Celery Group result ordering is not respected for groups

The Fix

pip install celery==5.6.0

Based on closed celery/celery issue #9909 · 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

@@ -1741,7 +1741,7 @@ def _prepared(self, tasks, partial_args, group_id, root_id, app,
     def _apply_tasks(self, tasks, producer=None, app=None, p=None,
                      add_to_parent=None, chord=None,
-                     args=None, kwargs=None, **options):
+                     args=None, kwargs=None, group_index=None, **options):
         """Run all the tasks in the group.

repro.py

from celery import group, shared_task

@shared_task
def add(x, y):
    return x + y

@shared_task(bind=True)
def group_replace(self):
    self.replace(group([add.s(1 ,1), add.s(2, 2)]))


print(group_replace.apply_async().get())  # <- should be [2, 4], sometimes is [4, 2]

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==5.6.0\nWhen NOT to use: This fix should not be used if the task replacement behavior is intended to maintain the original group order.\n\n

Why This Fix Works in Production

Trigger: - [x] I have included all related issues and possible duplicate issues
Mechanism: The group_index value passed to _apply_tasks overrides the expected ordering of tasks in a group
Why the fix works: Preserves group ordering when replacing a task with a group by preventing the passing of a `group_index` in the options. (first fixed release: 5.6.0).

Production impact:

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

Why This Breaks in Prod

The group_index value passed to _apply_tasks overrides the expected ordering of tasks in a group
Production symptom (often without a traceback): - [x] I have included all related issues and possible duplicate issues

Proof / Evidence

GitHub issue: #9909
Fix PR: https://github.com/celery/celery/pull/9910
First fixed release: 5.6.0
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

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

Jump to Sources Open on GitHub

“I reproduced this issue locally (in my case group tasks replacing with a group, with redis backend)”

@chrisbarber · 2025-09-30 · source

“I identified the issue to be here https://github.com/celery/celery/blob/70087e49888cefe092ac2068ceb37769d8e8dd4b/celery/canvas.py#L1793”

@simonepozzoli-pix4d · 2025-09-24 · source

Failure Signature (Search String)

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

Minimal Reproduction

repro.py

from celery import group, shared_task

@shared_task
def add(x, y):
    return x + y

@shared_task(bind=True)
def group_replace(self):
    self.replace(group([add.s(1 ,1), add.s(2, 2)]))


print(group_replace.apply_async().get())  # <- should be [2, 4], sometimes is [4, 2]

What Broke

Tasks in a group return results in random order instead of the expected sequence.

Why It Broke

The group_index value passed to _apply_tasks overrides the expected ordering of tasks in a group

Fix Options (Details)

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

pip install celery==5.6.0

When NOT to use: This fix should not be used if the task replacement behavior is intended to maintain the original group order.

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

Fix reference: https://github.com/celery/celery/pull/9910

First fixed release: 5.6.0

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the task replacement behavior is intended to maintain the original group order.

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
5.6.0	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 Group result ordering is not respected for groups replacing a task (Fix)