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Sweep 10 Nov 2026 · 02:18Z Build v2.1.322 539 read Stable v2.1.311 Latest v2.1.322 Next v2.1.322 Feeds RSS JSON llms.txt llms-full.txt Unofficial
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thinking changedbuild-with-claude/thinking

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11071107## Preserved thinking
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1109[Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) determines whether the model can use a thinking block that you send back from an earlier turn. Starting with Claude Fable 5.1, the API checks the `signature` of every `thinking` or `redacted_thinking` block in a request for two things:
1109[Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) determines whether the model can use a thinking block that you send back from an earlier turn. The API checks the `signature` of every `thinking` or `redacted_thinking` block in a request for two things:
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11111111* **The model that produced it.** 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. The API drops a block the current model can’t read, without an error and without billing it. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
11121112* **Everything sent before it.** A block stays valid only while the top-level `system` prompt, the `tools`, and the messages before it are unchanged. If any of them changes, that block and every later thinking block fail the prefix check. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check).
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