The signMcpbFile function appended a signature block to the ZIP file but
did not update the EOCD comment_length field. Strict ZIP parsers (like
Claude Desktop's) rejected these files because the declared comment
length didn't account for the appended signature data.
Now scans backwards for the EOCD magic bytes, reads the current
comment_length, and adds the signature block length before concatenating.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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