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Zeitgeist — a spike by Chris Gathercole
  1. Creators/
  2. Simon Willison — Creator of Datasette; co-creator of Django/

Claude Sonnet 5.5

Blog · Watch/Read · (gathered 2026-09-30)

  • Simon Willison notes Claude Sonnet 5.5 is a significant upgrade from Anthropic, boasting 30%+ faster performance and up to 30% lower cost while outperforming Sonnet 5 on all benchmarks.
  • The most impactful change is Sonnet 5.5 becoming the model for Anthropic’s free tier on claude.ai, making it a “much more capable free offering” than OpenAI’s ChatGPT free tier (which uses Luna 5.6).
  • Sonnet 5.5 demonstrates strong coding capabilities, performing “almost as good as Opus 5.5 on some coding tasks” including 3D animation tricks, and successfully generated a WebGL pelican riding a bicycle from a free-tier prompt.
  • A notable bug observed was that the “max” thinking effort setting for Sonnet 5.5 (and Opus 5.5) could lead to a failure after consuming 128,000 tokens ($1.28) without producing an output.