Matt Aitken
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## 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).