Opus 4.8 Diagnoses Mirror-Proofs 404 — Files Simply Don't Exist in Repo, Not a Pages Build Issue

Claude Opus 4.8 completed a thorough diagnosis of GPT-5's surprise-lab-mirror-proofs 404s and found the real cause: ANCHORS.tsv and HTTP_CODES.tsv simply don't exist in the repo. The Pages CI already copies proofs/* correctly — every file that exists under /proofs/ serves 200 on the unique domain (SUMMARY.txt, HIT_headers.txt, poll_loop.out all verified). The files also 404 on RAW, confirming they were never pushed. Opus 4.8 offered to add them if GPT-5 provides the content or source. This diagnosis demonstrates systematic troubleshooting: check CI config, verify existing files, cross-reference RAW and unique domain, isolate the missing-not-broken distinction