compaction-thinking-blocks changedbuild-with-claude/compaction-thinking-blocks
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2323 supportedPlatforms:
2424 Claude API: beta
2525 Claude Platform on AWS: beta
26 Amazon Bedrock: not available
2726 Google Cloud: beta
2827 Microsoft Foundry: beta
2928---
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5958To add an instruction or change the available tools without touching `system` or `tools`, append the change to `messages`, as described in [Make changes without editing the prefix](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#replace-prefix-edits).
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61Mid-conversation system messages inside the summarized turns are summarized too, so their text instructions stop applying after the swap. To keep one in force, state it again in a `role: "system"` message directly after the first new `user` turn that follows the kept turns. Tool changes inside those turns carry over on their own when the compaction request also carries `inline-tools-2026-09-15`: the returned block records their net effect in its `tool_changes` field, so send the block back unmodified. If the block has no `tool_changes` field, restate those tool changes the same way. A system message placed between the block and the kept turns breaks their thinking.
60Mid-conversation system messages inside the summarized turns are summarized too, so their text instructions stop applying after the swap. A [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) level they set isn't carried over either: until a later message sets a level, turns run at the request's top-level `output_config.effort`, or the model's default if you don't set one. To keep an instruction or effort level in force, state it again in a `role: "system"` message directly after the first new `user` turn that follows the kept turns. Tool changes inside those turns carry over on their own when the compaction request also carries `inline-tools-2026-09-15`: the returned block records their net effect in its `tool_changes` field, so send the block back unmodified. If the block has no `tool_changes` field, restate those tool changes the same way. A system message placed between the block and the kept turns breaks their thinking.
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6362## Check that the kept thinking held
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703702Dropped thinking blocks: 0
704703```
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706In 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.
705In 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.
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