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1919### September 14, 2026
2020
2121* The Messages API can now [compact a conversation on demand](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand) on the Claude API, in beta with the `compact-2026-09-04` beta header. Send the top-level `compaction` parameter, and the API returns a signed `compaction` block that summarizes the messages you sent. On later requests, send that block first, in place of those messages. You choose when to compact, the request can run in the background, and you can keep recent turns word for word after the summary. On models with preserved thinking, the thinking in those kept turns can stay valid.
22* With the `thinking-binding-controls-2026-08-01` beta header, the `input_transformations` response field gains a second entry type, `thinking_mismatch_allowed`. It names a thinking block that failed the prefix check on a request where the API doesn't enforce that check: on Claude Fable 5.1, for example, a request from an account created before August 31, 2026, with `prefix_mismatch_behavior` unset. The block still reaches the model unchanged. Log these entries to find history edits in production traffic before you opt into enforcement. See [Set the mismatch behavior and read `input_transformations`](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#preserved-thinking-controls).
2223
2324### September 10, 2026
2425