compaction-thinking-blocks changedbuild-with-claude/compaction-thinking-blocks
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## Availability ### API support #### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) ### Required beta headers
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1313 - claude-mythos-preview
1414 - claude-opus-5-5
1515 - claude-opus-5
16 - claude-sonnet-5-5
17 - claude-sonnet-5
18 - claude-haiku-5-5
1619 - claude-opus-4-8
1720 - claude-opus-4-7
1821 - claude-opus-4-6
19 - claude-sonnet-5-5
20 - claude-sonnet-5
2122 - claude-sonnet-4-6
22 - claude-haiku-5-5
2323 supportedPlatforms:
2424 Claude API: beta
2525 Claude Platform on AWS: beta
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691691```
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693693In production, `"drop_block"` keeps requests succeeding when a condition doesn’t hold, and reports each dropped block in `input_transformations` with `reason: "prefix_binding_mismatch"`. An entry whose `path` falls in a kept turn means that turn’s thinking didn’t hold. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes what is dropped and says how to alert on it. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. A request that sets it with `thinking: {"type": "between_tools"}` on Claude Sonnet 5.5, or with `thinking: {"type": "disabled"}` on Claude Haiku 5.5, returns a 400 error. On those requests, make sure every condition holds, or remove the `thinking` and `redacted_thinking` blocks from the kept turns.
694
695## Availability
696
697Compaction and preserved thinking are in beta on select models.
698
699### API support
700
701#### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages)
702
703* Availability: Available
704
705| Status | Cloud platforms | Models |
706| ------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
707| Beta | Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview), [Claude Opus 4.7](https://platform.claude.com/docs/en/models/opus-4-7/overview), [Claude Opus 4.6](https://platform.claude.com/docs/en/models/opus-4-6/overview), [Claude Sonnet 4.6](https://platform.claude.com/docs/en/models/sonnet-4-6/overview) |
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709### Required beta headers
710
711[Beta headers overview](https://platform.claude.com/docs/en/api/beta-headers)
712
713Beta headers allow you to access experimental features and new model capabilities before they become part of the standard API.
714
715* `compact-2026-09-04`
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