Anthropic released message threads in public beta on the Claude Platform on November 2. Instead of resending the full conversation history with every Messages API request, an agent sends the full conversation once, then only the new messages plus the previous response ID, while Anthropic holds the thread in memory between turns.

The gain is latency, not cost. With message threads on by default for most Claude Code users, Anthropic says requests carrying 500,000 to 1 million tokens see median time to first token fall by about 35%, and one reported long-conversation payload shrank from about 800 KB to 1.8 KB. Claude still reads the full conversation on every turn, so token usage, standard rates and rate limits are unchanged.

The limits are built into the design. Threads are held encrypted in memory, never written to disk, and expire five minutes after the last request, after which the client resends the full conversation to start a new one. The beta is available on the native Claude Platform, Claude Platform on AWS, Claude in Microsoft Foundry and Claude Models on Google Cloud, aimed at long-running coding agents, computer-use agents and document- and image-heavy workflows.