fix(ai, ai-client): stream durable responses live and hydrate once in Strict Mode - #1620
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With `durability` set, a chunk reached the client only after its batch was appended, and a batch was appended only at 32 chunks or the run end. A short reply showed up all at once at the end. The batch now also flushes when the producer sends no new chunk for `batchWaitMs` (default 50ms). `batch` stays the most chunks in one append. Set `batchWaitMs` on `durability` or on `toWebSocketStream`. Claude-Session: https://claude.ai/code/session_01APYv1qshKyjPPpkFyRZhfZ
React Strict Mode runs mount effects twice in dev: attach, detach, attach. Each attach of a `persistence: true` chat sent its own hydrate GET. A re-attach now reuses the GET that is still in flight, and that GET still paints the view when it returns. Claude-Session: https://claude.ai/code/session_01APYv1qshKyjPPpkFyRZhfZ
`/durable-persistence` in ts-react-chat runs `persistence: true` with a durable POST. Switches turn on the old batching and a slow model, and a counter shows how text chunks arrive. Reload mid-answer to see the run continue. Claude-Session: https://claude.ai/code/session_01APYv1qshKyjPPpkFyRZhfZ
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Included review availability: This review used your included allowance. Your plan provides up to 8 included reviews per hour; 6 remain after this review. 📝 WalkthroughWalkthroughThe changes add timed flushing for durable response batches, a durable-persistence chat example, and reuse of in-flight hydration requests during chat reattachment. ChangesDurable stream batching
Hydration request reuse
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant Source as Producer iterator
participant Durable as durableStreamSource
participant Adapter as Durability adapter
participant Reader as Stream reader
Source->>Durable: Yield chunks
Durable->>Adapter: Append buffered batch at size limit, wait limit, or stream boundary
Adapter->>Reader: Deliver appended chunks
Merge Risk: ⚪ Minimal · up to The change makes durable streams flush buffered text on a timer, and it ends the response cleanly if a timed append fails. No merge-blocking risk is evident from the supplied review material. Security Architecture ReviewSecurity architecture risk: 🟡 Moderate · up to Normal streaming preserves persistence-before-delivery, and hydration reuse retains its stale-result guards. However, timed flushing introduces failure paths that can delay durable-run cleanup or leave a source rejection temporarily unhandled. The new API is explicitly a demo and does not enforce user ownership; its deployment exposure is unknown. Retained concerns
Security review detailsSecurity Blast Radius
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Review comments at @packages/ai/src/stream-to-response.ts:
- Around line 574-603: In the `settlesWithin` timeout-flush path, abort
`abortController` with the flush error before rethrowing it if the signal is not
already aborted. This ensures a failed `flush()` does not leave the pending
producer pull running while `iterator.return()` waits.
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.changeset/chat-hydrate-strict-mode.md.changeset/durable-batch-live-flush.mddocs/config.jsondocs/resumable-streams/advanced.mdexamples/ts-react-chat/src/components/Header.tsxexamples/ts-react-chat/src/routeTree.gen.tsexamples/ts-react-chat/src/routes/api.durable-persistence.tsexamples/ts-react-chat/src/routes/durable-persistence.tsxpackages/ai-client/src/chat-client.tspackages/ai-client/tests/dispose-tail-leak.test.tspackages/ai/src/stream-to-response.tspackages/ai/src/stream-to-websocket.tspackages/ai/tests/stream-to-response-durability.test.tstesting/e2e/src/routeTree.gen.tstesting/e2e/src/routes/api.durable-delivery.tstesting/e2e/src/routes/client-mount-hydrate.tsxtesting/e2e/tests/delivery-durability.spec.tstesting/e2e/tests/persistence-durability.spec.ts
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A timed flush runs while the next chunk is still pending. If its append failed, the producer awaited `return()` on the source, and an async generator runs that only after its pending pull. So RUN_ERROR and the log close waited for the model's next chunk, maybe forever. After a failed timed flush, close the source in the background. The failure path then persists RUN_ERROR and ends the response at once. Claude-Session: https://claude.ai/code/session_01APYv1qshKyjPPpkFyRZhfZ
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With
durabilityontoServerSentEventsResponse,toHttpResponseor the WebSocket stream, live text stopped streaming: a reply came in lumps of 32 chunks, and a short reply came all at once at the end. This PR flushes the durability batch when the model sends no new chunk forbatchWaitMs(default 50ms), so text streams again withoutbatch: 1. It also fixes a second bug: apersistence: truechat sent two hydrateGETs when it mounted in React Strict Mode.🎯 Changes
@tanstack/ai: the durable producer flushes a non-empty batch when the next chunk does not arrive withinbatchWaitMs.batchstays the most chunks in oneappend. A burst of chunks still goes in oneappend, so a remote log (durableStream) does not get one write per token. If a timed flush fails while the next chunk is pending, the run still ends withRUN_ERRORat once.@tanstack/ai: newbatchWaitMsoption ondurability(SSE and NDJSON) and ontoWebSocketStream/toWebSocketResponse. Values from 0 to 2147483647 are valid. Other values throw, the same way a badbatchdoes.@tanstack/ai-client:attach()reuses a hydrateGETthat is still in flight. React Strict Mode runs attach, detach, attach before the firstGETreturns, and each attach sent its ownGET.docs/resumable-streams/advanced.mdshowsbatchWaitMsand when to raise or lower it./durable-persistenceinexamples/ts-react-chatrunspersistence: truewith a durablePOST. It has switches for the old batching and a slow model, and a counter of how text chunks arrive.Both fixes, the option and the demo are in one PR on purpose (maintainer choice). The
bugfix-prkeep gate asks for one fix per PR.✅ Checklist
pnpm run test:pr, or these tests do not apply to this pull request.docs/for this change, or this change is not user-facing.pnpm changeset), or this PR does not change a published package.pnpm test:pritself did not run: Nx in this local worktree reads the cache of another checkout and skips work. I ran each of its targets directly instead. See Testing.🚀 Release Impact
Root cause
Bug 1: durable text arrives in lumps
Issue. Any app that passes
durabilityto a response helper. Live text arrives in lumps of 32 chunks, and a reply with fewer than 32 chunks shows up only atRUN_FINISHED.batch: 1hides it, at one log write per chunk.Cause.
