fix(chat): serialize reconnect-driven resumes to stop activeResponse race (#1837)#1838
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…race (#1837) With `resume: true` (the default), `useAgentChat` re-probes the stream from its WebSocket `onAgentOpen` handler on every reconnect. The AI SDK's `Chat.makeRequest` has no concurrency guard: every resume shares the single mutable `this.activeResponse`, and its `finally` finalizer reads `this.activeResponse.state.message` with a bare (unguarded) read before clearing it (the adjacent `finishReason` field is optional-chained, the `message` field is not). Under a reconnect storm (flaky mobile link, or a Durable Object bounce on redeploy), a later resume could overwrite + clear `activeResponse` before an earlier resume's finalizer ran, so the earlier finalizer read `undefined` and threw a handled `TypeError: Cannot read properties of undefined (reading 'state')`. The previous guard did not close the window: - `customTransport.isAwaitingResume()` only covers the handshake — it flips false the instant `STREAM_RESUMING` resolves, but the AI SDK only sets status to "submitted" in a later microtask (it sits behind `await transport.reconnectToStream(...)`), and - `statusRef.current` is lagging React state that has not re-rendered yet. So a second `open` landing in that post-handshake / pre-status-propagation window sailed past both guards and launched an overlapping resume. Fix: serialize re-probe resumes with `resumeInFlightRef` — never issue a new `resumeStream()` while one is still outstanding. The flag is held for the whole resume lifetime and force-cleared in the socket-effect cleanup so an orphaned resume (agent swapped on a `_pk` change) can't leave the gate stuck closed. A `resumeGenerationRef` token prevents a stale, orphaned resume's late `.finally` from reopening the gate underneath a newer resume. The definitive activeResponse-local fix belongs upstream in Vercel `ai`; this stops the SDK from triggering the overlap. Adds a deterministic regression test (`resume-overlap-race.test.tsx`) that drives the real hook through a reconnect storm via a fake EventTarget agent and asserts the overlapping reconnect issues no second resume and the finalizer never reads a cleared activeResponse. Co-authored-by: Cursor <cursoragent@cursor.com>
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…#1837) A review suggested clearing `resumeInFlightRef` in `onAgentClose` for clarity. That is unsafe: the flag is owned by the in-flight resume and cleared only by its own `.finally` (or invalidated via the cleanup generation bump). Resetting it on close would set it false while the resume may still be mid-flight, re-coupling correctness to close/open task ordering and reopening the overlap window. It is also unnecessary — handshake-phase drops are gated by `isAwaitingResume()` and streaming-phase drops settle `makeRequest` (which clears the flag). Document the invariant inline so it isn't "hardened" away. Co-authored-by: Cursor <cursoragent@cursor.com>
This was referenced Jun 29, 2026
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Summary
Fixes #1837 —
useAgentChat({ resume: true })throwing a handledTypeError: Cannot read properties of undefined (reading 'state')from the AI SDK'sChat.makeRequestfinalizer during a reconnect storm.Root cause. The AI SDK's
Chat.makeRequesthas no concurrency guard — every resume shares the single mutablethis.activeResponse, and itsfinallyfinalizer readsthis.activeResponse.state.messagewith a bare (unguarded) read before clearing it (the adjacentfinishReasonfield is optional-chained;messageis not). The hook re-probes the stream from its WebSocketonAgentOpenhandler on every reconnect, and the existing guard didn't close the overlap window:customTransport.isAwaitingResume()only covers the handshake — it flips false the instantSTREAM_RESUMINGresolves, but the AI SDK only sets status to"submitted"in a later microtask (behindawait transport.reconnectToStream(...)), andstatusRef.currentis lagging React state that hasn't re-rendered yet.So a second socket
openlanding in that post-handshake / pre-status-propagation window (flaky mobile link, or a Durable Object bounce on redeploy) sailed past both guards and launched an overlapping resume. The later resume clearedactiveResponsebefore the earlier resume's finalizer ran → the earlier finalizer readundefined→ throw.Changes
resumeInFlightRef: never issue a newresumeStream()while one is still outstanding. The flag is held for the whole resume lifetime and force-cleared in the socket-effect cleanup so an orphaned resume (agent swapped on a_pkchange) can't leave the gate stuck closed.resumeGenerationReftoken prevents a stale, orphaned resume's late.finallyfrom reopening the gate underneath a newer resume on the next socket (mirrors the existing tool-continuation generation pattern in this file).packages/agents/src/react-tests/resume-overlap-race.test.tsx) drives the real hook through a reconnect storm via a fakeEventTargetagent and asserts the overlapping reconnect issues no second resume and the finalizer never reads a clearedactiveResponse. Verified it fails onmain(3 vs 2 resume requests + the exactstateTypeError) and passes with the fix.The definitive
activeResponse-local fix belongs upstream in Vercelai; this stops the SDK from triggering the overlap. The fix lives in the sharedagents/chatcore hook, so it covers both@cloudflare/thinkand@cloudflare/ai-chat.Residual (out of scope, same upstream root cause)
The narrower mount-resume-vs-reprobe and submit-vs-reprobe windows share the same unguarded-
activeResponsedefect; React event batching makes the submit case effectively closed, and both ultimately want the upstream finalizer fix.Test plan
pnpm run check(sherif + export checks + oxfmt + oxlint + typecheck across all 114 projects)vitest --project react(agent-tool-replay) — unchangedvitest --project chat(514 tests, incl. resume-handshake) — unchanged@cloudflare/thinkreact-tests (stream-resume, studio-chat) — unchangedMade with Cursor