starlette Session cookie triggers failure on websocket

The Fix

Upgrade to version 0.13.4 or later.

Based on closed Kludex/starlette issue #483 · 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

@@ -6,7 +6,7 @@
 
 from starlette.datastructures import MutableHeaders, Secret
-from starlette.requests import Request
+from starlette.requests import HTTPConnection
 from starlette.types import ASGIApp, Message, Receive, Scope, Send

repro.py

Traceback (most recent call last):
  File ".venv/lib/python3.7/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 146, in run_asgi
    asgi = self.app(self.scope)
  File ".venv/lib/python3.7/site-packages/starlette/applications.py", line 134, in __call__
    return self.error_middleware(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 88, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/uvicorn/middleware/proxy_headers.py", line 37, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/base.py", line 22, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/base.py", line 22, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/exceptions.py", line 49, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/routing.py", line 587, in __call__
    return route(scope)
  File ".venv/lib/python3.7/site-packages/starlette/routing.py", line 356, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/applications.py", line 134, in __call__
    return self.error_middleware(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 88, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/sessions.py", line 34, in __call__
    request = Request(scope)
  File ".venv/lib/python3.7/site-packages/starlette/requests.py", line 128, in __init__
    assert scope["type"] == "http"

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 0.13.4 or later.\nWhen NOT to use: This fix is not applicable if the application does not use session cookies with websocket connections.\n\nOption C — Workaround\nand it is working so far. I haven't done extensive testing.\nWhen NOT to use: This fix is not applicable if the application does not use session cookies with websocket connections.\n\n

Why This Fix Works in Production

Trigger: asgi = self.app(self.scope)
Mechanism: The session middleware incorrectly processes websocket connections as HTTP requests
Why the fix works: Resolves the issue by changing the session middleware to use HTTPConnection instead of Request, allowing proper handling of websocket connections. (first fixed release: 0.13.4).

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 session middleware incorrectly processes websocket connections as HTTP requests
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #483
Fix PR: https://github.com/kludex/starlette/pull/597
First fixed release: 0.13.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.29

Discussion

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

Jump to Sources Open on GitHub

“Can reproduce on Starlette 0.11.1. Is there any known workaround?”

@Taarek · 2019-05-18 · source

“Thanks folks. Can anyone confirm on the 0.12 prerelease?”

@lovelydinosaur · 2019-05-20 · source

“Ah yeah I think it's still an issue in master. Looks like this: https://github.com/encode/starlette/blob/master/starlette/middleware/sessions.py#L36 ought to be Connection not Request.”

@lovelydinosaur · 2019-05-20 · source

“Is there any progression on this issue ? I am currently evaluating frameworks (for a hobby project), and I would like to use Starlette API,…”

@TheBlusky · 2019-06-07 · source

Failure Signature (Search String)

asgi = self.app(self.scope)

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File ".venv/lib/python3.7/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 146, in run_asgi
    asgi = self.app(self.scope)
  File ".venv/lib/python3.7/site-packages/starlette/applications.py", line 134, in __call__
    return self.error_middleware(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 88, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/uvicorn/middleware/proxy_headers.py", line 37, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/base.py", line 22, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/base.py", line 22, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/exceptions.py", line 49, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/routing.py", line 587, in __call__
    return route(scope)
  File ".venv/lib/python3.7/site-packages/starlette/routing.py", line 356, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/applications.py", line 134, in __call__
    return self.error_middleware(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 88, in __call__
    return self.app(scope
... (truncated) ...

Minimal Reproduction

repro.py

Traceback (most recent call last):
  File ".venv/lib/python3.7/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 146, in run_asgi
    asgi = self.app(self.scope)
  File ".venv/lib/python3.7/site-packages/starlette/applications.py", line 134, in __call__
    return self.error_middleware(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 88, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/uvicorn/middleware/proxy_headers.py", line 37, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/base.py", line 22, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/base.py", line 22, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/exceptions.py", line 49, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/routing.py", line 587, in __call__
    return route(scope)
  File ".venv/lib/python3.7/site-packages/starlette/routing.py", line 356, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/applications.py", line 134, in __call__
    return self.error_middleware(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/errors.py", line 88, in __call__
    return self.app(scope)
  File ".venv/lib/python3.7/site-packages/starlette/middleware/sessions.py", line 34, in __call__
    request = Request(scope)
  File ".venv/lib/python3.7/site-packages/starlette/requests.py", line 128, in __init__
    assert scope["type"] == "http"

Environment

Python: 3.7

What Broke

Websocket connections fail with assertion errors when session cookies are enabled.

Why It Broke

The session middleware incorrectly processes websocket connections as HTTP requests

Fix Options (Details)

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

Upgrade to version 0.13.4 or later.

When NOT to use: This fix is not applicable if the application does not use session cookies with websocket connections.

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

Option C — Workaround Temporary workaround

and it is working so far. I haven't done extensive testing.

When NOT to use: This fix is not applicable if the application does not use session cookies with websocket connections.

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/597

First fixed release: 0.13.4

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

When NOT to Use This Fix

This fix is not applicable if the application does not use session cookies with websocket connections.

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
0.13.4	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 Session cookie triggers failure on websocket connection upgrade (Fix)