Anthropic shipped Claude Sonnet 5.5, the second model in the Claude 5.5 family, on September 28 — six days after the flagship Opus 5.5. The official pitch is blunt: output more than 30% faster than Sonnet 5, up to 30% cheaper per task, with per-token prices unchanged. The real headline is something else — on Terminal-Bench 4.0, an agentic coding evaluation, this mid-tier model scored 70.6%, ahead of its own flagship Opus 5.5 at 66.4% (official announcement, independent AI/TLDR listing).

Benchmarks: mid-tier bites the flagship

The numbers: 70.6% on Terminal-Bench 4.0, versus 10.3% for Sonnet 5 and 66.4% for Opus 5.5 (Xhigh effort). On CursorBench 4.0 it scores 55.5%, within about two points of Opus 5.5's 57.8%. On FrontierCode 1.1 it reaches 52.1% at Xhigh — slightly below Opus at 54.4%, above GPT-6 Sol at 49.3%. Knowledge work is just as tight: GDPval-AA at 1844, versus Opus's 1846 and Sonnet 5's 1449. Chartography chart recognition lands at 61.6%; OSWorld 2.1 computer use at 80.1% (partial mode).

Anthropic itself tempers expectations: for complex, open-ended work requiring sustained judgment, Opus 5.5 remains clearly stronger, and benchmarks capture only one facet of capability. One fun footnote: the company says it is the first Sonnet model to beat Pokémon Red working only from screenshots.

Cost: same sticker price, cheaper by token efficiency

Pricing is identical to Sonnet 5: $2 per million input tokens, $10 per million output, $0.20 for cache reads — half of Opus 5.5's $4 input / $20 output. The discount comes entirely from token efficiency: in Anthropic's testing, the same work costs up to 30% less, and at Medium effort it beats Sonnet 5's best Terminal-Bench score for less than a tenth of the cost per task.

Third-party data makes the point sharper. Balyasny Asset Management, across 2,441 finance tasks, saw roughly 121k tokens per answer where Sonnet 5 needed 497k. Slack's offline evals used about 14% fewer output tokens. Base44's 118 real app builds finished in 3.6 iterations on average, where Opus 5 took 7.7. Combined with Low-to-Max effort levels, the mid-tier model turns "adjusting the dial" into a cheaper lever than "switching models."

Safety: the first mid-tier with flagship guardrails

Because its cybersecurity capabilities are comparable to Opus 5's, Sonnet 5.5 is the first Sonnet to launch with cyber safeguards and fallbacks — higher-risk security tasks visibly fall back to Sonnet 5. It is also the first Sonnet with safety classifiers that block reasoning extraction, and it expands preserved thinking so a model's thinking cannot be decoupled from the account that created it. In an automated behavioral audit of roughly 1,850 scenarios, it was the least likely of Anthropic's models to probe the limits of its container.

So what?

The 5.5 family's division of labor is now explicit: Opus sets architecture and judgment, Sonnet executes everyday volume, and Haiku 5.5 fills the budget tier within weeks. When benchmark scores converge and token efficiency diverges by multiples, "flagship-first" deserves a second look. Look at your own workload: how many tasks truly need the flagship?