- Compress finalized Codex OAuth request bodies with zstd on the verified ChatGPT backend path, with size/benefit gating, header-body alignment, and one uncompressed retry on 400/415 rejection.
- Negotiate downstream HTTP response compression under server.response-compression (zstd/gzip with q-values, SSE-safe flushing, commit-time Vary merging) and enable negotiated permessage-deflate on Codex and downstream WebSockets.
- Advertise Accept-Encoding to Codex upstreams and decode gzip/deflate/br/zstd upstream responses through a shared decoder before parsing or relaying.
Closes: #6905
