Flask integration does not create transactions when called via Flask.wsgi_app (like Connexion 3.x)

[PetrusAsikainen]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

2.34.1

Steps to Reproduce

1. pip install 'connexion[flask]==3.*' sentry_sdk gunicorn uvicorn

2. Create app.py and openapi.yaml as below

   openapi.yaml

   openapi: 3.0.0
   info:
     title: Test
     version: 1.0.0
   paths:
     /api:
       get:
         operationId: app.test_op
         responses:
           '200':
             description: Test response
             content:
               application/json:
                 schema:
                   type: string

   app.py

   import connexion
   import sentry_sdk
   
   def debug(evt, hint):
       print(evt["contexts"]["trace"]["trace_id"])
       return evt
   
   sentry_sdk.init(debug=True, before_send=debug, traces_sample_rate=1.0)
   
   def test_op():
       return "Hello world"
   
   if __name__ == "__main__":
       app = connexion.FlaskApp("app")
       app.add_api("openapi.yaml")
       app.run(port=5000)

3. python app.py

4. Access http://localhost:5000/api a few times

pip freeze --all

a2wsgi==1.10.10
anyio==4.10.0
asgiref==3.9.1
attrs==25.3.0
blinker==1.9.0
certifi==2025.8.3
charset-normalizer==3.4.3
click==8.2.1
connexion==3.2.0
distlib==0.3.9
filelock==3.18.0
Flask==3.1.1
gunicorn==23.0.0
h11==0.16.0
httpcore==1.0.9
httpx==0.28.1
idna==3.10
inflection==0.5.1
itsdangerous==2.2.0
Jinja2==3.1.6
jsonschema==4.25.0
jsonschema-specifications==2025.4.1
MarkupSafe==3.0.2
packaging==25.0
pip==25.0.1
platformdirs==4.3.7
python-multipart==0.0.20
PyYAML==6.0.2
referencing==0.36.2
requests==2.32.4
rpds-py==0.27.0
sentry-sdk==2.34.1
sniffio==1.3.1
starlette==0.47.2
typing_extensions==4.14.1
urllib3==2.5.0
uvicorn==0.35.0
virtualenv==20.29.3
Werkzeug==3.1.3

Expected Result

Trace ID changes for each request, as transactions are created

Actual Result

Each request prints the same trace_id (and all events end up in the same transaction)

[linear]

PY-1808 Flask integration does not create transactions when called via Flask.wsgi_app (like Connexion 3.x)

[PetrusAsikainen]

Why this occurs:

We use Connexion's FlaskApp as our current API middleware. In Connexion v2, this is implemented as an ASGI wrapper that directly calls Flask.wsgi_app. This is also the official wrapping point recommended by Flask.

https://github.com/spec-first/connexion/blob/1844a2fb3f1d09a37edfeab7a7200d52e97abbd9/connexion/apps/flask.py#L129-L141

The Sentry Flask integration only hooks Flask.__call__ and therefore doesn't receive transactions from the wrapped Flask app.

We could probably change to using the ASGI integration, but routing happens within Flask, so that doesn't have proper transaction metadata when the transaction is created. Therefore, it might make sense to hook wsgi_app instead. (As a mitigation we'll probably internally monkey-patch wsgi_app.)

[sentrivana]

Hey @PetrusAsikainen, thanks for writing in. We can look into wrapping wsgi_app instead of __call__ -- I assume we're doing the latter mostly because the Flask integration is pretty old and at that point it was probably the best way to hook into it. If there's nothing we'd lose/break by hooking into wsgi_app instead, we can do that.

(As a mitigation we'll probably internally monkey-patch wsgi_app.)

If you tried this -- does everything work ok?

[PetrusAsikainen]

Hey @PetrusAsikainen, thanks for writing in. We can look into wrapping wsgi_app instead of __call__ -- I assume we're doing the latter mostly because the Flask integration is pretty old and at that point it was probably the best way to hook into it. If there's nothing we'd lose/break by hooking into wsgi_app instead, we can do that.

Flask.__call__ has been an alias to it since 2010, so I'd imagine there should at most be ordering issues with other similar hooks, or someone depending on random internals of Sentry.

(As a mitigation we'll probably internally monkey-patch wsgi_app.)

If you tried this -- does everything work ok?

Mostly. We're still getting some issues not being correctly attributed to requests, but my first guess is that's because we're not correctly setting up the asyncio integration yet.

[sentrivana]

So we apparently used to patch wsgi_app but switched to __call__ in 1a66198. Unsure atm if that was just to make testing easier. FWIW the tests pass with this trivial change, except for the one linked in the old commit:

diff --git a/sentry_sdk/integrations/flask.py b/sentry_sdk/integrations/flask.py
index f45ec6db2..dfdc575ae 100644
--- a/sentry_sdk/integrations/flask.py
+++ b/sentry_sdk/integrations/flask.py
@@ -91,7 +91,7 @@ class FlaskIntegration(Integration):
         request_started.connect(_request_started)
         got_request_exception.connect(_capture_exception)
 
-        old_app = Flask.__call__
+        old_app = Flask.wsgi_app
 
         def sentry_patched_wsgi_app(self, environ, start_response):
             # type: (Any, Dict[str, str], Callable[..., Any]) -> _ScopedResponse
@@ -111,7 +111,7 @@ class FlaskIntegration(Integration):
             )
             return middleware(environ, start_response)
 
-        Flask.__call__ = sentry_patched_wsgi_app
+        Flask.wsgi_app = sentry_patched_wsgi_app