starlette StreamingResponse doesn't support PEP-492

The Fix

Upgrade to version 0.14.0 or later.

Based on closed Kludex/starlette issue #793 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -1,6 +1,5 @@
 import hashlib
 import http.cookies
-import inspect
 import json
 import os

repro.py

ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/uvicorn/protocols/http/httptools_impl.py", line 385, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/fastapi/applications.py", line 142, in __call__
    await super().__call__(scope, receive, send)  # pragma: no cover
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/applications.py", line 134, in __call__
    await self.error_middleware(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/middleware/errors.py", line 178, in __call__
    raise exc from None
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/middleware/errors.py", line 156, in __call__
    await self.app(scope, receive, _send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/exceptions.py", line 73, in __call__
    raise exc from None
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/exceptions.py", line 62, in __call__
    await self.app(scope, receive, sender)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/routing.py", line 590, in __call__
    await route(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/routing.py", line 208, in __call__
    await self.app(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/routing.py", line 44, in app
    await response(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/responses.py", line 196, in __call__
    async for chunk in self.body_iterator:
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/concurrency.py", line 45, in iterate_in_threadpool
    yield await run_in_threadpool(_next, iterator)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/concurrency.py", line 25, in run_in_threadpool
    return await loop.run_in_executor(None, func, *args)
  File "/home/korijn/.pyenv/versions/3.6.8/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/contextvars/__init__.py", line 38, in run
    return callable(*args, **kwargs)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.py
... (truncated) ...

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.14.0 or later.\nWhen NOT to use: This fix is not applicable if the content is not an async iterator.\n\n

Why This Fix Works in Production

Trigger: ERROR: Exception in ASGI application
Mechanism: StreamingResponse does not handle custom async iterators correctly, leading to execution in the threadpool
Why the fix works: Allows custom async iterators to be passed to StreamingResponse by checking if the content is an instance of AsyncIterable. (first fixed release: 0.14.0).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.6 in real deployments (not just unit tests).
StreamingResponse does not handle custom async iterators correctly, leading to execution in the threadpool
Surfaces as: ERROR: Exception in ASGI application

Proof / Evidence

GitHub issue: #793
Fix PR: https://github.com/kludex/starlette/pull/1041
First fixed release: 0.14.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.28

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“I don't think there is builtin function we can call”

@Korijn · 2020-01-13 · confirmation · source

“Okay, I guess we'd need to change this test... https://github.com/encode/starlette/blob/5c43dde0ec0917673bb280bcd7ab0c37b78061b7/starlette/responses.py#L180 Any idea what the check ought to be instead?”

@lovelydinosaur · 2020-01-13 · source

“I need this issue to be fixed as well. Why not just go with something like:”

@witling · 2020-09-03 · source

Failure Signature (Search String)

ERROR: Exception in ASGI application

Error Message

Stack trace

error.txt

Error Message
-------------
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/uvicorn/protocols/http/httptools_impl.py", line 385, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/fastapi/applications.py", line 142, in __call__
    await super().__call__(scope, receive, send)  # pragma: no cover
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/applications.py", line 134, in __call__
    await self.error_middleware(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/middleware/errors.py", line 178, in __call__
    raise exc from None
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/middleware/errors.py", line 156, in __c
... (truncated) ...

Minimal Reproduction

repro.py

ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/uvicorn/protocols/http/httptools_impl.py", line 385, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/fastapi/applications.py", line 142, in __call__
    await super().__call__(scope, receive, send)  # pragma: no cover
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/applications.py", line 134, in __call__
    await self.error_middleware(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/middleware/errors.py", line 178, in __call__
    raise exc from None
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/middleware/errors.py", line 156, in __call__
    await self.app(scope, receive, _send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/exceptions.py", line 73, in __call__
    raise exc from None
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/exceptions.py", line 62, in __call__
    await self.app(scope, receive, sender)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/routing.py", line 590, in __call__
    await route(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/routing.py", line 208, in __call__
    await self.app(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/routing.py", line 44, in app
    await response(scope, receive, send)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/responses.py", line 196, in __call__
    async for chunk in self.body_iterator:
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/concurrency.py", line 45, in iterate_in_threadpool
    yield await run_in_threadpool(_next, iterator)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/starlette/concurrency.py", line 25, in run_in_threadpool
    return await loop.run_in_executor(None, func, *args)
  File "/home/korijn/.pyenv/versions/3.6.8/lib/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.pyenv/shims/python/lib/python3.6/site-packages/contextvars/__init__.py", line 38, in run
    return callable(*args, **kwargs)
  File "/home/korijn/.local/share/virtualenvs/aiotest--vQHOsY--/home/korijn/.py
... (truncated) ...

Environment

Python: 3.6

What Broke

Passing an async iterator to StreamingResponse causes application crashes and exceptions.

Why It Broke

StreamingResponse does not handle custom async iterators correctly, leading to execution in the threadpool

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 0.14.0 or later.

When NOT to use: This fix is not applicable if the content is not an async iterator.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/kludex/starlette/pull/1041

First fixed release: 0.14.0

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not applicable if the content is not an async iterator.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
0.14.0	Fixed

Related Issues

No related fixes found.

Cluster: starlette:configuration-error All hubs All clusters

Related clusters: Data consistency SSL handshake Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

starlette StreamingResponse doesn't support PEP-492 asynchronous iterators (Fix)