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Sweep 09 Oct 2026 · 17:27Z Build v2.1.296 517 read Stable v2.1.287 Latest v2.1.296 Next v2.1.296 Feeds RSS JSON llms.txt llms-full.txt Unofficial
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compaction-threshold changedbuild-with-claude/compaction-threshold

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First seen 21 Sep 2026 this site's first read of the page
Recorded edits8to this page, all time

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2222 - claude-sonnet-5-5
2323 - claude-sonnet-5
2424 - claude-sonnet-4-6
25 - claude-haiku-5-5
2526 supportedPlatforms:
2627 Claude API: beta
2728 Claude Platform on AWS: beta
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17131714* Keep the original messages in your list and let the API handle removing the compacted content
17141715* Manually drop the compacted messages and only include the compaction block onwards
17151716 
1716On Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, and Claude Sonnet 5.5, thinking blocks from before a `compaction` block aren't carried forward, so the summary is all the model has of that earlier work. If you write your own `instructions`, tell the model what the summary must retain; see [Tell the model what to preserve in compaction summaries](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#tell-the-model-what-to-preserve-in-compaction-summaries).
1717On Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, thinking blocks from before a `compaction` block aren't carried forward, so the summary is all the model has of that earlier work. If you write your own `instructions`, tell the model what the summary must retain; see [Tell the model what to preserve in compaction summaries](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#tell-the-model-what-to-preserve-in-compaction-summaries).
17171718 
17181719### Streaming
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28972898 ```
28982899</CodeGroup>
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2900On Claude Fable 5.1, Claude Opus 5.5, and Claude Sonnet 5.5, remove the `thinking` and `redacted_thinking` blocks from any assistant turn you re-insert after the compaction block, or send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` with the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). Those blocks were produced when the full history was present, so they no longer pass the [conversation check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Where the check is enforced, the continuation request is rejected with a 400 error. The preserved text and tool blocks can stay as they are. Letting the API summarize everything, without re-inserting earlier turns, avoids this. On Claude Sonnet 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With `between_tools`, remove the blocks instead.
2901On Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, remove the `thinking` and `redacted_thinking` blocks from any assistant turn you re-insert after the compaction block, or send `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` with the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). Those blocks were produced when the full history was present, so they no longer pass the [conversation check](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Where the check is enforced, the continuation request is rejected with a 400 error. The preserved text and tool blocks can stay as they are. Letting the API summarize everything, without re-inserting earlier turns, avoids this. On Claude Sonnet 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With `between_tools`, remove the blocks instead. On Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`, so with `thinking: {"type": "disabled"}`, remove the blocks instead.
29012902 
29022903Here's an example that uses `pause_after_compaction` to preserve the prior exchange and the current user message (three messages total) verbatim instead of summarizing them:
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