Celery The result of the task will prepend to the signature

The Fix

pip install celery==5.6.0

Based on closed celery/celery issue #9745 · 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.

Open PR/Commit

@@ -244,7 +244,7 @@ arguments:
 
 As expected this will first launch one task calculating :math:`2 + 2`, then
-another task calculating :math:`8 + 4`.
+another task calculating :math:`4 + 8`.

fix.md

Option A — Upgrade to fixed release\npip install celery==5.6.0\nWhen NOT to use: This fix is not applicable if the order of arguments does not impact task execution.\n\n

Why This Fix Works in Production

Trigger: - [x] I have included all related issues and possible duplicate issues in this issue
Mechanism: The documentation incorrectly stated the order of arguments in the callback section
Why the fix works: Updates the documentation to correct the calculation order in the callback section. (first fixed release: 5.6.0).

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 documentation incorrectly stated the order of arguments in the callback section
Production symptom (often without a traceback): - [x] I have included all related issues and possible duplicate issues in this issue

Proof / Evidence

GitHub issue: #9745
Fix PR: https://github.com/celery/celery/pull/9758
First fixed release: 5.6.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.76

Discussion

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

Jump to Sources Open on GitHub

“here is a PR https://github.com/celery/celery/pull/9758”

@auvipy · 2025-06-15 · source

Failure Signature (Search String)

- [x] I have included all related issues and possible duplicate issues in this issue
or possible duplicates to this issue as requested by the checklist above.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
- [x] I have included all related issues and possible duplicate issues in this issue
or possible duplicates to this issue as requested by the checklist above.

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
or possible duplicates to this issue as requested by the checklist above.

What Broke

Users may experience incorrect task execution order leading to unexpected results.

Why It Broke

The documentation incorrectly stated the order of arguments in the callback section

Fix Options (Details)

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

pip install celery==5.6.0

When NOT to use: This fix is not applicable if the order of arguments does not impact task execution.

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)

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/9758

First fixed release: 5.6.0

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

When NOT to Use This Fix

This fix is not applicable if the order of arguments does not impact task execution.
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.

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
5.6.0	Fixed

Related Issues

No related fixes found.

Cluster: celery:configuration-error Celery hub Celery best practices All hubs All clusters

Related clusters: Data consistency Duplicates Duplicate processing

Sources

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

Celery The result of the task will prepend to the signature arguments in callback, not append. (Fix)