Unsloth Studio Streaming Chat Crashes on Python 3.13 (anyio cancel scope)
Streaming chat completions from Unsloth Studio's OpenAI-compatible endpoint crash with a RuntimeError about exiting a cancel scope in a different task, on macOS with Python 3.13. The anyio/Starlette streaming machinery regressed under Python 3.13's asyncio changes; downgrading the runtime Python fixes it.
Streaming chat completions from Unsloth Studio's OpenAI-compatible endpoint crash with a RuntimeError about exiting a cancel scope in a different task, on macOS with Python 3.
What this failure is
Unsloth Studio Streaming Chat Crashes on Python 3.13 (anyio cancel scope) is a Environment failure seen during ML training runs. Streaming chat completions from Unsloth Studio's OpenAI-compatible endpoint crash with a RuntimeError about exiting a cancel scope in a different task, on macOS with Python 3.13. The anyio/Starlette streaming machinery regressed under Python 3.13's asyncio changes; downgrading the runtime Python fixes it. Common tags: Unsloth, Python 3.13, Anyio, Asyncio.
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Why it happens (the mechanism)
Python 3.13 changed asyncio task/cancellation semantics, and anyio's cancel-scope handling for streaming responses enters and exits the scope across different tasks. The SSE generator therefore violates anyio's same-task invariant and raises on teardown. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- POST /v1/chat/completions with stream=true crashes the server
- Exception spam on every streamed token
- Only happens on Python 3.13
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| RuntimeError: Attempted to exit cancel scope in a different task than it was entered in | Python 3.13 changed asyncio task/cancellation semantics, and anyio's cancel-scope handling for streaming responses enters and exits the scope across different tasks |
| anyio/Starlette traceback during the SSE response | The SSE generator therefore violates anyio's same-task invariant and raises on teardown |
| Stable on Python 3.11/3.12, broken on 3.13 | Python 3.13 changed asyncio task/cancellation semantics, and anyio's cancel-scope handling for streaming responses enters and exits the scope across different tasks |
Which systems are affected
- Unsloth Studio OpenAI-compatible server on Python 3.13
- FastAPI/Starlette + anyio streaming responses
- macOS serving setups
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: RuntimeError: Attempted to exit cancel scope in a different task than it was entered in
- ✓Verified signal present: anyio/Starlette traceback during the SSE response
- ✓Verified signal present: Stable on Python 3.11/3.12, broken on 3.13
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Root cause
- Python 3.13 changed asyncio task/cancellation semantics, and anyio's cancel-scope handling for streaming responses enters and exits the scope across different tasks
- The SSE generator therefore violates anyio's same-task invariant and raises on teardown
The fix and how to prevent it
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