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Sweep 22 Sep 2026 · 17:19Z Build v2.1.280 501 read Stable v2.1.267 Latest v2.1.280 Next v2.1.280 Feeds RSS JSON llms.txt Unofficial
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sent_count = len(messages sent in the compaction request) changed

build-with-claude/compaction

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First seen 14 Aug 2026 this site's first read of the page
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34183418Compacting this way gives you three things. First, you decide when to compact. Second, the summarization request can run in the background while the conversation continues on its full history, and you swap the block in when it arrives. This is often called async or background compaction. Third, you can keep recent turns word for word after the summary, which is often called keep-tail compaction. Models with [preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) check earlier thinking blocks against the conversation. On those models, the thinking in turns that follow the summary, from either pattern, can stay valid after the swap, under the conditions in [Continue from the summary](https://platform.claude.com/docs/en/build-with-claude/compaction#continue-from-the-summary). That lets a long-running agent keep its train of thought. Use threshold compaction when you want the API to manage context inside ordinary requests. Use the `compaction` parameter when your application needs to control when compaction happens, can't pause while a summary is written, or must keep recent turns and their thinking after the summary.
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3420Send the `compact-2026-09-04` beta header on the request that asks for the summary and on every later request that carries the signed block. On-demand compaction is available on the Claude API only, on 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, and Claude Sonnet 4.6. You can also call the [Models API](https://platform.claude.com/docs/en/api/beta/models/list) with the beta header and read each model's `capabilities.compaction`. You can't combine `compaction` with `context_management` on one request.
3420Send the `compact-2026-09-04` beta header on the request that asks for the summary and on every later request that carries the signed block. On-demand compaction is available on the Claude API but not on Amazon Bedrock or Google Cloud. It works on 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, and Claude Sonnet 4.6. You can also call the [Models API](https://platform.claude.com/docs/en/api/beta/models/list) with the beta header and read each model's `capabilities.compaction`. You can't combine `compaction` with `context_management` on one request.
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34223422### Request a summary
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