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Claude Opus 5.5 lists 20 percent below Claude Opus 5 on input and output, at $4 and $20 per million tokens. Its cache reads fall 60 percent, to $0.20, because the multiplier changed from 0.1x to 0.05x. Thinking cannot be disabled, and 25 percent more output tokens erases the output cut.
4 min read
Claude Opus 5.5 launched September 22, 2026 with four breaking changes that return a 400 error: disabled thinking, forced tool_choice, computer_20251124 on the API and Google Cloud, and replayed thinking blocks on newer accounts. A fifth change returns 200: tool-call progress text goes quiet by default, and the effort parameter's default silently drops from high to medium.
5 min read