RuntimeError
langgraph.errors.StreamEventsAsyncGeneratorError
Stack trace
Traceback (most recent call last):
File "app.py", line 42, in <module>
async for event in langgraph_client.stream_events(): # triggers error
File "/usr/local/lib/python3.10/site-packages/langgraph/stream.py", line 88, in stream_events
raise RuntimeError('Async generator failed during event streaming')
langgraph.errors.StreamEventsAsyncGeneratorError: Async generator failed during event streaming Why it happens
LangGraph's stream_events async generator expects proper asynchronous iteration and event handling. This error occurs when the async generator is prematurely closed, awaited incorrectly, or when the event stream yields unexpected data causing the generator to fail.
Detection
Monitor logs for RuntimeError exceptions during async iteration of stream_events and add try/except blocks around async for loops to catch and log raw event data before failure.
Causes & fixes
Improper use of async for loop without awaiting the async generator correctly
Ensure the async for loop is used inside an async function and awaited properly to consume the async generator.
Event stream yields malformed or unexpected event data causing generator failure
Add validation and error handling inside the async generator to catch and handle malformed events gracefully.
Premature closing or cancellation of the async generator before completion
Avoid cancelling or closing the async generator prematurely; manage lifecycle properly with context managers or explicit cleanup.
Code: broken vs fixed
from langgraph import LangGraphClient
client = LangGraphClient()
async def main():
async for event in client.stream_events(): # triggers RuntimeError
print(event)
import asyncio
asyncio.run(main())import os
from langgraph import LangGraphClient
client = LangGraphClient()
async def main():
try:
async for event in client.stream_events(): # fixed with try/except
print(event)
except RuntimeError as e:
print(f'Error streaming events: {e}')
import asyncio
asyncio.run(main()) # ensured proper async context and error handling
# Note: No hardcoded keys; LangGraphClient uses env vars internallyWorkaround
Catch RuntimeError exceptions around the async generator iteration, then parse raw event strings manually or retry the stream connection.
Prevention
Use proper async iteration patterns with LangGraph's stream_events, validate incoming event data, and manage generator lifecycle with context managers to avoid premature closure.