Claude Sonnet 5.5 changelog

Every change to Claude Sonnet 5.5 that Modelpeers has recorded, newest first, each with the source it came from. The record starts Sep 1, 2026.

All (14)Releases (4)Breaking changes (3)Prices/limits (1)Benchmarks (6)

October 2026

Claude Sonnet 5.5, Opus 5.5, Gemini 4 Argon

5 models from Anthropic and Google and OpenAI

Benchmark1w

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.

Gemini 4 Argon, Claude Opus 5.5, Claude Opus 5

6 models from Google and Anthropic and OpenAI

Benchmark1w

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.

Claude Opus 5.5, GPT-6 Astra, Sonnet 5.5

6 models from Anthropic and OpenAI and Google

Benchmark1w

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.

Claude Opus 5.5, Sonnet 5.5, GPT-6 Astra

6 models from Anthropic and OpenAI and Google

Benchmark1w

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.

Claude Sonnet 5.5 and Claude Code

2 products from Anthropic

Benchmark3w

Sonnet 5.5 in Claude Code scores 61.8% on Terminal-Bench 4.0

Claude Code with Sonnet 5.5 at max effort resolved 61.82% (plus or minus 2.9, 330 trials, $7,333.64 total cost), second on the leaderboard. Posted October 6, 2026.

Claude Opus 5.5, Sonnet 5.5 and GPT-6 Astra

3 models from Anthropic and OpenAI

BenchmarkOct 1, 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.

3 incidents are listed on the Claude Sonnet 5.5 page. Get this changelog in a feed reader with the RSS feed.