Celery task replace chord inside chord not working. (Fix)

The Fix

pip install celery==4.4.0rc5

Based on closed celery/celery issue #4368 · 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

@@ -11,7 +11,8 @@
 from celery._state import _task_stack
 from celery.canvas import signature
-from celery.exceptions import Ignore, MaxRetriesExceededError, Reject, Retry
+from celery.exceptions import (Ignore, ImproperlyConfigured,
+                               MaxRetriesExceededError, Reject, Retry)

repro.py

@app.task(bind=True, ignore_result=True)
def some_task(self, args=None, **kwargs):
    #add_to_chord not working too
    #self.add_to_chord(chord([echo.s(name='t1'), echo_s.s(name='t2'), echo.s(name='t3')], echo.s(name='inner_chord_end')))
    raise self.replace(chord([echo.s(name='t1'), echo.s(name='t2'), echo.s(name='t3')], echo.s(name='inner_chord_end')))

@app.task(bind=True, ignore_result=True)
def echo(self, args=None, **kwargs):
    print(kwargs['name'])
    return kwargs['name']


result = chord([some_task.s(), echo.s(name='task')], echo.s(name='total end')).apply_async()

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==4.4.0rc5\nWhen NOT to use: Do not use this fix if the task is not part of a chord.\n\n

Why This Fix Works in Production

Trigger: The outer chord body was not called, leading to skipped tasks.
Mechanism: Replacing a task in a chord with another chord did not trigger the outer chord body
Why the fix works: Fixes the issue where replacing a task in a chord with another chord did not work, preventing the outer chord body from being called. (first fixed release: 4.4.0rc5).

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

Replacing a task in a chord with another chord did not trigger the outer chord body
Production symptom (often without a traceback): The outer chord body was not called, leading to skipped tasks.

Proof / Evidence

GitHub issue: #4368
Fix PR: https://github.com/celery/celery/pull/4369
First fixed release: 4.4.0rc5
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.52

Discussion

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

Jump to Sources Open on GitHub

“## Steps to reproduce if we trying to replace task in a chord by another chord we have a problem - outer chord body not called add_to_chord not working too for example ## Expected behavior echo.s(name='total end') called ## Actual behavior”

Issue thread · issue description · source

Failure Signature (Search String)

The outer chord body was not called, leading to skipped tasks.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
The outer chord body was not called, leading to skipped tasks.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
The outer chord body was not called, leading to skipped tasks.

Minimal Reproduction

repro.py

@app.task(bind=True, ignore_result=True)
def some_task(self, args=None, **kwargs):
    #add_to_chord not working too
    #self.add_to_chord(chord([echo.s(name='t1'), echo_s.s(name='t2'), echo.s(name='t3')], echo.s(name='inner_chord_end')))
    raise self.replace(chord([echo.s(name='t1'), echo.s(name='t2'), echo.s(name='t3')], echo.s(name='inner_chord_end')))

@app.task(bind=True, ignore_result=True)
def echo(self, args=None, **kwargs):
    print(kwargs['name'])
    return kwargs['name']


result = chord([some_task.s(), echo.s(name='task')], echo.s(name='total end')).apply_async()

What Broke

The outer chord body was not called, leading to skipped tasks.

Why It Broke

Replacing a task in a chord with another chord did not trigger the outer chord body

Fix Options (Details)

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

pip install celery==4.4.0rc5

When NOT to use: Do not use this fix if the task is not part of a chord.

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/4369

First fixed release: 4.4.0rc5

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

When NOT to Use This Fix

Do not use this fix if the task is not part of a chord.

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
4.4.0rc5	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.