Anthropic Opens Public Beta of Message Threads in the Messages API

Anthropic has opened the public beta of message threads on the Messages API. The API caches conversations on the server side, so developers only need to send the new message and the previous message ID, without resending the full conversation. The feature has been integrated into Claude Code; on requests of 500K to 1M tokens, median time to first token dropped by about 35%, and in long conversations the median request size fell from about 800 KB to 1.8 KB. To use it, add the thread parameter and beta header to requests. The feature is available on Claude Platform, Claude Platform on AWS, Claude in Microsoft Foundry, and Claude on Google Cloud, and is billed at standard token prices.