Anthropic says message threads cut median time to first token by about 35% for requests of 500k to 1M tokens

Message threads is in public beta on the Claude Platform. Today a coding agent resends every earlier message and tool result with each request. With a thread, the first request carries the full conversation and starts the thread. Every later request carries only the new messages and the ID of the previous response. Anthropic says each thread is held in memory, encrypted and not written to disk, and expires 5 minutes after its last request, after which the client resends the full conversation. The post says the feature is on by default for most Claude Code users and that requests of 500k to 1M tokens see "median TTFT drop by about 35%." It also lists availability on Claude Platform on AWS, Claude in Microsoft Foundry, and Claude Models on Google Cloud (Anthropic managed).

The 35% figure is Anthropic's own, and we did not test it. The post states that Claude still reads the full conversation on every turn, so token usage stays the same, and that standard token rates and rate limits apply.

Why it matters: The gain is latency, not cost. For a harness that runs many tool-call turns against a long context, the saving belongs to active loops. A pause longer than the 5-minute expiry, such as a person reviewing a diff, means the next request goes back to a full resend.