Last verified: October 2, 2026 (Pacific).
Direct answer: Anthropic released Claude Sonnet 5.5 on September 28, 2026 — the second model in the Claude 5.5 family, six days after Opus 5.5. The headline: $2 per million input tokens and $10 per million output tokens — the exact same price as Sonnet 5 — while Anthropic's benchmarks put it near Opus 5.5 on most evals and ahead of it on Terminal-Bench 4.0. A typical workload costs about 30% less than Sonnet 5, because the model uses fewer tokens per task.
Same price, different model
Anthropic describes Sonnet 5.5 as "a faster, lower-cost complement to Claude Opus 5.5," strongest at well-scoped everyday tasks, bug fixing, and creating polished documents, slides, and spreadsheets. The price table:
| Sonnet 5.5 | Sonnet 5 | Opus 5.5 | |
|---|---|---|---|
| Input / output per 1M | $2 / $10 | $2 / $10 | $4 / $20 |
| Cache reads per 1M | $0.20 | — | $0.20 |
| Typical workload vs predecessor | ~30% less | — | ~40% less |
Cache writes run $2.50 per million (5-minute) and $4.00 (1-hour). The Batch API halves input and output to $1/$5. The full 1M-token context window is billed at the standard rate — no long-context surcharge.
The benchmarks, with Anthropic's own caveat attached
These are Anthropic's numbers from its launch table. The company's own caveat is worth quoting in full: "Opus 5.5 remains clearly stronger at complex, open-ended work requiring sustained judgment." Read the table as a cheaper model closing most of the gap, not as a Sonnet that replaces Opus.
| Benchmark | Sonnet 5.5 | Sonnet 5 | Opus 5.5 |
|---|---|---|---|
| Terminal-Bench 4.0 (agentic coding) | 70.6% | 10.3% | 66.4% |
| CursorBench 4.0 | 55.5% | 34.1% | 57.8% |
| GDPval-AA v2.1 (knowledge work, Elo) | 1844 | 1449 | 1846 |
| OSWorld 2.1 (computer use) | 80.1% | 57.0% | 81.8% |
| Humanity's Last Exam (with tools) | 64.5% | 54.9% | 67.7% |
The Terminal-Bench number deserves a pause: 70.6% for a $2/$10 model beats not just Sonnet 5's 10.3% but Opus 5.5's 66.4%. That is the single most aggressive price-performance move in this release cycle. As always, validate against your own tasks before rerouting production traffic.
Switching: what breaks
Moving from Sonnet 5 to Sonnet 5.5 is not drop-in. Five documented breaking changes: disabled thinking, forced tool use, replaying thinking blocks across accounts, the old computer_20251124 tool on the API and Google Cloud, and using older models as advisors in the advisor tool. Test before you cut over.
Separately: Sonnet 4.5 retires November 30, 2026, and Anthropic names Sonnet 5.5 as its replacement. If you still have 4.5 in a pipeline, that migration has a date on it. Sonnet 5 is not listed for retirement as of October 1.
Worth knowing
- Effort defaults differ by surface. High on the Claude Platform; medium in Claude Code and the Claude apps. Like Opus 5.5, effort is the first dial — tune it before rewriting prompts.
- First Sonnet with Opus-class cyber safeguards. Anthropic says Sonnet 5.5's cyber capabilities are a large jump over Sonnet 5, so it ships with safeguards previously reserved for top models. Higher-risk cybersecurity prompts get detected and routed to Sonnet 5 instead; ordinary coding and bug-fixing are unaffected.
- Where it runs. Claude apps, Claude Code, Claude Platform, Amazon Bedrock, Google Cloud, Microsoft Azure. Model ID
claude-sonnet-5-5. 1M-token context, 128K max output, June 2026 knowledge cutoff.
The honest read: Anthropic kept the sticker price flat and moved the performance. For everyday agentic coding and knowledge work, Sonnet 5.5 is now the default answer — until your workload is complex and open-ended enough that Opus 5.5's sustained judgment earns its 2x price. For the Opus side of that tradeoff, see our Claude Opus 5.5 release coverage.
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