The Fix
pip install celery==5.5.0
Based on closed celery/celery issue #9360 · PR/commit linked
Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.
@@ -11,7 +11,6 @@
import click
import click.exceptions
-from click.types import ParamType
from click_didyoumean import DYMGroup
from click_plugins import with_plugins
repro.py
from celery import Celery
from celery.loaders.app import AppLoader
class ExampleAppLoader(AppLoader):
def on_worker_init(self):
print("ExampleAppLoader.on_worker_init() called")
app = Celery("example")
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\npip install celery==5.5.0\nWhen NOT to use: This fix should not be applied if the loader option is not intended to be used.\n\n
Why This Fix Works in Production
- Trigger: - [x] I have included all related issues and possible duplicate issues in this issue (If there are none, check this box anyway).
- Mechanism: The `celery --loader` option was ignored due to incorrect application import timing
- Why the fix works: Fixes the issue where the `celery --loader` option was ignored by importing the Celery application after setting the necessary environment variables. (first fixed release: 5.5.0).
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
- The `celery --loader` option was ignored due to incorrect application import timing
- Production symptom (often without a traceback): - [x] I have included all related issues and possible duplicate issues in this issue (If there are none, check this box anyway).
Proof / Evidence
- GitHub issue: #9360
- Fix PR: https://github.com/celery/celery/pull/9361
- First fixed release: 5.5.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.69
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“# Checklist - [x] I have verified that the issue exists against the main branch of Celery. - [ ] ~This has already been asked to the discussions forum first.~ - [x] I have read the relevant section in the contribution guide on reporting bug”
Failure Signature (Search String)
- - [x] I have included all related issues and possible duplicate issues in this issue (If there are none, check this box anyway).
- https://github.com/celery/celery/blob/03e3359320c9f72507a32a51ab61605db503da98/celery/app/base.py#L391-L397
Copy-friendly signature
signature.txt
Failure Signature
-----------------
- [x] I have included all related issues and possible duplicate issues in this issue (If there are none, check this box anyway).
https://github.com/celery/celery/blob/03e3359320c9f72507a32a51ab61605db503da98/celery/app/base.py#L391-L397
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
- [x] I have included all related issues and possible duplicate issues in this issue (If there are none, check this box anyway).
https://github.com/celery/celery/blob/03e3359320c9f72507a32a51ab61605db503da98/celery/app/base.py#L391-L397
Minimal Reproduction
repro.py
from celery import Celery
from celery.loaders.app import AppLoader
class ExampleAppLoader(AppLoader):
def on_worker_init(self):
print("ExampleAppLoader.on_worker_init() called")
app = Celery("example")
What Broke
The loader field displays the wrong loader instead of the specified one, causing confusion.
Why It Broke
The `celery --loader` option was ignored due to incorrect application import timing
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install celery==5.5.0
When NOT to use: This fix should not be applied if the loader option is not intended to be used.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option D — Guard side-effects with OnceOnly Guardrail for side-effects
Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.
- Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
- Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
- Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
Show example snippet (optional)
onceonly.py
from onceonly import OnceOnly
import os
once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)
# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..." # replace
key = f"stripe:webhook:{event_id}"
res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
return {"status": "already_processed"}
# Safe to execute the side-effect exactly once.
handle_event(event_id)
When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
Fix reference: https://github.com/celery/celery/pull/9361
First fixed release: 5.5.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if the loader option is not intended to be used.
- Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
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
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Version Compatibility Table
| Version | Status |
|---|---|
| 5.5.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.