Claude Sonnet 5.5 (Anthropic)

The Rundown AI reports Anthropic launched Claude Sonnet 5.5 on September 28, 2026, positioning the middle tier close to Opus 5.5 on several tests at half the flagship's input and output token prices. At maximum effort, Artificial Analysis gave Sonnet 5.5 a score of 56 on its Intelligence Index, behind only Opus 5.5 at 58, and 64% on Terminal-Bench 4.0 versus roughly 60% for Opus 5.5 and GPT-6 Astra. AA's results were provisional pending reruns due to a prerelease structured-output bug. Sonnet 5.5 is priced at $2 per million input tokens and $10 per million output tokens, with output at least 30% faster than Sonnet 5 and task costs up to 30% lower. At maximum effort, AA measured roughly $7.60 per task, about 50% more than Sonnet 5. Anthropic gives Opus the edge on complex work requiring judgment, while Sonnet 5.5 suits well-scoped everyday tasks like bug fixes and document production.

Trending Topics reports Claude Sonnet 5.5 launched on September 28, 2026, landing at No. 2 on the Artificial Analysis Intelligence Index with 56 points, two behind Anthropic's own Opus 5.5 at 58. OpenAI's GPT-6 Astra follows at 53 and GPT-6 Sol at 48, with Sonnet 5 trailing at 38 points. The 18-point jump over its predecessor gives Anthropic the top two positions on the independent ranking. On agentic and office work, Sonnet 5.5 reaches 64% on Terminal-Bench 4.0 ahead of Opus 5.5 and GPT-6 Astra at 60%, and nearly matches Opus on GDPval-AA (1,844 vs 1,846) and AA-Briefcase (1,811 vs 1,822). It hallucinates less than Opus (47% vs 59%) but answers fewer AA-Omniscience questions correctly (54% vs 66%). Anthropic designed Sonnet 5.5 for well-scoped everyday work where speed and clear writing matter.

Claude Sonnet 5.5 launched on September 28, 2026, six days after Claude Opus 5.5, as Anthropic's middle-tier model in the Claude 5.5 family. The aggregator notes it generates output more than 30% faster than Sonnet 5 and costs up to 30% less per task, with a 1-million-token context window, up to 128K output tokens on the synchronous Messages API, and 300K output on the Message Batches API via the output-300k-2026-03-24 beta header. Adaptive thinking is enabled by default with the effort parameter defaulting to high, and the tokenizer matches Sonnet 5's. Pricing stays at $2 per million input tokens and $10 per million output tokens, half of Opus 5.5's $4/$20, with $0.20 per million cache reads and 50% off on the Batch API. The model is also the first Sonnet to launch with Anthropic's cyber safeguards, and several breaking changes affect code written for Sonnet 5, including disabled thinking being replaced by between thinking blocks.

Anthropic launched Claude Sonnet 5.5 on September 28, 2026, the second model in its Claude 5.5 family and a complement to the flagship Opus 5.5. The company describes it as a clear upgrade over Sonnet 5, with output generated more than 30% faster and task costs up to 30% lower than its predecessor. Pricing remains unchanged at $2 per million input tokens, $10 per million output tokens, and $0.20 per million for cache reads. Sonnet 5.5 scores 70.6% on Terminal-Bench 4.0 versus 10.3% for Sonnet 5 and is the first Sonnet to beat Pokémon Red using only screenshots. It also ships with cyber safeguards and fallbacks comparable to those for Anthropic's most capable models. Anthropic positions Sonnet 5.5 for well-scoped everyday work such as bug fixes, polished documents, slides, and spreadsheets, where speed and clean collaboration matter more than complex judgment.