anyio
Async compatibility layer and structured concurrency library for Python — runs on both asyncio and trio backends, provides task groups, cancellation scopes, threading integration, and synchronization primitives. anyio features: TaskGroup for structured concurrency (similar to trio nurseries), CancelScope with deadline/cancel, from_thread.run_sync() for calling async from sync, to_thread.run_sync() for offloading to thread pool, async file I/O, sleep()/get_current_time(), move_on_after()/fail_after() context managers, Event/Lock/Semaphore/Condition for async synchronization, and socket/stream abstractions. Used by httpx, FastAPI, and Starlette internally.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Async concurrency library. No network calls. Proper cancellation handling prevents resource leaks. to_thread.run_sync with user-controlled callables: validate inputs. CancelScope can interfere with cleanup if not properly shielded.
⚡ Reliability
Best When
Libraries and agents that must work with both asyncio and trio, or code needing structured concurrency (TaskGroup) with proper cancellation propagation.
Avoid When
Asyncio-only code (use asyncio directly), trio-specific features, or when asyncio.gather() with gather(return_exceptions=True) is sufficient.
Use Cases
- • Agent task group — async with anyio.create_task_group() as tg: tg.start_soon(fetch_data, url1); tg.start_soon(process, item1) — structured concurrency; agent runs multiple coroutines concurrently; all tasks complete or all cancelled on error; exception in any task cancels group; structured vs asyncio.gather() which doesn't propagate exceptions
- • Agent timeout — async with anyio.move_on_after(30) as cancel_scope: result = await slow_operation(); if cancel_scope.cancelled_caught: handle_timeout() — timeout handling; agent imposes deadline on operations; move_on_after() continues after timeout (cancelled_caught=True); fail_after() raises TimeoutError
- • Agent thread to async — from anyio import from_thread; def sync_callback(data): from_thread.run_sync(lambda: async_queue.put_nowait(data)) — thread bridge; agent runs async code from synchronous thread; from_thread.run() for awaiting coroutines; essential for integrating sync callbacks with async agent loops
- • Agent async to thread — async def process(): result = await anyio.to_thread.run_sync(blocking_function, arg1) — thread offload; agent offloads CPU-intensive or blocking operations to thread pool without blocking event loop; limiter parameter controls max concurrent threads; cancellable=True allows thread cancellation
- • Agent cancel scope — async with anyio.CancelScope() as scope: try: await operation(); finally: if scope.cancel_called: cleanup() — cancellation; agent implements fine-grained cancellation; scope.cancel() cancels contained code; deadline= sets absolute time limit; shield=True protects from outer cancellation
Not For
- • Trio-specific features — anyio provides compatibility layer; code using Trio-specific APIs (trio.from_thread.run_in_worker_thread) won't work via anyio
- • Simple asyncio code — if targeting asyncio only, use asyncio directly; anyio adds abstraction overhead
- • CPU-bound parallelism — anyio threads work but for CPU parallelism use multiprocessing or concurrent.futures ProcessPoolExecutor
Interface
Authentication
No auth — async concurrency library.
Pricing
anyio is MIT licensed. Free for all use.
Agent Metadata
Known Gotchas
- ⚠ TaskGroup errors use ExceptionGroup — if multiple tasks raise, anyio raises ExceptionGroup containing all exceptions; Python 3.11+ has native ExceptionGroup; older Python uses anyio's backport; agent code: use except* SomeError: handlers or anyio.abc.TaskGroup.start() with explicit error collection; single exception still wrapped in ExceptionGroup
- ⚠ move_on_after vs fail_after — move_on_after(5) cancels and CONTINUES execution after timeout (no exception); fail_after(5) raises TimeoutError; agent code: use fail_after() when timeout should stop processing; use move_on_after() when timeout is acceptable and should continue; check cancel_scope.cancelled_caught for move_on_after
- ⚠ from_thread.run() requires anyio event loop — from_thread.run(coro) works only inside thread started by anyio (to_thread.run_sync); not for arbitrary threads; agent code with external threads: use from_thread.run_sync() for sync functions; for arbitrary threads calling into anyio: use anyio.from_thread.start_blocking_portal() context manager
- ⚠ Backend selection happens at startup — anyio.run(main, backend='asyncio') or backend='trio'; cannot switch backend mid-execution; agent code: set backend at top-level anyio.run() call; libraries using anyio work on either backend; FastAPI/Starlette use asyncio backend by default
- ⚠ CancelScope.shield=True prevents cancellation propagation — scope inside shielded scope cannot be cancelled from outside; used for cleanup code that must complete; agent code: use shield=True in finally blocks for resource cleanup; shielded scope can still be cancelled by its own cancel() call
- ⚠ to_thread.run_sync limiter controls concurrency — anyio.to_thread.run_sync(fn, limiter=limiter) where limiter=anyio.CapacityLimiter(10) limits concurrent threads; default: 40 threads; agent code with many concurrent thread offloads: create shared CapacityLimiter; pass same limiter to all to_thread.run_sync calls
Alternatives
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Scores are editorial opinions as of 2026-03-06.