fix(sdk): preserve partial assistant message on chat stream failure by matt-aitken · Pull Request #4348 · triggerdotdev/trigger.dev

matt-aitken · 2026-07-23T13:58:34Z

Summary

When a chat.agent (or chat.createSession) turn's model stream fails mid-response (e.g. a transport timeout like UND_ERR_BODY_TIMEOUT), the assistant output that already streamed was dropped: onTurnComplete fired with responseMessage: undefined, and the manual loop's turn.complete() rethrew without keeping the partial. Apps that register hydrateMessages are hit hardest, since boot-time tail-replay recovery is off by design.

This preserves the streamed-so-far assistant output while still reporting the turn as errored, so persistence and recovery keep it.

Scope of behavior change

Only the error path changes. Successful turns are unaffected: the same chunks stream to the client in the same order, and backpressure/cancel behave as before. Everything here is a correctness improvement on a turn that hit a source-stream failure.

What it does

Follow-up to #4304 (chat.pipeAndCapture), extending the same partial-recovery to the two loops that lacked it:

chat.agent: taps the response stream (via a TransformStream, so pass-through backpressure and cancel are preserved) to buffer chunks, and on a source-stream failure reconstructs the partial (preferring the onFinish message). It's surfaced on the error-path onTurnComplete (responseMessage, rawResponseMessage, uiMessages, newUIMessages, newMessages) and committed to the accumulator so the next turn and the reboot snapshot keep it.
chat.createSession / turn.complete(): the reconstructed partial is accumulated (so turn.uiMessages reflects it and the caller can persist after catching) before turn.complete() rethrows.

onBeforeTurnComplete stays skipped on the error path (it hands out a writer for a stream that has already broken).

Correctness properties (each covered by a regression test)

Each test below was confirmed to fail without its fix:

The recovered partial reaches onTurnComplete and the next turn's accumulated messages.
An already-committed (possibly enriched) response is not overwritten if a post-response hook then throws.
Incomplete tool parts are cleaned from the recovered partial (text kept), so the UI and model views agree and the next turn isn't poisoned.
A prior turn's model-only compaction survives an errored turn (append only the new tail, don't reconvert the full history).
A reconstructed fragment that reuses an existing message id does not clobber the complete message.
Queued chat.response data parts are folded into the recovered partial, matching the success path.
newMessages (model delta) stays symmetric with newUIMessages.

Tests

New chat-agent-source-stream-error.test.ts covers the cases above. The full @trigger.dev/sdk unit suite passes and the package build is green across all supported runtimes (Node 20 to 26, Bun, Deno, Cloudflare Workers).

When a chat turn's model stream fails mid-response (e.g. a transport timeout), the streamed-so-far output is no longer dropped. chat.agent passes the recovered partial to onTurnComplete, and chat.createSession accumulates it before turn.complete() rethrows, so it survives for persistence even when hydrateMessages disables boot-time replay recovery. The turn is still reported as errored.

changeset-bot · 2026-07-23T13:58:42Z

🦋 Changeset detected

Latest commit: 0316b05

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 26 packages

Name	Type
@trigger.dev/sdk	Patch
@trigger.dev/python	Patch
@internal/dashboard-agent	Patch
@internal/sdk-compat-tests	Patch
@trigger.dev/build	Patch
@trigger.dev/core	Patch
@trigger.dev/react-hooks	Patch
@trigger.dev/redis-worker	Patch
@trigger.dev/rsc	Patch
@trigger.dev/schema-to-json	Patch
@trigger.dev/database	Patch
@trigger.dev/otlp-importer	Patch
@trigger.dev/rbac	Patch
@trigger.dev/sso	Patch
trigger.dev	Patch
@internal/cache	Patch
@internal/clickhouse	Patch
@internal/llm-model-catalog	Patch
@internal/redis	Patch
@internal/replication	Patch
@internal/run-engine	Patch
@internal/run-store	Patch
@internal/schedule-engine	Patch
@internal/testcontainers	Patch
@internal/tracing	Patch
@internal/tsql	Patch

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coderabbitai · 2026-07-23T13:59:01Z

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Walkthrough

The SDK buffers streamed chunks and captures partial assistant responses during chat turns. When a source stream fails, chat.agent reconstructs or reuses the partial response and includes it in completion data while reporting an error. chat.createSession accumulates the partial assistant message before rethrowing. Tests cover managed-loop and session behavior, and a patch changeset documents the update.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title is concise and accurately summarizes the main SDK change: preserving partial assistant messages on stream failure.
Description check	✅ Passed	The description covers summary, scope, implementation details, and testing; only some optional template sections are omitted.

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Commit unit tests in branch fix/chat-preserve-partial-on-stream-failure

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Commit the recovered partial to the canonical accumulator on the chat.agent error path so the next turn and the reboot snapshot both carry it, matching the success path. Fold queued response parts into the manual-loop error partial too. Add a continuation regression test.

