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uvicorn reload flag causes failure to start (Fix)

Fixed
Spec
Package
uvicorn
Fixed In
0.11.4
Python
3.6
Verdict
Status
Fixed
Safe for prod
No
Risk
Medium
Last verified
2026-02-09
Own content ratio
0.28
#374 ai tls Updated: 2026-02-13
Issue #374

The Fix

Upgrade to version 0.11.4 or later.

Based on closed Kludex/uvicorn issue #374 · 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
@@ -141,10 +141,14 @@ def __init__( self.loaded = False + @property + def is_ssl(self) -> bool: + return self.ssl_keyfile or self.ssl_certfile
repro.py
docker run -it -p 8000:8000 ubuntu bash apt update apt install --yes python3 python3-dev python3-pip neovim pip3 install uvicorn nvim example.py # paste in the example code from README.md export LC_ALL=C.UTF-8 # fix system encoding -- this is not required if you run this in a proper VM or on bare metal export LANG=C.UTF-8 uvicorn example:app # this works -- visit 127.0.0.1:8000 uvicorn example:app --reload # this causes an error, see below
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: This fix should not be used if SSL context is not properly configured.\n\n

Why This Fix Works in Production

  • Trigger: root@4a74f603e723:/# uvicorn example:app --reload
  • Mechanism: The Config class did not initialize the ssl attribute, causing an AttributeError when using the --reload flag
  • Why the fix works: Added a property to the Config class to check for SSL context, resolving the AttributeError when using the --reload flag. (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 Config class did not initialize the ssl attribute, causing an AttributeError when using the --reload flag
  • Surfaces as: root@4a74f603e723:/# uvicorn example:app --reload

Proof / Evidence

  • GitHub issue: #374
  • Fix PR: https://github.com/encode/uvicorn/pull/375
  • First fixed release: 0.11.4
  • Reproduced locally: No (not executed)
  • Last verified: 2026-02-09
  • Confidence: 0.75
  • 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
“Same issue here”
@kthwaite · 2019-06-18 · repro detail · source
“+1 Just started happening. Issue does not exist in 0.7.2 both --reload and --debug flags are affected.”
@gnat · 2019-06-18 · source
“Meh. What's our options here - do we just have to revert #362 / #352?”
@lovelydinosaur · 2019-06-18 · source
“Does Config.bind_socket need an actual SSL context, or just a check equivalent to the one being made in Config.load? i.e”
@kthwaite · 2019-06-18 · source

Failure Signature (Search String)

  • root@4a74f603e723:/# uvicorn example:app --reload

Error Message

Stack trace
error.txt
Error Message ------------- root@4a74f603e723:/# uvicorn example:app --reload Traceback (most recent call last): File "/usr/local/bin/uvicorn", line 11, in <module> sys.exit(main()) File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 764, in __call__ return self.main(*args, **kwargs) File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 717, in main rv = self.invoke(ctx) File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 956, in invoke return ctx.invoke(self.callback, **ctx.params) File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 555, in invoke return callback(*args, **kwargs) File "/usr/local/lib/python3.6/dist-packages/uvicorn/main.py", line 257, in main run(**kwargs) File "/usr/local/lib/python3.6/dist-packages/uvicorn/main.py", line 270, in run socket = config.bind_socket() File "/usr/local/lib/python3.6/dist-packages/uvicorn/config.py", line 222, in bind_socket protocol_name = "https" if self.ssl else "http" AttributeError: 'Config' object has no attribute 'ssl'
Stack trace
error.txt
Error Message ------------- Traceback (most recent call last): File "/home/kthwaite/.virtualenvs/test/bin/uvicorn", line 10, in <module> sys.exit(main()) File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/click/core.py", line 764, in __call__ return self.main(*args, **kwargs) File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/click/core.py", line 717, in main rv = self.invoke(ctx) File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/click/core.py", line 956, in invoke return ctx.invoke(self.callback, **ctx.params) File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/click/core.py", line 555, in invoke return callback(*args, **kwargs) File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/uvicorn/main.py", line 257, in main run(**kwargs) File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/uvicorn/main.py", line 274, in run socket = config.bind_socket() File "/home/kthwaite/.virtualenvs/test/local/lib/python3.6/dist-packages/uvicorn/config.py", line 222, in bind_socket protocol_name = "https" if self.ssl else "http" AttributeError: 'Config' object has no attribute 'ssl'

Minimal Reproduction

repro.py
docker run -it -p 8000:8000 ubuntu bash apt update apt install --yes python3 python3-dev python3-pip neovim pip3 install uvicorn nvim example.py # paste in the example code from README.md export LC_ALL=C.UTF-8 # fix system encoding -- this is not required if you run this in a proper VM or on bare metal export LANG=C.UTF-8 uvicorn example:app # this works -- visit 127.0.0.1:8000 uvicorn example:app --reload # this causes an error, see below

Environment

  • Python: 3.6

What Broke

Uvicorn fails to start with an AttributeError when using the --reload flag.

Why It Broke

The Config class did not initialize the ssl attribute, causing an AttributeError when using the --reload flag

Fix Options (Details)

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

Upgrade to version 0.11.4 or later.

When NOT to use: This fix should not be used if SSL context is not properly configured.

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

Fix reference: https://github.com/encode/uvicorn/pull/375

First fixed release: 0.11.4

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

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When NOT to Use This Fix

  • This fix should not be used if SSL context is not properly configured.

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

VersionStatus
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.

Source & Disclaimer: Analysis derived from GitHub Issue #374. Not affiliated with the library maintainers. Verify before production use.