Celery Chain with one task doesn't run (Fix)

The Fix

pip install celery==4.4.0rc5

Based on closed celery/celery issue #4498 · 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

@@ -779,10 +779,9 @@ class chain(_chain):
         # This forces `chain(X, Y, Z)` to work the same way as `X | Y | Z`
         if not kwargs and tasks:
-            if len(tasks) == 1 and is_list(tasks[0]):
-                # ensure chain(generator_expression) works.
-                tasks = tasks[0]

repro.py

import celery

@celery.task
def foo():
    print("hello, world")

f = foo.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 chaining behavior is intentionally designed to be different.\n\n

Why This Fix Works in Production

Trigger: (celery) ➜ myapp git:(master) ./manage.py test_cmd
Mechanism: The chain implementation did not handle single task chains correctly
Why the fix works: Fixes the issue where a chain with one task does not run as expected by modifying the chain implementation. (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

Shows up under Python 3.6 in real deployments (not just unit tests).
The chain implementation did not handle single task chains correctly
Surfaces as: (celery) ➜ myapp git:(master) ./manage.py test_cmd

Proof / Evidence

GitHub issue: #4498
Fix PR: https://github.com/celery/celery/pull/4730
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: 0.25

Discussion

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

Jump to Sources Open on GitHub

“@xbogdan - chain reduces to X | Y | Z if there are no kwargs, which means that chain(AddTask().si(1,2)) will return that signature object, not…”

@pachewise · 2018-01-28 · source

“Hi I think it's not just chains, here's a simpler test case: Output:”

@davmlaw · 2018-02-01 · source

“@davmlaw can you try binding the task, just to check if this gives us more insight:”

@georgepsarakis · 2018-02-03 · source

Failure Signature (Search String)

(celery) ➜ myapp git:(master) ./manage.py test_cmd

Error Message

Stack trace

error.txt

Error Message
-------------
(celery) ➜  myapp git:(master) ./manage.py test_cmd
Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/Users/admin/Projects/myapp/cel/management/commands/test_cmd.py", line 10, in handle
    c = chain(AddTask().si(1, 2), AddTask().si(1, 2))()
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/celery/canvas.py", line 533, in __call__
    return self.apply_async(args, kwargs)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/celery/canvas.py", line 559, in apply_async
    dict(self.options, **options) if options else self.options))
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/celery/canvas.py", line 586, in run
    first_task
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
(celery) ➜  myapp git:(master) ./manage.py test_cmd
Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/Users/admin/Projects/myapp/cel/management/commands/test_cmd.py", line 10, in handle
    c = chain(AddTask().si(1, 2))()
  File "/Users/admin/Envs/celery/lib/python3.6/site-packages/celery/canvas.py", line 178, in __call__
    return self.type(*args, **kwargs)
TypeError: object() takes no parameters

Stack trace

error.txt

Error Message
-------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-686caed5db4b> in <module>()
----> 1 f = foo.apply_async()

/usr/local/lib/python3.5/dist-packages/celery/app/task.py in apply_async(self, args, kwargs, task_id, producer, link, link_error, shadow, **options)
    530             args = args if isinstance(args, tuple) else tuple(args or ())
    531             args = (self.__self__,) + args
--> 532         shadow = shadow or self.shadow_name(args, kwargs, options)
    533 
    534         preopts = self._get_exec_options()

TypeError: shadow_name() missing 1 required positional argument: 'options'

Minimal Reproduction

repro.py

import celery

@celery.task
def foo():
    print("hello, world")

f = foo.apply_async()

Environment

Python: 3.6

What Broke

Tasks in a chain with one task do not execute as expected, leading to failures.

Why It Broke

The chain implementation did not handle single task chains correctly

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 chaining behavior is intentionally designed to be different.

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/4730

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 chaining behavior is intentionally designed to be different.
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

Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

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.

Celery Chain with one task doesn't run (Fix)

The Fix

Why This Fix Works in Production

Why This Breaks in Prod

Proof / Evidence

Discussion

Failure Signature (Search String)

Error Message

Minimal Reproduction

Environment

What Broke

Why It Broke

Fix Options (Details)

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

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Get updates

When NOT to Use This Fix

Verify Fix

Did This Fix Work in Your Case?

Prevention

Version Compatibility Table

Related Issues

Sources