Anthropic adds on-demand conversation compaction to the Messages API

Anthropic's Messages API can now compact a conversation on demand, in beta under the compact-2026-09-04 header. Sending a top-level compaction parameter returns a signed compaction block summarizing prior messages, which can be sent in place of those messages on later requests; compaction can run in the background, recent turns can be kept verbatim, and preserved thinking in kept turns can remain valid on models that support it.

Significance 2: An incremental first-party API feature for managing long-running agent context; useful but not landscape- or verdict-moving.

Sources

• primary Claude Platform release notes retrieved 2026-09-15

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