Messages can now be stamped with Projects thread state

What

• Claude Code can now attach a "thread state" (how long since the last human message, and how many replies have happened since then) to queued messages in Projects threads, via a stampThreadState flag.
  • Cron-scheduled prompts that fire on a non-peer session now carry stampThreadState: true, and a thread_state attachment can be pushed onto the message list during prompt processing.
  • Incoming user messages in genuine (non-overview) Projects threads get the same stampThreadState flag added when building the queued prompt.
  • Replies composed to artifact comment threads (for example via MCP) can also be stamped with thread state, gated behind a dedicated flag.
  • "thread_state" was added as a recognized category/content-block type alongside existing ones like "prefix_delta", "deferred_tools_delta", and "diagnostics", with the stampThreadState flag threaded through the relevant message-construction and result-merging functions.

Why

Tagging messages with thread state lets Claude Code, and anything consuming its output such as automated replies, know how stale or active a conversation thread is, which is useful context for deciding how to respond in scheduled or automated flows.