Claude Sonnet 5.5

Overview

Anthropic introduced Claude Sonnet 5.5 on September 28, 2026. It is the second model in the Claude 5.5 family after Opus 5.5 — a clear upgrade over Claude Sonnet 5 that runs 30%+ faster and costs up to 30% less for most work. The model ID is claude-sonnet-5-5 and pricing is $2 / $10 per million tokens, the same as Sonnet 5.

Key features

• A faster, lower-cost complement to Opus 5.5

  Where Opus 5.5 is built for complex work requiring careful judgment, Sonnet 5.5 is strongest at well-scoped everyday tasks, fixing bugs, and creating polished documents, slides, and spreadsheets. Its speed makes it well suited to fast iteration on less complex tasks, and like Opus 5.5, it writes more clearly than the previous generation of models.

• Same price, fewer tokens

  Sonnet 5.5 is priced the same as Sonnet 5 at $2 per million input tokens, $10 per million output tokens, and $0.20 per million tokens for cache reads, but it typically needs far fewer tokens to do the same work. In Anthropic’s testing, it costs up to 30% less per task than its predecessor.

• The fastest Sonnet model to date

  Sonnet 5.5 generates outputs 30%+ faster than Sonnet 5, making it Anthropic’s fastest Sonnet model to date.

• Gains on agentic coding and knowledge work

  Sonnet 5.5 scores 70.6% on Terminal-Bench 4.0, an agentic coding evaluation, compared to Sonnet 5’s 10.3%. It scores two points below Opus 5.5 on GDPval-AA, a test of real-world work across a variety of occupations. Key results are below.

  Benchmark	Sonnet 5.5	Sonnet 5	Opus 5.5
  Terminal-Bench 4.0	70.6%	10.3%	66.4%
  CursorBench 4.0	55.5%	34.1%	57.8%
  OSWorld 2.1 (partial)	80.1%	57.0%	81.8%
  GDPval-AA v2.1	1844	1449	1846

Notes

• Switch to between_tools if you run with thinking off — if you run Sonnet with thinking off, you’ll need to switch to the new between_tools setting, which keeps up-front thinking off, before moving to Sonnet 5.5. See Anthropic’s migration guide for details.
• Higher-risk cybersecurity tasks fall back to Sonnet 5 — Sonnet 5.5’s cyber capabilities are a large improvement over Sonnet 5’s, so it ships with safeguards similar to those on Opus 5.5. Users can still find and fix bugs in their code as part of routine software development, but higher-risk cybersecurity tasks will visibly fall back to Sonnet 5.
• Same biology safeguards as Sonnet 5 — most research, education, and clinical work is unaffected, though some microbiology and virology requests may be flagged in error.
• Protection against reasoning extraction — Sonnet 5.5 is the first Sonnet model to launch with safety classifiers that prevent reasoning extraction. It also expands preserved thinking, so Claude’s thinking cannot be decoupled from the account that created it. Most developers won’t notice a change.
• Zero data retention — as with Opus 5.5 and Sonnet 5, Claude Sonnet 5.5 is available with zero data retention.
• Default Sonnet model in Claude Code — starting with Claude Code 2.1.284, Sonnet 5.5 is the default Sonnet model on the Anthropic API.
• Haiku 5.5 in the coming weeks — Claude Haiku 5.5, built for high-volume and cost-sensitive applications, will join the Claude 5.5 family in the coming weeks.