Claude Sonnet 5.5 (Reasoning)
by Anthropic
Claude Sonnet 5.5 (Reasoning) is a reasoning model from Anthropic, released on September 28, 2026. It has a 1M-token context window and can generate up to 128K output tokens. API pricing is $2 per 1M input tokens and $10 per 1M output tokens. It ranks #2 of 220 models by Intelligence Score with a score of 58.3.
Claude Sonnet 5.5 is the second model in Anthropic's Claude 5.5 family and a faster, lower-cost complement to Claude Opus 5.5. It targets well-scoped everyday tasks, coding, and polished documents, slides, and spreadsheets, and runs 30%+ faster than Claude Sonnet 5 while typically using far fewer tokens at the same $2/$10 per million input/output prices. It accepts text and image input with text output, supports multilingual and vision workloads, tool use, and adaptive thinking (Claude Platform default effort high; Claude Code and apps default medium). Context window is 1M tokens with 128K max output on the sync Messages API; reliable knowledge and training-data cutoffs are June 2026. Cache reads are $0.20 and 5-minute cache writes are $2.50 per million tokens. Capability scores are self-reported from the Claude Sonnet 5.5 System Card (Table 8.1.A and §8) and the launch announcement. Available on the Claude API as `claude-sonnet-5-5`.
Key specs
| Provider | Anthropic |
|---|---|
| Release date | 2026-09-28 |
| License | proprietary |
| Context window | 1M tokens |
| Max output | 128K tokens |
| Input price | $2 / 1M tokens |
| Output price | $10 / 1M tokens |
| Intelligence Score | 58.3 |
| Coding index | 48.6 |
Quick answers
What is the context window of Claude Sonnet 5.5 (Reasoning)?
Claude Sonnet 5.5 (Reasoning) has a context window of 1M tokens, with a maximum output of 128K tokens per response.
How much does Claude Sonnet 5.5 (Reasoning) cost?
Claude Sonnet 5.5 (Reasoning) costs $2 per 1M input tokens and $10 per 1M output tokens through the API.
Is Claude Sonnet 5.5 (Reasoning) open source?
No. Claude Sonnet 5.5 (Reasoning) is a proprietary model from Anthropic (proprietary license).
When was Claude Sonnet 5.5 (Reasoning) released?
Anthropic released Claude Sonnet 5.5 (Reasoning) on September 28, 2026.
How does Claude Sonnet 5.5 (Reasoning) compare to Claude Opus 5.5 (Reasoning)?
Claude Sonnet 5.5 (Reasoning) scores 58.3 on the Intelligence Score versus 60.5 for Claude Opus 5.5 (Reasoning), the current #1. Input pricing is $2 vs $4 per 1M tokens.
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