Claude Sonnet 5.5

Claude Sonnet 5.5, released September 28, 2026, was the second model of Anthropic's 5.5 generation, six days after Claude Opus 5.5 and three months after Claude Sonnet 5 opened the 5-series at the mid tier. Anthropic positioned it as the faster, cheaper complement to Opus 5.5: strongest at well-scoped everyday work such as bug fixes and polished documents, slides and spreadsheets, while Opus 5.5 kept the complex, open-ended tasks that need sustained judgment. The price stayed at Sonnet 5's $2 per million input tokens and $10 per million output, with cache reads at $0.20, but Anthropic said the model needed far fewer tokens for the same work, for up to 30% lower cost per task, and generated output more than 30% faster, which made it the fastest Sonnet at the time. It shipped with a 1-million-token context window, 128,000 tokens of maximum output and a June 2026 knowledge cutoff, and was the first Sonnet to launch with cyber safeguards, because its cybersecurity capabilities were comparable to Opus 5's.

The launch table's biggest jump was on agentic coding: 70.6% on Terminal-Bench 4.0 against Sonnet 5's 10.3%, and above the 66.4% Anthropic reported for Opus 5.5 at xhigh effort. On FrontierCode v1.1 it scored 52.1% at xhigh effort and 46.2% at max, against Sonnet 5's 42.4% and Opus 5.5's 54.4%; Anthropic attributed the drop at max effort to the model running more subagent code reviews that made edits outside the task's scope. On knowledge work it came within a few points of Opus 5.5, with a GDPval-AA v2.1 Elo of 1844 against 1846 and an AA-Briefcase v1.1 Elo of 1811 against 1822, both about 400 points above Sonnet 5. It also scored 64.5% on Humanity's Last Exam with tools, 80.1% partial completion on OSWorld 2.1 computer use and 61.6% on Chartography without tools, up from Sonnet 5's 15.6%. It was the first Sonnet model to beat Pokémon Red working only from screenshots. At launch it was available through Anthropic's API as claude-sonnet-5-5 and on Amazon Bedrock, Google Cloud Vertex AI and Microsoft Foundry, with Claude Haiku 5.5 announced to follow in the coming weeks.