Replace a same-id continuation partial in place instead of dropping it as a dup, commit the errored user message unconditionally so it reaches the next live turn, and push only the appended partial's model messages to preserve a prior turn's compaction (reconvert only when replacing or folding).

pkg-pr-new · 2026-07-23T14:22:34Z

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commit: 0316b05

…tep throws The chat.agent try block spans the whole turn, so a throw from a post-response step (a customer onBeforeTurnComplete/onTurnComplete hook, a late conversion) lands in the error handler after the response was already committed. Track a per-turn responseCommitted flag and keep capturedPartialResponse pointed at the enriched committed message, so the error path reports it without re-recovering a raw partial and clobbering the committed message, its queued data parts, or a prior turn's compaction. Also use truthy id checks to match the success path.

matt-aitken · 2026-07-23T16:27:57Z

Additional hardening from an adversarial self-review of the error path:

Fixed (real): the chat.agent try spans the whole turn, so a throw from a post-response step (a customer onBeforeTurnComplete/onTurnComplete hook, or a late toModelMessages) lands in the error handler after the response was already committed. The error path would then re-recover a raw partial and overwrite the committed (possibly enriched) message, dropping queued chat.response data parts and re-inflating a prior turn's compaction. Now a per-turn responseCommitted flag skips re-recovery in that case and capturedPartialResponse tracks the enriched committed message. Regression test added (fails without the fix).

Considered, no change needed:

Committing an incomplete-tool-call partial can't poison the next turn: toModelMessages uses ignoreIncompleteToolCalls: true, so dangling tool calls are dropped from the model messages. Incomplete parts are intentionally kept in the UI message (the point is to preserve what streamed), matching the success path which only runs cleanupAbortedParts on a user stop.
Per-turn chunk buffering retains chunk refs for the turn's duration; this mirrors the existing chat.pipeAndCapture buffering and is freed when the turn ends.

319 SDK tests pass, 0 typecheck errors.

The error-path onTurnComplete added the partial to newUIMessages/uiMessages but left newMessages (the model-message delta) empty, so apps persisting the model delta missed the partial. Populate it from the same conversion.

… and desync Drop a reconstructed-from-chunks partial that reuses an existing message id (a fragment can't be safely merged into a complete message the way an onFinish message can), and make the error-path accumulator update atomic so a failed model-message conversion can't leave the UI and model accumulators out of sync.

Tap the response stream with a TransformStream instead of an async generator. The generator forced ensureReadableStream down its eager-drain wrapper, which enqueues without honoring desiredSize (dropping backpressure to the model stream) and never propagates consumer cancel to the source. piping through a TransformStream keeps the ReadableStream pass-through, so native backpressure and stop/cancel propagation are retained. Also stamp an id on a recovered partial that lacks one, matching the success path.

Apply cleanupAbortedParts to the partial recovered on a source-stream failure, in both the managed and manual loops. A transport error leaves the same incomplete-part state as a user stop (streaming text not finalized, dangling tool calls), so the partial is cleaned the same way: text is kept and unfinished tool parts are dropped, keeping the persisted UI history and the model view consistent. Also reset the error event's newMessages delta when the model-message conversion fails, so it can't advertise messages that were rolled back.

On a successful turn the buffered chunks are dead weight (the recovered-partial fallback prefers the onFinish message), yet they were retained through the end-of-turn idle wait. Clear the buffer once the response is committed so a completed turn no longer holds a full copy of its streamed output while idle.

When committing the errored turn's state to the accumulator, append only the new tail's model messages (errored user message and/or recovered partial) instead of reconverting the whole UI history, so a prior turn's model-only compaction survives an errored turn. Skip re-adding the partial once the response was already committed (a later compaction may have folded it into a summary), and roll the event/snapshot views back if the model conversion fails so they never advertise a partial the accumulator didn't take.

…ssages The error-path onTurnComplete reported the errored user message in newUIMessages but not in newMessages (the model-message delta), so a customer persisting the model delta lost it. Derive newMessages from the same set as newUIMessages so the two stay symmetric.

The managed error path now folds data parts queued via chat.response / writer.write() into the recovered partial before persisting, matching the success path and the manual loop, so a data part queued just before a source-stream failure isn't dropped.

…error partial The error-path UI-message array construction tripped the tshy build's stricter check (partialResponse not narrowed by the includePartial flag). Assert non-null where includePartial already guarantees it.

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fix(sdk): preserve partial assistant message on chat stream failure#4348

fix(sdk): preserve partial assistant message on chat stream failure#4348
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fix/chat-preserve-partial-on-stream-failure

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Summary

Scope of behavior change

What it does

Correctness properties (each covered by a regression test)

Tests

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matt-aitken commented Jul 23, 2026 •

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