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Sweep 06 Nov 2026 · 01:32Z Build v2.1.319 537 read Stable v2.1.311 Latest v2.1.319 Next v2.1.319 Feeds RSS JSON llms.txt llms-full.txt Unofficial
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sent_count = len(messages sent in the compaction request) changedbuild-with-claude/compaction

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88- Status: Beta
99- [Beta header](https://platform.claude.com/docs/en/api/beta-headers): `compact-2026-01-12`
1010- [ZDR](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention): eligible (excludes [Covered Models](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements))
11- Supported models: `claude-fable-5`, `claude-mythos-5`, `claude-mythos-preview`, `claude-opus-5`, `claude-opus-4-8`, `claude-opus-4-7`, `claude-opus-4-6`, `claude-sonnet-5`, `claude-sonnet-4-6`
11- Supported models: `claude-fable-5-1`, `claude-mythos-5-1`, `claude-fable-5`, `claude-mythos-5`, `claude-mythos-preview`, `claude-opus-5`, `claude-opus-4-8`, `claude-opus-4-7`, `claude-opus-4-6`, `claude-sonnet-5`, `claude-sonnet-4-6`
1212- Platforms: Claude API (beta), Claude Platform on AWS (beta), Amazon Bedrock (beta), Google Cloud (beta), Microsoft Foundry (beta)
1313 
1414<Tip>
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707707 ```
708708</CodeGroup>
709709 
710On Claude Fable 5.1 and Claude Mythos 5.1, a request with custom `instructions` summarizes from the visible conversation only: earlier thinking blocks are not part of the summarizer's input.
711 
710712### Pausing after compaction
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712714Use `pause_after_compaction` to pause the API after generating the compaction summary. This allows you to add additional content blocks (such as preserving recent messages or specific instruction-oriented messages) before the API continues with the response.
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16901692* Keep the original messages in your list and let the API handle removing the compacted content
16911693* Manually drop the compacted messages and only include the compaction block onwards
16921694 
1695On Claude Fable 5.1 and Claude Mythos 5.1, 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).
1696 
16931697### Streaming
16941698 
16951699The compaction block streams differently from text blocks. You receive a `content_block_start` event, followed by a single `content_block_delta` with the complete summary content (no intermediate streaming), and then a `content_block_stop` event.
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28302834 puts chat(client, messages, "Now add rate limiting and error handling")
28312835 ```
28322836</CodeGroup>
2837 
2838On Claude Fable 5.1, 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.
28332839 
28342840Here'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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