Celery task serializer ignored with CELERY_ALWAYS

The Fix

pip install celery==4.4.0rc5

Based on closed celery/celery issue #5548 · 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

@@ -531,6 +531,18 @@ def apply_async(self, args=None, kwargs=None, task_id=None, producer=None,
                 check_arguments(*(args or ()), **(kwargs or {}))
 
+        if self.__v2_compat__:
+            shadow = shadow or self.shadow_name(self(), args, kwargs, options)
+        else:

repro.py

def test_always_eager_with_task_serializer_option(self):
        self.app.conf.task_always_eager = True

        @self.app.task(serializer='pickle')
        def task(*args, **kwargs):
            pass
        task.apply_async((1, 2, 3, 4, {1}))

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 you rely on the previous behavior of ignoring the serializer.\n\n

Why This Fix Works in Production

Trigger: Error `kombu.exceptions.EncodeError: Object of type set is not JSON serializable`
Mechanism: The task serializer is ignored when CELERY_ALWAYS_EAGER is set to True
Why the fix works: Fixes the issue where the task serializer is ignored when CELERY_ALWAYS_EAGER is set to True. (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

The task serializer is ignored when CELERY_ALWAYS_EAGER is set to True
Surfaces as: Error `kombu.exceptions.EncodeError: Object of type set is not JSON serializable`

Proof / Evidence

GitHub issue: #5548
Fix PR: https://github.com/celery/celery/pull/5549
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: 0.61

Discussion

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

Jump to Sources Open on GitHub

“Would be really really nice if the fix could go out as 4.3.1 unless the 4.4.0 release is imminent. Our task with custom serialiser is…”

@ippeiukai · 2019-05-28 · source

Failure Signature (Search String)

Error `kombu.exceptions.EncodeError: Object of type set is not JSON serializable`

Error Message

Stack trace

error.txt

Error Message
-------------
Error `kombu.exceptions.EncodeError: Object of type set is not JSON serializable`

Minimal Reproduction

repro.py

def test_always_eager_with_task_serializer_option(self):
        self.app.conf.task_always_eager = True

        @self.app.task(serializer='pickle')
        def task(*args, **kwargs):
            pass
        task.apply_async((1, 2, 3, 4, {1}))

What Broke

Tasks with custom serializers fail to serialize correctly in eager mode.

Why It Broke

The task serializer is ignored when CELERY_ALWAYS_EAGER is set to True

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 you rely on the previous behavior of ignoring the serializer.

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/5549

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 you rely on the previous behavior of ignoring the serializer.

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.

Celery task serializer ignored with CELERY_ALWAYS_EAGER (Fix)