Claude Sonnet 5.5
- ID
- 29529
- Status
- summarized
- Published
- 29 Sep 2026, 6:07 AM
- Fetched
- 29 Sep 2026, 7:08 AM
- Provider
- Simon Willison
- Category
- developer-ai
- Original URL
- https://simonwillison.net/2026/Sep/28/claude-sonnet-5-5/
- Source URL
- https://simonwillison.net/atom/everything/
Summary
- Score
- 7.0
- Created
- 29 Sep 2026, 7:08 AM
- Tags
- Audience
- developersvibe_codersai_ml_learnersai_agent_userssaas_founders
What happened
Anthropic released Claude Sonnet 5.5, which per Anthropic "runs 30%+ faster, and costs up to 30% less for most work" while priced the same as Sonnet 5, and in Simon Willison's hands-on tests it beat Sonnet 5 on every benchmark and came close to Opus 5.5 on some coding tasks. Sonnet 5.5 is now the model behind the free tier on claude.ai, which Willison notes makes Anthropic's free offering more capable than ChatGPT's free tier running Luna 5.6. He also reproduced an Opus 5.5 failure mode: at "max" thinking effort the model burned 128,000 tokens (~$1.28) and failed to produce an SVG, while "xhigh" effort produced output in 41 seconds for 5.74 cents; Haiku 5.5 is still promised "in the coming weeks".
Why it matters
If you pay for Sonnet-tier API calls, the same price now buys a model that is roughly 30% faster and cheaper to run, and Willison reports it nearly matching Opus 5.5 on coding tasks — a concrete reason to re-run your evals before defaulting to a pricier model. If you prototype on free tiers, claude.ai's free tier now serves Sonnet 5.5 rather than a weaker small model, so the WebGL-pelican-style prompt he tested is a free way to gauge output quality before spending. Set a thinking-token ceiling: his "max" run spent $1.28 and 128,000 tokens and still returned nothing.
Discussion angle
Run the same prompt against claude.ai's free tier (Sonnet 5.5) and your paid model, and compare cost, latency, and whether it actually finishes — then decide what still justifies paying, given the max-effort run that cost $1.28 and produced nothing.