Claude Sonnet 5.5 changelog

Full Claude Platform available on Google Cloud (Anthropic managed)

Claude Models on Google Cloud (Anthropic managed) is generally available with US and Global endpoints. It gives Google Cloud customers the full Claude Platform feature set with Google Cloud IAM, billing and commitment drawdown. Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5 are available, and fast mode for Opus 5.5 costs $8 / $40 per million input and output tokens there.

Who is affected:

Teams that buy Claude through Google Cloud and want features that the Google-managed offering lacks.

What to do:

No action needed. Google Cloud customers who need same-day Claude API features can switch to the Anthropic-managed offering.

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.

Antigravity 2.24 adds model choice for automations and Claude 5.5 models through Vertex AI

Antigravity app 2.24.0 lets automations run on Flash Lite, Flash or Pro, adds /compact and a redesigned side panel. With Application Default Credentials sign-in, the model picker offers Claude Opus 5.5 and Claude Sonnet 5.5 when the Google Cloud project has access to them.

Claude Sonnet 5.5 cache reads cut in half

Prompt cache reads on Sonnet 5.5 dropped from $0.20 to $0.10 per million tokens. Anthropic says this makes Sonnet 5.5 about 20% cheaper on most agentic work.

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.

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.

Claude Code makes Sonnet 5.5 the default Sonnet model

Claude Code 2.1.284 added Claude Sonnet 5.5 as the default Sonnet model on the Anthropic API.

Claude Sonnet 5.5 released

Claude Sonnet 5.5 launched at the same $2 / $10 per million tokens as Sonnet 5, with cache reads at $0.10 per million tokens after the October 7 cut.

Who is affected:

Developers and subscribers using Claude Sonnet 5.

What to do:

Test claude-sonnet-5-5. Switch thinking disabled to between_tools and stop forcing tool_choice.

Claude Sonnet 5.5 changes thinking and tool use in ways that break Sonnet 5 code

To turn off up-front thinking on Sonnet 5.5 you send between_tools instead of disabled. Forced tool use returns a 400 error. Thinking blocks only work in the account that produced them.

Who is affected:

Code moving from Claude Sonnet 5 to Sonnet 5.5.

Action needed:

Follow the Sonnet 5.5 migration guide before switching model IDs.

Claude Sonnet 5.5 available in GitHub Copilot

Claude Sonnet 5.5 is available on Copilot Pro, Pro+, Max, Business and Enterprise at provider list price.

New API accounts can no longer edit context around thinking blocks

To counter distillation, the API now checks that a thinking block is sent back with the same system prompt, tools and messages. On Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5, accounts created on or after August 31, 2026 get an error unless they opt into dropping the block.

Who is affected:

Agents that compact or rewrite earlier turns, especially on new API accounts.

Action needed:

Use server-side compaction or mid-conversation tool changes, or opt into non-strict mode.