Message Threads

Claude Code Sends 838 Bytes Where It Sent 193,000

Anthropic opened message threads on the Messages API as a public beta on 2 November 2026. Claude Code 2.1.319 already uses them by default on a claude.ai login against api.anthropic.com, with no changelog mention. Through a local logging proxy, a three-request Haiku 5.5 turn sent 185-198 KB per request stateless (about 575 KB total) and 189,090 then 839 and 838 bytes with threads (create, then continue with previous_message_id, no tools, a 111-byte billing-tag system block). Cached input tokens read were the same either way (72-74K), and at that context size median time to first byte was 1.12 s threaded vs 0.96 s stateless over eight requests each. The gate is first-party API only: off on Bedrock/Vertex/Foundry, behind any non-Anthropic ANTHROPIC_BASE_URL, and for ZDR and HIPAA orgs. Undocumented CLAUDE_CODE_TETHER_LIVE=0/1 switches it; CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=1 also does but dropped deferred tool loading and produced a ~1.11M-token request on a machine with many MCP servers. Claude Code tracks the 5-minute lifetime itself: after 315 s idle it sent a full create (194,828 bytes, 11 messages) with no 404.