whats-new-opus-5-5 changedmodels/opus-5-5/whats-new-opus-5-5
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6565### Fast mode
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67[Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) (research preview) is available for Claude Opus 5.5 on the Claude API only; it is not available on Amazon Bedrock, Claude Platform on AWS, Google Cloud, or Microsoft Foundry. Set `speed: "fast"` with the `fast-mode-2026-02-01` beta header. See [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) for access, supported models, and pricing.
67[Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) is available for Claude Opus 5.5 on the Claude API only; it is not available on Amazon Bedrock, Claude Platform on AWS, Google Cloud, or Microsoft Foundry. On the Claude API, set `speed: "fast"` on your request. Claude Opus 5.5 doesn't need a beta header. See [Fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode) for supported models, rate limits, and pricing.
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6969### Define tools in a message (beta)
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