Anthropic adds on-demand conversation compaction to the Claude Messages API (beta)

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

Significance 2: An incremental but genuinely useful API capability for long-running agent sessions; a changelog feature, not a new model or benchmark result.

Sources

• primary Claude Platform release notes retrieved 2026-09-16

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