fix(langchain): emit finish and finish-step events for LangGraph streams #11624
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Previously, `toUIMessageStream` only emitted `finish` events for model and streamEvents stream types, but not for LangGraph streams. This caused the stream to go from `text-end` straight to `[DONE]` without proper lifecycle events. This fix ensures that: - LangGraph streams now emit `finish-step` when a step was started - LangGraph streams now emit `finish` event at the end of the stream This addresses the issue where multi-turn conversations with LangGraph agents would fail due to missing protocol events.
lgrammel
approved these changes
Jan 8, 2026
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Background
When using
@ai-sdk/langchainwith LangGraph agents, the stream was missingfinishandfinish-stepevents. ThetoUIMessageStreamfunction only emitted these lifecycle events formodelandstreamEventsstream types, but not forlanggraphstreams.This caused the stream to go from
text-endstraight to[DONE]without proper protocol events, which breaks the expected AI SDK UI Message Stream protocol and can cause issues in multi-turn conversations.Summary
finishevent emission for LangGraph streams at the end of the streamfinish-stepevent emission for LangGraph streams when a step was started (tracked vialangGraphState.currentStep)finisheventsManual Verification
streamMode: ['values', 'messages']start-stepwhen a step beginsfinish-stepwhen a step ends or at stream completionfinishat the end of the streamChecklist
pnpm changesetin the project root)Related Issues
Fixes #11417
Related reproduction: https://github.com/tuntisz/ai-sdk-langchain-bug-repro