durableStreamSourcesends a chunk to the client only after its batch is appended to the log, so that a reconnect can replay what the client saw. Before this PR, it appended a batch only when 32 chunks were waiting or at a flush boundary (RUN_STARTED,TOOL_CALL_END, terminals, mostCUSTOMevents). Text chunks waited while the model was still working. This has been the behavior since resumable streams were added (#955), so it is not a regression from a recent merge.Fix. The producer loop now reads the source by hand. Before it waits for the next chunk, it races that chunk against the batch deadline (
batchWaitMsafter the first buffered chunk). If the deadline wins, the batch is appended and sent first. Thefinallycloses the source on an early exit, likefor awaitdid.Bug 2: two hydrate
GETs in Strict ModeIssue. A
useChat({ persistence: true })chat that mounts on the client in dev sends the hydrateGETtwice.Cause.
ChatClient.attach()callshydrateFromServer()each time. Strict Mode replays a client mount's effects: attach, detach, attach.detach()does not cancel the firstGET, so the secondattach()sent another one. A chat hydrated from the server render gets no replay, which is why the existing E2E page did not show it.Fix.
ChatClientremembers thehistoryGenerationof the hydrateGETin flight.attach()skips a newGETwhile one for the same generation is still out. ThatGETpaints the view, because a view is attached again when it returns. Afterclear(), a re-attach still starts a freshGET.Possible alternatives
batchto 1. Every chunk is sent at once. This PR did not take it, because a remote log would get one write per token.id:that the log does not have yet, and exact resume breaks.batchWaitMs: 0). Zero added latency, but almost one write per token from a real model. It is still possible withbatchWaitMs: 0.GETindetach(). The Strict Mode remount would still send a secondGET. This PR did not take it.GenerationClient). A real view switch must hydrate again to see new turns, so this PR keeps oneGETper attach and only reuses the one in flight.Testing
Commands run
main@6d8e6485fand on this branch):delivery durability (live text)andsends one hydrate GET when a chat mounts under React Strict Mode. Both pass. With each fix turned off, each fails: the first text came 1.4ms before the end (expected more than 800ms), and the page sent 2GETs (expected 1).playwright test --workers=2): 438 passed, 3 skipped, 3 failed. None are from this PR. The Gemini structured-output test passed on a rerun. The boxd and Cloudflare tests failed because those packages were not built in the worktree. After the build, Cloudflare passed. Boxd still fails on Windows withERR_UNSUPPORTED_ESM_URL_SCHEME, because the test imports a rawF:\path.@tanstack/ai(test:oxlint,test:types,test:build,test:lib1959 passed) and@tanstack/ai-client(test:oxlint,test:types,test:build,test:lib872 passed).@tanstack/ai-reacttest:lib248 passed.test:sherif,test:knip,test:docs,test:kiira,test:dts.test:typesforexamples/ts-react-chatandtesting/e2e.Manual test
pnpm --filter ts-react-chat dev:viteand openhttp://localhost:3000/durable-persistence(needsOPENAI_API_KEY).durable-persistence, and reload. One hydrateGET ...?threadId=per load.How this PR makes testing easy
packages/ai/tests/stream-to-response-durability.test.tsandpackages/ai-client/tests/dispose-tail-leak.test.ts. The first also covers a timed flush whoseappendfails (from review).testing/e2e/tests/delivery-durability.spec.ts(newslowscenario on/api/durable-delivery) andtesting/e2e/tests/persistence-durability.spec.ts(new/client-mount-hydratepage)./durable-persistenceinexamples/ts-react-chat.Risk / rollback
batch: 1, and a remote log gets more writes than with the old size-only batches.batchWaitMstunes both.for await. Itsfinallykeeps the same close rules (close on an early exit, not after the source finished or threw). After a failed timed flush, it closes the source in the background, so the error path does not wait for the model. The existing abort, detach and terminal tests pass.toEncodedStreamand the same E2E harness files, in other hunks. Expect at most a small merge conflict.Public API change
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Summary by CodeRabbit
New Features
0to append each chunk separately.Bug Fixes
Documentation