A 607 KB Codex request was estimated at 189776 tokens (window 200000), sent untrimmed and rejected by Claude; the gateway only said "upstream 400", so Codex saw a dropped stream. Claude targets now use 2.6 bytes/token, and that bare 400 on a request filling most of the window becomes context_length_exceeded. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>