errors changedapi/errors
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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), Claude Opus 5.5, 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:
478On Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, 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
481481"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
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520520### Forced tool use not supported
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522Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://anthropic.com/glasswing) don't support forced tool use. Sending `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` to any of these models, including on the [token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting), returns a 400 `invalid_request_error`:
522Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) don't support forced tool use. Sending `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` to any of these models, including on the [token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting), returns a 400 `invalid_request_error`:
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524524```text wrap
525525tool_choice: type "tool" and "any" are not supported for this model.
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