Claude Opus 5.5 changelog

Livenerf finds no change in Opus 5.5 accuracy in its first 10-day window

Window 1 (days 11 to 20, from October 4) scored 0.592 against a 0.584 launch-week baseline, a difference of +0.007 plus or minus 0.018 over 780 samples, which the pre-registered rule calls no change. Median output rose from 588 to 606 tokens. Day 19 ran on API credits because the Max weekly meter was exhausted.

GitHub issue claims Opus 5.5 quality fell back to Opus 5 level

A single Claude Code GitHub issue on version 2.1.284 claims Opus 5.5 quality degraded to the level of Opus 5. It was labeled needs-info and has no Hacker News thread. Livenerf found no change in the same period.

Livenerf starts a 30-day test of whether Opus 5.5 gets worse after launch

Livenerf runs a frozen 78-question panel (from GPQA Diamond, MMLU-Pro, competition math and AIME) through headless Claude Code on a Max plan, 90 samples a day for 30 days starting September 24, 2026, with a pinned CLI and exact-match grading. The Hacker News launch thread drew 926 points and 394 comments.

Who is affected:

Claude Max subscribers who suspect Opus 5.5 has been quietly downgraded.

What to do:

No action needed. Watch for the first decision around October 24, 2026.

MarginLab Claude Code tracker collects an Opus 5.5 baseline, with detection paused

As of its September 29, 2026 update, MarginLab's Claude Code tracker (Opus 5.5 at high effort, from September 24) showed a 77% daily pass rate on 50 tasks, 76% over 7 days (350 tasks) and 81% over 30 days (1,200 tasks). Degradation detection is paused until a baseline exists.

Who is affected:

People watching Claude Code with Opus 5.5 for silent regressions.

What to do:

No action needed. Check the tracker again once the baseline is set.