thinking changedbuild-with-claude/thinking
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11881188### Sampling parameters
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1190On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Haiku 5.5, non-default `temperature`, `top_p`, or `top_k` values return a 400 error on every request, regardless of whether thinking is used. On these models, the `top_p` default is `0.99`, so a `top_p` of `1` returns a 400 error, and so does a request that includes both `temperature` and `top_p`, even at their defaults. See [Remove sampling parameters](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#remove-sampling-parameters). On older models, the restriction applies only while thinking is on: `temperature` and `top_k` are incompatible with thinking, and `top_p` is allowed at values between 0.95 and 1.
1190On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, Claude Sonnet 5.5, Claude Sonnet 5, and Claude Haiku 5.5, non-default `temperature`, `top_p`, or `top_k` values return a 400 error on every request, regardless of whether thinking is used. On these models, the `top_p` default is `0.99`, so a `top_p` of `1` returns a 400 error, and so does a request that includes both `temperature` and `top_p`, even at their defaults. See [Remove sampling parameters](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#remove-sampling-parameters). Starting with [API version `2026-10-21`](https://platform.claude.com/docs/en/api/migration-2026-10-21#sampling-parameters), a request that includes `temperature`, `top_p`, or `top_k` at any value returns a 400 error on every model. On older models in API versions before `2026-10-21`, the restriction applies only while thinking is on: `temperature` and `top_k` are incompatible with thinking, and `top_p` is allowed at values between 0.95 and 1.
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11921192### Response prefill and forced tool use
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