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5757## A 400 error says `"thinking.type.disabled"` is not supported
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59The request fails with a 400 error whose message reads:
59The request fails with a 400 error. On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, and Claude Mythos 5, the message reads:
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6161```text wrap
62"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
63```
64
65On Claude Mythos Preview, the only one of these models that accepts extended thinking, the message reads:
66
67```text wrap
6268"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.
6369```
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65This happens on models where thinking is always on: Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, and Claude Mythos Preview reject `"disabled"`. All of these except Claude Mythos Preview also reject the error text's suggested `"thinking.type.enabled"`.
71This happens because thinking is always on for all of these models (see the [per-model configuration table](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#rejected-configurations)).
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6773Omit the `thinking` parameter; these models think without any configuration. If your goal was to keep thinking text out of responses, use `display: "omitted"` instead of disabling thinking; see [Controlling thinking display](https://platform.claude.com/docs/en/build-with-claude/thinking#controlling-thinking-display).
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