Python asyncio: 'RuntimeError: asyncio.run() cannot be called from a running event loop' or 'This event loop is already running' (Jupyter, FastAPI, LangChain, async SDKs). How do I fix it?

Cause: asyncio.run() creates a new event loop and runs it until the coroutine finishes. It refuses to run inside a thread that already has a running loop. Common places that already run a loop: Jupyter/IPython notebooks, async web frameworks (FastAPI, aiohttp, Quart), async agent frameworks and test runners like pytest-asyncio. The error also appears when sync helper functions inside a library call asyncio.run() or loop.run_until_complete() and are then used from async code. Fix (by situation): 1. In Jupyter/IPython: don't call asyncio.run(); use top-level await directly in a cell: result = await main(). 2. Inside async code (FastAPI endpoints, async functions): await the coroutine (await client.fetch()) instead of wrapping it with asyncio.run(). Make the calling function async if needed. 3. Calling blocking sync code from async code: run it in a thread with await asyncio.to_thread(func, *args) so it doesn't block the loop. 4. Calling async code from sync code that runs inside a thread with no loop (for example a worker thread): asyncio.run() is fine there; from a different thread than the running loop, use asyncio.run_coroutine_threadsafe(coro, loop).result(). 5. For scripts, keep one entry point: asyncio.run(main()) at the bottom, and everything else async inside main(). Reuse one asyncio.Runner (Python 3.11+) if you need several top-level calls. 6. Avoid nest_asyncio as a permanent fix; it patches the loop to allow re-entry and can hide real design problems. 7. If the error comes from a library's sync wrapper, use the library's async API (often named a<method> or available on an Async client class) when you're already in async code. Verify: run with PYTHONASYNCIODEBUG=1 (or asyncio debug mode) to catch slow callbacks and never-awaited coroutines; the error should be gone and no 'coroutine was never awaited' warnings should appear. Sources: https://docs.python.org/3/library/asyncio-runner.html https://docs.python.org/3/library/asyncio-dev.html