The whole hunk
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476476### Thinking cannot be disabled
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478On Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), and [Claude Mythos Preview](https://anthropic.com/glasswing), thinking is always on. Sending `thinking: {"type": "disabled"}` to any of these models returns a 400 `invalid_request_error`:
478On Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), and [Claude Mythos Preview](https://anthropic.com/glasswing), thinking is always on. Sending `thinking: {"type": "disabled"}` to any of these models returns a 400 `invalid_request_error`. On all of these models except Claude Mythos Preview, the message reads:
479479
480480```text wrap
481"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
482```
483
484On Claude Mythos Preview, the only one of these models that accepts extended thinking, the message reads:
485
486```text wrap
481487"thinking.type.disabled" is not supported for this model. Thinking defaults to adaptive mode when not specified; use "thinking.type.enabled" with "budget_tokens" for extended thinking.
482488```
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484On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, and Claude Mythos 5, the error message's own suggestion of `"thinking.type.enabled"` is also rejected. Omit the `thinking` parameter and the request runs with adaptive thinking. To keep thinking content out of responses without turning thinking off, set `display: "omitted"` on the thinking configuration. See [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-type-disabled).
490Omit the `thinking` parameter and the request runs with adaptive thinking. To keep thinking content out of responses without turning thinking off, set `display: "omitted"` on the thinking configuration. See [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-type-disabled).
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486492### Forced tool use not supported
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