Make long-running agents faster with message threads

Anthropic announced that message threads is in public beta on the Claude Platform, letting agents send only new messages plus the previous response ID instead of resending full conversation history each turn. With message threads on by default for most Claude Code users, requests of 500k to 1M tokens saw median time to first token drop by about 35%, though token usage stays the same since Claude still reads the full conversation each turn. Threads are held encrypted in memory, expire 5 minutes after the last request, and are available on the native Claude Platform, Claude Platform on AWS, Claude in Microsoft Foundry, and Claude Models on Google Cloud (Anthropic managed).