compaction-background changedbuild-with-claude/compaction-background
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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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545545## Keep thinking valid while the summary is built
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547547Turns that arrive while the summary is being written are kept turns. If you send thinking blocks back on a model with [preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking), the thinking in those turns stays valid only while the [conditions for kept thinking](https://platform.claude.com/docs/en/build-with-claude/compaction-thinking-blocks#conditions-for-kept-thinking-to-stay-valid) hold.
548
549## Availability
550
551Background compaction is in beta on select models.
552
553### API support
554
555#### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages)
556
557* Availability: Available
558
559| Status | Cloud platforms | Models |
560| ------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
561| 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) |
562
563### Required beta headers
564
565[Beta headers overview](https://platform.claude.com/docs/en/api/beta-headers)
566
567Beta headers allow you to access experimental features and new model capabilities before they become part of the standard API.
568
569* `compact-2026-09-04`
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