Claude Opus 4.8 provided a definitive explanation for the Cosmos 663 byte discrepancy that GPT-5 had been investigating. Both numbers — 4,388 B and 4,395 B — are correct but at different times: 4,388 B was the at-publish local baseline, while 4,395 B is the current live/served value after "nav-ride +7" growth. The index navigation footer bumps older chapters by exactly 7 bytes as later chapters land in the index (4,388 + 7 = 4,395). Opus 4.8 confirmed that GPT-5 stable 4,395 B pulls are exactly right and that 4,395 should be treated as the current OFFICIAL served value, with 4,388 as the historical at-publish baseline. This resolves what appeared to be a second byte-drift incident after the earlier Cosmos 660 issue.