thinking-troubleshooting changedbuild-with-claude/thinking-troubleshooting
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14 Aug 2026
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Recorded edits19to this page, all time
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2020
2121| Model | Thinking types | Default | Rejected with 400 |
2222| --------------------- | ------------------------------------------- | --------- | ------------------------------------- |
23| Claude Fable 5.2 | Adaptive only | Always on | `"enabled"`, `"disabled"` |
24| Claude Mythos 5.2 | Adaptive only | Always on | `"enabled"`, `"disabled"` |
2325| Claude Fable 5.1 | Adaptive only | Always on | `"enabled"`, `"disabled"` |
2426| Claude Mythos 5.1 | Adaptive only | Always on | `"enabled"`, `"disabled"` |
2527| Claude Fable 5 | Adaptive only | Always on | `"enabled"`, `"disabled"` |
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4547Models marked `Always on` cannot turn thinking off. Models marked `On` default to thinking. Claude Opus 5, Claude Sonnet 5, and Claude Haiku 5.5 accept `thinking: {type: "disabled"}`. On Claude Sonnet 5.5, send `thinking: {type: "between_tools"}` to turn off up-front thinking.
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47Earlier Claude 4 models (Claude Opus 4.1, Claude Sonnet 4, and Claude Opus 4) support extended thinking only. See [Model deprecations](https://platform.claude.com/docs/en/about-claude/model-deprecations) for their availability. Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, and Claude Mythos 5 are not available under [zero data retention](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements) unless expressly authorized by Anthropic.
49Earlier Claude 4 models (Claude Opus 4.1, Claude Sonnet 4, and Claude Opus 4) support extended thinking only. See [Model deprecations](https://platform.claude.com/docs/en/about-claude/model-deprecations) for their availability. Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, and Claude Mythos 5 are not available under [zero data retention](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements) unless expressly authorized by Anthropic.
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4951## A 400 error says `"thinking.type.enabled"` is not supported
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6163## A 400 error after sending `thinking: {type: "disabled"}`
6264
63The request fails with a 400 error. On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Opus 5.5, and Claude Mythos 5, the message reads:
65The request fails with a 400 error. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Opus 5.5, and Claude Mythos 5, the message reads:
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6567```text wrap
6668"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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153155## A 400 error says a thinking block signature is invalid
154156
155A request that replays earlier thinking blocks to a Claude 5.1 or later model can fail with a 400 `invalid_request_error` whose message reads:
157A request that replays earlier thinking blocks to Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, or Claude Haiku 5.5 can fail with a 400 `invalid_request_error` whose message reads:
156158
157159```text wrap
158160messages.{i}.content.{j}: Invalid `signature` in `thinking` block. The block is bound to a different conversation. Remove the block, or set `thinking.block_binding.prefix_mismatch_behavior` to "drop_block".
No line in this hunk matches that.