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## Availability ### API support #### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) #### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
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22title: Preserved thinking
33url: https://platform.claude.com/docs/en/build-with-claude/preserved-thinking
44description: Preserved thinking lets a model use a thinking block from an earlier turn only if that model or one of a fixed set of other models produced it and nothing before the block has changed.
5featureMetadata:
6 status: ga
7 supportedModels:
8 - claude-fable-5-1
9 - claude-mythos-5-1
10 - claude-opus-5-5
11 - claude-sonnet-5-5
12 - claude-haiku-5-5
13 supportedPlatforms:
14 Claude API: ga
15 Claude Platform on AWS: ga
16 Claude Models on Google Cloud (Anthropic managed): ga
17 Claude Models on Google Cloud (Google managed): ga
18 Microsoft Foundry: ga
519---
620
21**API support**: Usage differs by product.
22
23* **Messages API**: Always on. Keep requests append-only: send earlier turns back exactly as returned, and do not change `system`, `tools` or earlier messages.
24* **Managed Agents**: Always on. Managed Agents builds the requests.
25
726Preserved thinking is a property of newer Claude models that guards against distillation. It determines whether the model can use a thinking block that you send back from an earlier turn. Starting with Claude Fable 5.1, when a `thinking` or `redacted_thinking` block comes back in a request, the API checks the block’s `signature` for two things:
827
928* **The model can read the block.** Each model reads its own thinking blocks and those of a fixed set of other models. Claude Fable 5.1 reads blocks from Claude Opus 5 and, on the Claude API, from Claude Opus 5.5; neither Claude Opus 5 nor Claude Opus 5.5 reads blocks from Claude Fable 5.1. If the current model can’t read a block, the API drops it from that request without an error. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
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1601
1602## Availability
1603
1604Preserved thinking is available on select models.
1605
1606### API support
1607
1608#### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages)
1609
1610* Availability: Always on
1611* Usage: Keep requests append-only: send earlier turns back exactly as returned, and do not change `system`, `tools` or earlier messages.
1612
1613| Status | Cloud platforms | Models |
1614| --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1615| Available | 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 Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview) |
1616
1617#### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
1618
1619* Availability: Always on
1620* Usage: Managed Agents builds the requests.
1621
1622| Status | Cloud platforms | Models |
1623| --------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1624| Available | Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed) | [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 Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview) |
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