Async and Concurrency Style Guide

I/O belongs in async def: network calls, file system access, database queries, and subprocess waits all go in async functions. ADK runs everything on one event loop, so a synchronous call inside it stalls every concurrent agent, not just the caller.
Don't block the event loop: no synchronous HTTP clients, time.sleep, or blocking file reads inside async code.
Wrap synchronous I/O in asyncio.to_thread when a library offers no async API (open(), pathlib, most cloud SDK clients):

python

async def save_data(path: Path, data: bytes) -> None:
  # Wrap the blocking write so the event loop stays free.
  await asyncio.to_thread(path.write_bytes, data)

No hook checks this — a blocking call passes CI and shows up later as unexplained latency under concurrency, so it is worth catching in review.