Anthropic adds server-side message threads to cut long-context Claude API latency

Anthropic launched server-side message threads in public beta on the Claude Messages API (beta header message-threads-2026-08-12) on 2 November 2026. Long-running coding and tool-use agents typically re-upload hundreds of thousands of conversation, system and tool-schema tokens on every turn, creating network upload, JSON parsing and validation overhead before inference even starts.

Creating a server-side thread stores the conversation history, system prompt and tool definitions in encrypted server RAM without writing them to disk. Follow-up calls pass previous_message_id and transmit only the new user or tool-result messages, supporting up to four cache_control breakpoints. Thread state expires after a rolling five-minute idle window and is permanently deleted within one hour; if a thread expires, the API returns a 404 thread_not_found error and Anthropic’s updated SDKs automatically replay the full local history.

Key numbers

• Roughly 35% lower median time to first token on 500k to 1M token requests.
• Rolling 5-minute idle expiry in encrypted RAM, with hard deletion within 1 hour and zero disk writes.
• Up to 4 cache_control breakpoints per threaded request.
• Standard Messages API per-token rates apply, with Zero Data Retention and HIPAA workloads excluded.