Celery getting celery.exceptions.Reject:

The Fix

pip install celery==5.3.0a1

Based on closed celery/celery issue #9174 · 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.

Jump to Verify Open PR/Commit

@@ -286,4 +286,5 @@ Patrick Zhang, 2017/08/19
 kronion, 2021/08/26
 Gabor Boros, 2021/11/09
-Tizian Seehaus, 2022/02/09
\ No newline at end of file
+Tizian Seehaus, 2022/02/09

repro.py

@CELERY_APP.task(base=BaseCurrentUserTask, bind=True, name="test_current_user", retry_kwargs={"max_retries": 2})
def test_current_user(self):
    from time import sleep

    try:
        if self.request.retries < self.max_retries:
            sleep(2)
            raise Exception()
        else:
            return "Last retry, no exception raised"
    except Exception as e:
        raise self.retry(exc=e)

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==5.3.0a1\nWhen NOT to use: This fix is not applicable if the task does not use custom headers.\n\n

Why This Fix Works in Production

Trigger: [2024-08-05 14:49:52,482: WARNING/ForkPoolWorker-7] Task test_current_user[606303f0-16fc-4eff-9dea-8355f503ed8f] reject requeue=False: 'headers'
Mechanism: Custom headers were not properly propagated during task retries in Celery
Why the fix works: Fixes the issue where custom headers were not properly propagated during task retries in Celery. (first fixed release: 5.3.0a1).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Shows up under Python 3.9 in real deployments (not just unit tests).
Custom headers were not properly propagated during task retries in Celery
Surfaces as: [2024-08-05 14:49:52,482: WARNING/ForkPoolWorker-7] Task test_current_user[606303f0-16fc-4eff-9dea-8355f503ed8f] reject requeue=False: 'headers'

Proof / Evidence

GitHub issue: #9174
Fix PR: https://github.com/celery/celery/pull/7555
First fixed release: 5.3.0a1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.38

Discussion

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

Jump to Sources Open on GitHub

“well I made silly mistake forgot include the headers argument in the base class apply_async method, adding it sovled my issue.”

@aashayamballi · 2024-08-05 · source

“I have observed some inconsistencies in the documentation for the apply_async method of the Celery Task class”

@aashayamballi · 2024-08-05 · source

Failure Signature (Search String)

[2024-08-05 14:49:52,482: WARNING/ForkPoolWorker-7] Task test_current_user[606303f0-16fc-4eff-9dea-8355f503ed8f] reject requeue=False: 'headers'

Error Message

Stack trace

error.txt

Error Message
-------------
[2024-08-05 14:49:52,482: WARNING/ForkPoolWorker-7] Task test_current_user[606303f0-16fc-4eff-9dea-8355f503ed8f] reject requeue=False: 'headers'
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/celery/app/autoretry.py", line 38, in run
    return task._orig_run(*args, **kwargs)
  File "/code/projects/tasks.py", line 71, in test_current_user
    raise Exception()
Exception

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/celery/app/task.py", line 753, in retry
    S.apply_async()
  File "/usr/local/lib/python3.9/site-packages/celery/canvas.py", line 400, in apply_async
    return _apply(args, kwargs, **options)
  File "/code/core/celery.py", line 324, in apply_async
    super().apply_async(args, kwargs, task_id, producer, link, link_error, shadow, headers=headers, **options)
KeyError: 'headers'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/celery/app/trace.py", line 453, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/sentry_sdk/integrations/celery.py", line 306, in _inner
    reraise(*exc_info)
  File "/usr/local/lib/python3.9/site-packages/sentry_sdk/_compat.py", line 127, in reraise
    r
... (truncated) ...

Minimal Reproduction

repro.py

@CELERY_APP.task(base=BaseCurrentUserTask, bind=True, name="test_current_user", retry_kwargs={"max_retries": 2})
def test_current_user(self):
    from time import sleep

    try:
        if self.request.retries < self.max_retries:
            sleep(2)
            raise Exception()
        else:
            return "Last retry, no exception raised"
    except Exception as e:
        raise self.retry(exc=e)

Environment

Python: 3.9

What Broke

Tasks failed with KeyError when custom headers were set and retried.

Why It Broke

Custom headers were not properly propagated during task retries in Celery

Fix Options (Details)

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

pip install celery==5.3.0a1

When NOT to use: This fix is not applicable if the task does not use custom headers.

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.
This does NOT fix data corruption; it only prevents duplicate side-effects.

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)

See OnceOnly SDK

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/7555

First fixed release: 5.3.0a1

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

When NOT to Use This Fix

This fix is not applicable if the task does not use custom headers.
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
5.3.0a1	Fixed

Related Issues

No related fixes found.

Cluster: celery:data-consistency Celery hub Celery best practices All hubs All clusters

Related clusters: Configuration error Duplicates Duplicate processing

Sources

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

Celery getting celery.exceptions.Reject: (KeyError('headers'), False) when custom headers is set (Fix)