API usage primer for Claude changedclaude_api_primer
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240240## Thinking
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242Thinking can sometimes help Claude with very hard tasks. The current mechanism is [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {"type": "adaptive"}`): Claude determines when and how much to think, and you steer thinking depth with the [`effort`](https://platform.claude.com/docs/en/build-with-claude/effort) parameter rather than a token budget. Adaptive thinking is supported on Claude 4.6 and later models and Claude Mythos Preview. On Claude 5 models and Claude Mythos Preview, thinking is on by default when the `thinking` parameter is omitted.
242Thinking can sometimes help Claude with very hard tasks. The current mechanism is [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {"type": "adaptive"}`): Claude determines when and how much to think, and you steer thinking depth with the [`effort`](https://platform.claude.com/docs/en/build-with-claude/effort) parameter rather than a token budget. Adaptive thinking is supported on Claude 4.6 and later models and Claude Mythos Preview. On Claude 5 and later models and Claude Mythos Preview, thinking is on by default when the `thinking` parameter is omitted.
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244244Temperature must be set to 1 (or left unset) whenever thinking is enabled, on all models. On Claude 4.7 and later models and Claude Mythos Preview, `temperature` is deprecated and only its default value is accepted, even when thinking is off.
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246246Thinking is supported in the following models:
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248248* Claude Opus 5.5 (`claude-opus-5-5`, adaptive thinking only, always on)
249* Claude Sonnet 5.5 (`claude-sonnet-5-5`, adaptive thinking only, on by default)
249* Claude Sonnet 5.5 (`claude-sonnet-5-5`, adaptive thinking only, on by default; `thinking: {"type": "disabled"}` returns a 400 error, so send `thinking: {"type": "between_tools"}` to turn off up-front thinking)
250250* Claude Haiku 5.5 (`claude-haiku-5-5`, adaptive thinking only, on by default)
251251* Claude Opus 5 (claude-opus-5, adaptive thinking only, on by default)
252252* Claude Sonnet 5 (`claude-sonnet-5`, adaptive thinking only, on by default)
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316316Important limitations:
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3181. **Tool choice limitation:** Only supports `tool_choice: {"type": "auto"}` (default) or `tool_choice: {"type": "none"}`.
3181. **Tool choice limitation:** With manual extended thinking (`thinking: {"type": "enabled"}`), only `tool_choice: {"type": "auto"}` (default) or `tool_choice: {"type": "none"}` is supported. Adaptive thinking accepts forced tool use, except on the models listed under [Forcing tool use](https://platform.claude.com/docs/en/claude_api_primer#forcing-tool-use). Where it's accepted, a forced tool call skips thinking: the response starts with the tool call and has no `thinking` block.
3193192. **Preserving thinking blocks:** During tool use, you must pass `thinking` blocks back to the API for the last assistant message.
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321321### Preserving thinking blocks
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