The Fix
Upgrade to version 0.24.0 or later.
Based on closed Kludex/starlette issue #472 · 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%.
@@ -2,8 +2,9 @@
import anyio
+from starlette.background import BackgroundTask
from starlette.requests import Request
-from starlette.responses import Response, StreamingResponse
repro.py
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.routing import Mount, Route
from starlette.staticfiles import StaticFiles
from starlette.templating import Jinja2Templates
from starlette.testclient import TestClient
templates = Jinja2Templates(directory="templates")
async def homepage(request):
return templates.TemplateResponse("index.html", {"request": request})
routes = [
Route("/", endpoint=homepage),
Mount("/static", StaticFiles(directory="static"), name="static"),
]
class CustomMiddleware(BaseHTTPMiddleware):
async def dispatch(self, request, call_next):
response = await call_next(request)
return response
app = Starlette(debug=True, routes=routes, middleware=[Middleware(CustomMiddleware)])
def test_homepage():
client = TestClient(app)
response = client.get("/")
assert response.status_code == 200
assert response.template.name == "index.html"
assert "request" in response.context
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 0.24.0 or later.\nWhen NOT to use: This fix should not be used if the application relies on specific middleware behavior.\n\nOption C — Workaround\nused in https://github.com/atviriduomenys/spinta/pull/22 but I'd rather not override such an important piece\nWhen NOT to use: This fix should not be used if the application relies on specific middleware behavior.\n\n
Why This Fix Works in Production
- Trigger: E AssertionError
- Mechanism: The AssertionError occurs due to the middleware not handling the response type correctly
- Why the fix works: Implements the Debug extension to resolve the AssertionError encountered with middleware and TemplateResponse in tests. (first fixed release: 0.24.0).
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 3.7 in real deployments (not just unit tests).
- The AssertionError occurs due to the middleware not handling the response type correctly
- Surfaces as: tests/test_templates.py:29: in test_templates
Proof / Evidence
- GitHub issue: #472
- Fix PR: https://github.com/kludex/starlette/pull/1991
- First fixed release: 0.24.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.33
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“The Debug extension was merged on asgiref. :pray:”
“A slightly neater solution, which can be injected into any test which already has the client injected: Keeping my fingers crossed that this problem gets…”
“btw: I think in general more verbose asserts should be used, e.g. in this case: > assert message["type"] == "http.response.start", message or > assert message["type"]…”
“Here's my hacky workaround to this for now in case it helps anyone: I then inject client_without_middleware into tests which render Jinja2 templates.”
Failure Signature (Search String)
- E AssertionError
Error Message
Stack trace
error.txt
Error Message
-------------
tests/test_templates.py:29: in test_templates
response = client.get("/")
.venv/lib/python3.7/site-packages/requests/sessions.py:546: in get
return self.request('GET', url, **kwargs)
starlette/testclient.py:421: in request
json=json,
.venv/lib/python3.7/site-packages/requests/sessions.py:533: in request
resp = self.send(prep, **send_kwargs)
.venv/lib/python3.7/site-packages/requests/sessions.py:646: in send
r = adapter.send(request, **kwargs)
starlette/testclient.py:238: in send
raise exc from None
starlette/testclient.py:235: in send
loop.run_until_complete(self.app(scope, receive, send))
/usr/lib/python3.7/asyncio/base_events.py:584: in run_until_complete
return future.result()
starlette/applications.py:134: in __call__
await self.error_middleware(scope, receive, send)
starlette/middleware/errors.py:122: in __call__
raise exc from None
starlette/middleware/errors.py:100: in __call__
await self.app(scope, receive, _send)
starlette/middleware/base.py:25: in __call__
response = await self.dispatch_func(request, self.call_next)
tests/test_templates.py:21: in noop
response = await call_next(request)
starlette/middleware/base.py:47: in call_next
assert message["type"] == "http.response.start"
E AssertionError
Minimal Reproduction
repro.py
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.routing import Mount, Route
from starlette.staticfiles import StaticFiles
from starlette.templating import Jinja2Templates
from starlette.testclient import TestClient
templates = Jinja2Templates(directory="templates")
async def homepage(request):
return templates.TemplateResponse("index.html", {"request": request})
routes = [
Route("/", endpoint=homepage),
Mount("/static", StaticFiles(directory="static"), name="static"),
]
class CustomMiddleware(BaseHTTPMiddleware):
async def dispatch(self, request, call_next):
response = await call_next(request)
return response
app = Starlette(debug=True, routes=routes, middleware=[Middleware(CustomMiddleware)])
def test_homepage():
client = TestClient(app)
response = client.get("/")
assert response.status_code == 200
assert response.template.name == "index.html"
assert "request" in response.context
Environment
- Python: 3.7
What Broke
Tests fail with AssertionError when using middleware with TemplateResponse.
Why It Broke
The AssertionError occurs due to the middleware not handling the response type correctly
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.24.0 or later.
When NOT to use: This fix should not be used if the application relies on specific middleware behavior.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option C — Workaround Temporary workaround
used in https://github.com/atviriduomenys/spinta/pull/22 but I'd rather not override such an important piece
When NOT to use: This fix should not be used if the application relies on specific middleware behavior.
Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.
Fix reference: https://github.com/kludex/starlette/pull/1991
First fixed release: 0.24.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the application relies on specific middleware behavior.
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
- Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
- Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.24.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.