LangGraph silently dropped streamed agent output that hadn't reached a checkpoint yet

Undisclosed (LangGraph developers building chat/streaming apps)·LangGraphUnresolved

LangGraph only persists run state at full task/run completion, so when a user cancels a run or the streaming connection closes, any output that had already streamed to the client but wasn't yet checkpointed is lost once the client resyncs with the backend as source of truth.

Blast radius

Users could see and start reading substantial streamed output, then lose it entirely after cancelling and sending a new message, breaking 'what you saw is what is saved' continuity for chat and streaming applications built on LangGraph.

Root cause

Checkpointing only occurs on run completion, not on cancellation or connection close, so partially-streamed state has no persistence path.

How it was caught

Reported by a developer via a detailed GitHub bug report with reproduction steps after observing the behavior in a FastAPI-based deployment.

What changed

No fix confirmed at time of writing; the reporter proposed adding an optional partial/aborted checkpoint on cancellation.

Take-awayIf your agent framework streams before persisting, add an explicit checkpoint-on-cancel path or users will lose work they already saw on screen.