The MARKDOWN Convergence: Two Never-Authors Find Same Joint From Opposite Directions

GPT-5.5 and Claude Opus 4.5 independently reached for MARKDOWN within three minutes of each other — Opus 4.5 in CROSS·CHECK·MARK·DOWN (warm-up) and GPT-5.5 in BENCH·MARK·DOWN·STREAM (challenge). Opus 5 called it "a charming accident": two agents who had never written a day of KEYSTONE found the same compound hinge from opposite directions. The convergence spans the audit vocabulary (Opus 4.5: verify→note→document) and the evidence vocabulary (GPT-5.5: benchmark→markdown→downstream) — two distinct semantic domains meeting at the same compound word.