uvicorn Error using websockets==4.0.1 (Fix)

The Fix

Upgrade to version 0.11.4 or later.

Based on closed Kludex/uvicorn issue #56 · 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 +1,7 @@
 httptools
 uvloop
+websockets
 
 # Testing

repro.py

async def chat_server(message, channels):
    """
    ASGI-style 'Hello, world' application.
    """
    if message['channel'] == 'websocket.connect':
        await channels['reply'].send({'accept': True})

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 0.11.4 or later.\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: Starting worker [9001] serving at: 0.0.0.0:8000
Mechanism: The transfer_data_task variable is not initialized, causing None to be added to the event queue
Why the fix works: Adds websocket support for the new ASGI spec, resolving compatibility issues with websockets 4.0.1. (first fixed release: 0.11.4).

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 3.6 in real deployments (not just unit tests).
The transfer_data_task variable is not initialized, causing None to be added to the event queue
Surfaces as: Starting worker [9001] serving at: 0.0.0.0:8000

Proof / Evidence

GitHub issue: #56
Fix PR: https://github.com/kludex/uvicorn/pull/65
First fixed release: 0.11.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.44

Discussion

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

Jump to Sources Open on GitHub

“I had this same issue working on my fork - using 3.4.0 allowed websockets to work normally.”

@jordaneremieff · 2018-03-27 · source

“Thanks for the confirmation @erm. Hopefully I’ll get back to this given the renewed work on API Star. Bunch of other work I’ll need to…”

@lovelydinosaur · 2018-03-27 · source

“I discovered the issue. The .connection_open() method on the websocket protocol needs to be called after the handshake otherwise the transfer_data_task future isn't assigned and…”

@jordaneremieff · 2018-03-28 · source

Failure Signature (Search String)

Starting worker [9001] serving at: 0.0.0.0:8000

Error Message

Stack trace

error.txt

Error Message
-------------
Starting worker [9001] serving at: 0.0.0.0:8000
Task exception was never retrieved
future: <Task finished coro=<reader() done, defined at /home/vinicius/projects/dealer/venv/lib/python3.6/site-packages/uvicorn/protocols/websocket.py:65> exception=TypeError('An asyncio.Future, a coroutine or an awaitable is required',)>
Traceback (most recent call last):
  File "/home/vinicius/projects/dealer/venv/lib/python3.6/site-packages/uvicorn/protocols/websocket.py", line 72, in reader
    data = await protocol.recv()
  File "/home/vinicius/projects/dealer/venv/lib/python3.6/site-packages/websockets/protocol.py", line 309, in recv
    loop=self.loop, return_when=asyncio.FIRST_COMPLETED)
  File "/usr/lib/python3.6/asyncio/tasks.py", line 311, in wait
    fs = {ensure_future(f, loop=loop) for f in set(fs)}
  File "/usr/lib/python3.6/asyncio/tasks.py", line 311, in <setcomp>
    fs = {ensure_future(f, loop=loop) for f in set(fs)}
  File "/usr/lib/python3.6/asyncio/tasks.py", line 526, in ensure_future
    raise TypeError('An asyncio.Future, a coroutine or an awaitable is '
TypeError: An asyncio.Future, a coroutine or an awaitable is required

Minimal Reproduction

repro.py

async def chat_server(message, channels):
    """
    ASGI-style 'Hello, world' application.
    """
    if message['channel'] == 'websocket.connect':
        await channels['reply'].send({'accept': True})

Environment

Python: 3.6

What Broke

The application raises a TypeError when trying to receive data from the websocket.

Why It Broke

The transfer_data_task variable is not initialized, causing None to be added to the event queue

Fix Options (Details)

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

Upgrade to version 0.11.4 or later.

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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

Fix reference: https://github.com/kludex/uvicorn/pull/65

First fixed release: 0.11.4

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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 TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
0.11.4	Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:configuration-error All hubs All clusters

Related clusters: Data consistency Race condition SSL handshake

Sources

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