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

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5252## Change the system prompt or tools
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54A later request can use a different `system`, different `tools`, or a different model than the compaction request, and the API still accepts the block. Such a change can invalidate the thinking in the kept turns, but it has no other effect.
54A later request can use a different `system`, different `tools`, or a different model from the compaction request, and the API still accepts the block. Such a change can invalidate the thinking in the kept turns, but it has no other effect.
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5656To change `system` or `tools` without invalidating any kept thinking, compact the whole conversation first, so no turns are kept. Then change them on the next request.
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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). Setting the field also opts the request into the check on accounts where the check isn't on by default.
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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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