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compaction-thinking-blocks changedbuild-with-claude/compaction-thinking-blocks

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3939* **The kept turns directly follow the summarized messages, and you send them unchanged.** Send each kept message exactly as it is in your history. Don’t skip or add a message between the last summarized message and the first kept one. The first kept message must also have a different role from the last summarized message, and it can’t be a mid-conversation `role: "system"` message. Otherwise, the API merges it into the last summarized message. One way to get the first kept message right is to compact exactly the `messages` of a request you already sent. The kept turns then start with Claude’s reply to it.
4040* **`system` and the `tools` not marked `defer_loading: true` don’t change.** They are the same on the compaction request as on the requests that produced the kept thinking, and they stay the same on the requests that follow. [Change the system prompt or tools](https://platform.claude.com/docs/en/build-with-claude/compaction-thinking-blocks#change-the-system-prompt-or-tools) covers how to change them safely.
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42If a condition doesn’t hold, nothing fails when you compact, and the API accepts the block on later requests either way. The failure comes on the first later request that sends the kept thinking where the API enforces the check: a 400 error by default, or dropped thinking blocks if the request sets `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"`. In the Message Batches API, an item that leaves the field unset doesn’t fail. Where the check applies by default, the API drops the blocks instead. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes both outcomes, and [When the API enforces the check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) says which requests are checked.
42If a condition doesn’t hold, nothing fails when you compact, and the API accepts the compaction block on later requests either way. What fails is the kept thinking, on the first later request that sends it. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes what happens then, and [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) covers requests that don’t set the field, including Message Batches items.
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4444## Compact again without breaking older thinking
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6464The compaction response doesn’t say whether the kept thinking holds. The first request after the swap does. To check in your tests:
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661. Have a short conversation with thinking on. Use a model on which the API runs the check (see [When the API enforces the check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement)), and use it for every step, because a model that can’t read a thinking block drops it with no error.
661. Have a short conversation with thinking on. Use a model that runs the prefix check (see [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check)), and use it for every step, because a model that can’t read a thinking block drops it with no error.
67672. Compact the older turns, and keep at least one turn that holds a thinking block.
68683. Send the next request, with the block first, then the kept turn, then a new `user` message, and with `thinking.block_binding.prefix_mismatch_behavior` set to `"error"`.
69694. Read the result. A 200 response whose `input_transformations` array is empty means no thinking block failed the check or was dropped. A 400 error that says the block is bound to a different conversation means one did. The message starts with the path of the first block that failed, and [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) shows it in full.
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71The `prefix_mismatch_behavior` field needs the `thinking-binding-controls-2026-08-01` beta header in addition to the [`compact-2026-09-04` beta header](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand#request-a-summary). Setting the field also opts the request into the check on accounts where the check isn’t on by default.
71The `prefix_mismatch_behavior` field needs the `thinking-binding-controls-2026-08-01` beta header in addition to the [`compact-2026-09-04` beta header](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand#request-a-summary). The value a request sets applies on any account.
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7373The following program runs the four steps. It prints how many thinking blocks the kept turn holds and how many entries `input_transformations` has; no entries means the kept thinking held:
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690690Dropped thinking blocks: 0
691691```
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693In production, `"drop_block"` keeps requests succeeding when a condition doesn’t hold, and reports each dropped block in `input_transformations` with `reason: "prefix_binding_mismatch"`. An entry whose `path` falls in a kept turn means that turn’s thinking didn’t hold. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes what is dropped and says how to alert on it. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. A request that sets 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. On those requests, make sure every condition holds, or remove the `thinking` and `redacted_thinking` blocks from the kept turns.
693In production, `"drop_block"` keeps requests succeeding when a condition doesn’t hold. The response lists each dropped block in `input_transformations` with `reason: "prefix_binding_mismatch"`. An entry whose `path` falls in a kept turn means a condition didn’t hold for that turn’s thinking. [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) describes what is dropped and says how to alert on it. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With any other thinking setting on those models, you can’t send `block_binding`, so make sure every condition holds or strip the `thinking` and `redacted_thinking` blocks from the kept turns.
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695695## Availability
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