Anthropic Launches Message Threads for Long Agent Sessions

What happened

Until now, an agent resent every earlier message and tool result on each turn, and once a conversation reached hundreds of thousands of tokens, the resend itself slowed down the model's first response. Anthropic's Message Threads, launched Nov 2, targets exactly this step: the first request sends the full conversation and opens a thread, and later turns send only new messages plus the previous response ID.

Anthropic says the feature is already on by default for most Claude Code users, and on requests of 500,000 to 1 million tokens, median time to first token falls by about 35%. In long conversations, the median request shrinks from about 800 KB to 1.8 KB, because history is no longer re-uploaded.

Threads are held in server memory, encrypted and never written to disk, and expire 5 minutes after the last request, after which the client resends the full conversation. Anthropic notes the model still reads the full conversation each turn, so token usage, pricing and rate limits are unchanged; the feature is not available to zero-data-retention or HIPAA organizations or those with inference hooks enabled.

Key facts

1. Launch and status: Anthropic, Nov 2, 2026, Message Threads in public beta on the Messages API
2. How it works: the first request sends the full conversation, system prompt and tools with the thread parameter; later requests send only new messages and the previous response ID
3. Latency effect: median time to first token falls about 35% on 500K–1M-token requests; the median request in long conversations drops from about 800 KB to 1.8 KB
4. Retention: threads live only in encrypted memory, never on disk, expire after 5 minutes of inactivity, and have no list, read or delete API
5. Platforms: the native Claude Platform, Claude Platform on AWS, Claude in Microsoft Foundry, and Claude Models on Google Cloud under Anthropic management
6. Billing and limits: token usage and pricing are unchanged; Message Batches, on-demand compaction and most context editing are not supported