Anthropic Launches Message Threads Beta for Long-Running Agents

Anthropic introduced message threads in public beta on November 2, 2026, allowing Messages API applications on the Claude Platform to send only new messages after an initial full-conversation request. The feature is intended for active sessions where latency matters, including long-running coding agents, computer-use agents whose screenshots remain with the thread, and document- and image-heavy workflows. Anthropic reports that, with message threads on by default for most Claude Code users, requests containing 500,000 to 1 million tokens see median time to first token fall by about 35%.

Without a thread, each turn uploads the entire conversation again, and the API validates and tokenizes all of it again. With a thread, most of that work applies only to the new turn before the API rebuilds the conversation for the model. Claude still reads the full conversation on every turn, so token usage remains unchanged.

Standard token rates and existing rate limits apply. The official documentation divides the lifecycle into create and continue requests. A create request sends the full conversation, system prompt and tools with the thread parameter; the API stores…