## 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.
Eric Allam
2b6d2492fe