Claude Opus 5.5 changelog

Vals Index: Sonnet 5.5 edges Opus 5.5 and Gemini 4 Argon

Vals AI's GDP-weighted index of finance, coding, legal and tax tasks ranks Claude Sonnet 5.5 first at 70.85%, Opus 5.5 at 70.18%, Gemini 4 Argon 69.24%, GPT-6 Astra 65.02% and GPT-6.1 Sol 63.94% as of October 21, 2026.

Arena text leaderboard: Gemini 4 Argon first, Opus 5.5 second, GPT-6 Astra 73rd

Arena's October 20, 2026 text leaderboard ranks Gemini 4 Argon (High) first at 1532 (4,900 votes), Claude Opus 5.5 (High) second at 1516, Claude Opus 5 (Max) fourth at 1509, Claude Sonnet 5.5 (xHigh) 28th at 1473, GPT-6.1 Sol (Max) 66th at 1443 and GPT-6 Astra (Max) 73rd at 1441.

Arena WebDev leaderboard: Opus 5.5 first, GPT-6 Astra second

Arena's October 19, 2026 WebDev leaderboard ranks Claude Opus 5.5 (Max) first at 1807 (2,925 votes), GPT-6 Astra (Max) second at 1774, Claude Sonnet 5.5 (xHigh) third at 1768, GPT-6.1 Sol (Max) fourth at 1753, Claude Opus 5 (Max) eighth at 1689 and Gemini 4 Argon (High) tenth at 1678.

Vals AI Terminal-Bench 4.0 with one harness: Opus 5.5 65.2%, Astra 59.6%, Argon 57.6%

Running every model in the same Mini-SWE-agent harness, Vals AI reports Claude Opus 5.5 at 65.15%, Sonnet 5.5 64.14%, GPT-6 Astra 59.60%, Gemini 4 Argon 57.58%, GPT-6.1 Sol 55.05% and Opus 5 53.53%. GPT-6.1 Sol cost $1.72 per test against $13.20 for Opus 5.5.

SlopCodeBench: GPT-6 Astra leads strict pass rate, Opus 5.5 does better at medium than xhigh

On 36 problems and 196 checkpoints, GPT-6 Astra at xhigh had the best strict pass rate (16.33%), GPT-6.1 Sol medium 14.80% and Opus 5.5 medium 14.29%. Opus 5.5 at xhigh scored lower (12.76%) at 4.22 times the cost. The Hacker News thread drew 303 points and 120 comments.

Who is affected:

Teams choosing effort levels for long coding tasks with changing requirements.

What to do:

Test medium effort before paying for xhigh.

Opus 5.5 in Claude Code tops the official Terminal-Bench 4.0 leaderboard at 64.8%

Claude Code with Opus 5.5 at max effort resolved 64.85% of Terminal-Bench 4.0 tasks (plus or minus 3.1, 330 trials, $4,718.65 total cost), ranked first. The entry was posted October 6, 2026.

Epoch: Opus 5.5 takes the top spot on the Epoch Capabilities Index

Epoch AI reported Opus 5.5 first on its Epoch Capabilities Index with 167, narrowly ahead of GPT-6 Astra, and Sonnet 5.5 roughly matching Fable 5.1 (165). Epoch's current ECI data lists Opus 5.5 at 168.83, GPT-6 Astra 168.81, GPT-6.1 Sol 168.37, Gemini 4 Argon 166.45, Sonnet 5.5 166.18 and Opus 5 163.94.