preserved-thinking changedbuild-with-claude/preserved-thinking

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Recorded here 9 Nov 2026 18:07 UTC

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First seen 1 Sep 2026 this site's first read of the page

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8888

8989## Thinking blocks stay with the account that produced them

9090

91Thinking blocks that Claude Sonnet 5.5 or Claude Haiku 5.5 produces work only in the account that produced them or in an account linked to it. When another account sends one of these blocks, the API drops the block before the model sees it, and the request succeeds. The model answers without the reasoning in the dropped blocks. Blocks from earlier models aren’t affected.

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9393On 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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176176* A top-level `input_transformations` array on every response

177177* A `block_binding` object on the `thinking` configuration, whose one field is `prefix_mismatch_behavior`

178178

179`block_binding` is accepted alongside `thinking.type: "adaptive"` and `thinking.type: "enabled"`. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. Sending 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. Sending it without the beta header returns a 400 error whose message ends `block_binding: Extra inputs are not permitted`. Models that don’t run the prefix check accept the object and report only blocks dropped because the model can’t read them, so one request body works across models. The API reference calls the prefix check the conversation check.

179The API’s request format allows `block_binding` alongside `thinking.type: "adaptive"` and `thinking.type: "enabled"`. Every model that runs the prefix check accepts it with `"adaptive"`, and none of them accepts `"enabled"`. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. Sending 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. Sending it without the beta header returns a 400 error whose message ends `block_binding: Extra inputs are not permitted`. Models that don’t run the prefix check accept the object and report only blocks dropped because the model can’t read them, so one request body works across models. The API reference calls the prefix check the conversation check.

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181181The following request opts into dropping rather than rejecting. On a first turn there’s nothing to replay, so `input_transformations` comes back empty:

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445445

446446Each row compares a request with the one before it. “Later thinking blocks” means the blocks already in the history that come after the changed content:

447447

448| Change between requests | Later thinking blocks |

449| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |

450| Append messages at the end | Valid |

451| Add a tool with `defer_loading: true` that nothing has referenced yet | Valid |

452| Remove `thinking` blocks from the start of the history, from the end, or all of them | Valid (the model loses that reasoning) |

453| Change any request parameter outside `model`, `system`, `tools`, and `messages` (`effort`, `max_tokens`, `output_config`, `tool_choice`, `metadata`, `thinking.display`, and so on) | Valid |

454| Change `model` | Valid for blocks that the new model can read; the API drops the others without an error (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)) |

455| Add, move, or remove `cache_control` markers | Valid |

456| A rotating signed URL that returns the same bytes | Valid |

457| Server-side compaction or context editing removes or replaces content | Valid (the check compares what you sent, not the server’s edited copy) |

458| A cleared [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#per-turn-reminders) left in place | Valid |

459| Edit, reorder, or delete any earlier `user`, `assistant`, or `system` message | Invalid, except when the signed block from [on-demand compaction](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand) replaces the messages it summarizes, under the [conditions for kept thinking](https://platform.claude.com/docs/en/build-with-claude/compaction-thinking-blocks#conditions-for-kept-thinking-to-stay-valid) |

460| Re-render the context you put in the first user message with a changed value | Invalid for every thinking block |

461| Clear or shorten an earlier `tool_result`, re-encode an earlier image, or change an earlier `tool_use` input | Invalid for every later thinking block |

462| Add a text block to an earlier user turn, or remove one you added last time | Invalid |

463| Change the top-level `system` string or blocks | Invalid |

464| Add, remove, rename, or edit a tool in `tools` | Invalid |

465| Remove a `thinking` block from the middle of the history and keep later ones | Invalid for every later thinking block |

466| Put back a `thinking` block you removed on an earlier request | Invalid for thinking blocks produced while it was gone |

467| An image or document URL that returns different bytes on the next request | Invalid |

468| The same turn-scoped message deleted or reworded on a later request | Invalid |

448| Change between requests | Later thinking blocks |

449| ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |