Anthropic Messages API Adds Server-Side Threads

This is a meaningful infrastructure upgrade for agent builders, turning stateless API calls into a more practical stateful primitive. The tradeoff is that thread lifecycle, privacy, retries, and cost semantics now become application concerns.

• –Create a thread once, then continue it with only incremental messages
• –Faster responses and lower client bandwidth for long-running conversations
• –SDK support makes threaded agents easier to build and maintain
• –Applications need fallback logic for expired or missing server-side threads
• –Beta users should instrument behavior and avoid assuming automatic token-cost savings