preserved-thinking changedbuild-with-claude/preserved-thinking
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7474On the Claude API and Claude Models on Google Cloud (Google managed), with the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers), the response lists each dropped block in `input_transformations` as a `thinking_dropped` entry with `reason: "organization_binding_mismatch"`. Without the header, the drop is silent.
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76The API can also leave out a thinking block that another account produced, if that account is no longer in good standing. This applies to requests to Claude Fable and Claude Mythos models, Claude Opus 5, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5. The request still succeeds, and the model answers without that block's reasoning.
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7678## Keeping the prefix unchanged
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7880On Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, a thinking block stays valid only while everything you sent before it is unchanged on later requests. The checked prefix has three parts:
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