Anthropic opens Message Threads beta for long-running agents

What is changing?

Anthropic has opened Message Threads in public beta for the Messages API on the Claude Platform. A thread receives the model, system prompt, and initial conversation on the first request; on later turns, it preserves the conversation state on the server and accepts only the new message instead of requiring the client to resend the full history.

Beta helpers in the TypeScript and Python SDKs support adding turns with `thread.send()` and running tool loops on the same thread with `thread.toolRunner()` or `thread.tool_runner()`. The official API reference shows that the feature uses a beta header and exposes server-side event streams for long-running agent sessions.

The design may reduce repeated context transfer in long conversations, coding agents, and document- or image-heavy sessions. Teams should still evaluate beta changes, retention policies, recovery behavior, and the lifecycle of server-held conversation data before production use.