` chunk with non-zero length.\n- **Check order:** the CRC is checked before the chunk's contents are interpreted, so ancillary chunks also get their CRC verified before being skipped.\n- **Missing `END `** reports the input length when the input stops cleanly between chunks. If it stops partway through a chunk, that's reported as truncation at the chunk's offset.\n- **Bytes after `END `** report the offset of the first byte after the `END ` chunk.\n\n**Writer:** it follows the spec as written.\n\n**Where the spec was silent, I chose:**\n1. **Empty META payload:** read as no entries rather than a bad line. The writer never produces one, so this only matters for files from other writers. It's a one-line change in `_parse_meta` if you want the stricter reading.\n2. **Strict zlib checks:** a `DATZ` chunk whose compressed stream is incomplete, or has extra bytes after the end of the stream, counts as bad zlib data. By default `zlib` would quietly ignore the extra bytes.\n3. **Writer input checks:** `write` raises `ValueError` for input that can't be stored safely:\n   - `chunk_size` of 0 or less;\n   - a META key that is empty or contains `=` or a newline;\n   - a META value that contains a newline;\n   - a chunk too long for a 5-byte length field.\n\n   Without these checks, `write` would produce files that read back wrong or not at all.",
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