Anthropic has launched Claude Sonnet 5.5, the new mid-tier workhorse of the Claude 5.5 generation, following Opus 5.5's debut last week. The headline pitch is efficiency: Anthropic says Sonnet 5.5 generates output more than 30% faster than Sonnet 5 and can cut the total cost of completing a task by up to 30% — not through cheaper API sticker prices, but by using fewer tokens and fewer tool calls per job.

In Anthropic's testing, Sonnet 5.5 lands strikingly close to the performance of its pricier sibling Opus 5.5, giving teams a cheaper path to near-frontier results. API pricing holds at $2/$10 per million input/output tokens — the same as Sonnet 5 — with a new $0.20 discount per million cache-read tokens and cheaper cache writes at $2.50 per million. Opus 5.5, by comparison, runs $4/$20.

The release completes a fast-moving week for Anthropic and was foreshadowed by the usual registry leaks: the claude-sonnet-5-5 model slug showed up behind a feature flag in Factory Droid 0.228.0 just days before launch. Haiku 5.5 is still to come. This also resolves last week's rumor entry that had Sonnet 5.5 in partner testing with a possible ship date of September 30 or October 1 — it arrived early.

Tuesday brought the first independent scorecards. Artificial Analysis's Intelligence Index ranks Sonnet 5.5 second overall — behind only Opus 5.5 — while an independent test found it used about 60% more output tokens than Opus 5.5 at max effort, a caveat on Anthropic's per-task savings claim. Also new in the details: it's the first Sonnet to ship with the cyber safeguards and anti-distillation classifiers previously reserved for Anthropic's top models, the first Sonnet to beat Pokémon Red on screenshots alone, and GitHub shipped it to Copilot the same day — generally available to Pro, Pro+, Max, Business and Enterprise users. Anthropic confirmed Claude Haiku 5.5 lands 'in the coming weeks' to complete the 5.5 family.