Eric Allam
2b6d2492fe
## Summary Three related fixes for `chat.headStart` and continuation boots, found while investigating customer reports. **1. `chat.headStart` now works with `hydrateMessages`.** The turn-0 handover splice only ran on the default accumulation path, so agents registering `hydrateMessages` silently lost the warm route's step-1 response: pure-text turns fired `onTurnComplete` with no assistant message (and an empty durable write), tool-call turns re-ran step 1 from scratch under a fresh `messageId`, and the head-start user message never reached the hydrate hook at all. The first-turn history now reaches `hydrateMessages` as `incomingMessages`, and the splice runs after both accumulation branches, deduplicated by the handover `messageId`. **2. Reasoning parts survive the handover.** The synthesized partial only mapped text and tool-call parts, so an extended-thinking model's step-1 reasoning streamed to the browser but never reached durable history. Reasoning parts now map through with provider metadata, so Anthropic thinking signatures survive a UIMessage round trip on hydrate replays. **3. Continuation boots no longer stall for ~10 seconds.** The `.in` resume cursor was found by draining an SSE subscription that only closes after its 5 second inactivity window, and the scan ran twice per boot. It is now a non-blocking records read of the latest turn-complete header, runs at most once per boot, the boot reads run concurrently, and chat snapshots carry the cursor so subsequent boots skip the scan entirely. Measured locally on a cancel-then-continue repro: pre-turn continuation latency dropped from ~11s to ~0.5s. Every fix was verified red-green: new unit tests reproduced each failure before the fix, and end-to-end smoke tests against a live local stack covered both handover legs, reasoning persistence with extended thinking (including a follow-up turn that round-trips the persisted signed reasoning back to the provider), and the boot timing comparison. ## Rollout SDK-only; no server change required. A new SDK against a server that does not serialize record headers degrades to the existing no-cursor fallback. Old SDKs ignore the new snapshot field, and new SDKs fall back to the records scan on snapshots written before it existed.
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