Celery Can't inspect with destination option (Fix)

The Fix

pip install celery==5.0.1

Based on closed celery/celery issue #6399 · 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

@@ -168,7 +168,7 @@ class CommaSeparatedList(ParamType):
 
     def convert(self, value, param, ctx):
-        return set(text.str_to_list(value))
+        return text.str_to_list(value)

repro.py

Traceback (most recent call last):
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/bin/celery", line 8, in <module>
    sys.exit(main())
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/__main__.py", line 15, in main
    sys.exit(_main())
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/bin/celery.py", line 150, in main
    return celery(auto_envvar_prefix="CELERY")
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/bin/control.py", line 125, in inspect
    replies = ctx.obj.app.control.inspect(timeout=timeout,
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/app/control.py", line 97, in _request
    return self._prepare(self.app.control.broadcast(
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/app/control.py", line 477, in broadcast
    return self.mailbox(conn)._broadcast(
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/kombu/pidbox.py", line 319, in _broadcast
    raise ValueError(
ValueError: destination must be a list/tuple not <class 'set'>
Sentry is attempting to send 0 pending error messages
Waiting up to 2 seconds
Press Ctrl-C to quit

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==5.0.1\nWhen NOT to use: This fix should not be applied if the application relies on set behavior for CommaSeparatedList.\n\n

Why This Fix Works in Production

Trigger: sys.exit(main())
Mechanism: CommaSeparatedList returns a set instead of a list, causing compatibility issues
Why the fix works: Fixes the issue by returning a list instead of a set in the CommaSeparatedList class, addressing compatibility with the _broadcast method of kombu Mailbox. (first fixed release: 5.0.1).

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 3.8 in real deployments (not just unit tests).
CommaSeparatedList returns a set instead of a list, causing compatibility issues
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #6399
Fix PR: https://github.com/celery/celery/pull/6400
First fixed release: 5.0.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.29

Discussion

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

Jump to Sources Open on GitHub

“https://github.com/celery/celery/blob/8a92b7128bc921d4332fe01486accc243115aba8/celery/bin/base.py#L171 CommaSeparatedList return set instead list”

@aszubarev · 2020-10-07 · source

Failure Signature (Search String)

sys.exit(main())

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/bin/celery", line 8, in <module>
    sys.exit(main())
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/__main__.py", line 15, in main
    sys.exit(_main())
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/bin/celery.py", line 150, in main
    return celery(auto_envvar_prefix="CELERY")
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(sel
... (truncated) ...

Minimal Reproduction

repro.py

Traceback (most recent call last):
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/bin/celery", line 8, in <module>
    sys.exit(main())
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/__main__.py", line 15, in main
    sys.exit(_main())
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/bin/celery.py", line 150, in main
    return celery(auto_envvar_prefix="CELERY")
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/bin/control.py", line 125, in inspect
    replies = ctx.obj.app.control.inspect(timeout=timeout,
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/app/control.py", line 97, in _request
    return self._prepare(self.app.control.broadcast(
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/celery/app/control.py", line 477, in broadcast
    return self.mailbox(conn)._broadcast(
  File "/Users/aszubarev/Library/Caches/pypoetry/virtualenvs/service-notification-py-xHAuYcng-py3.8/lib/python3.8/site-packages/kombu/pidbox.py", line 319, in _broadcast
    raise ValueError(
ValueError: destination must be a list/tuple not <class 'set'>
Sentry is attempting to send 0 pending error messages
Waiting up to 2 seconds
Press Ctrl-C to quit

Environment

Python: 3.8

What Broke

Users cannot inspect tasks with the destination option, leading to operational disruptions.

Why It Broke

CommaSeparatedList returns a set instead of a list, causing compatibility issues

Fix Options (Details)

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

pip install celery==5.0.1

When NOT to use: This fix should not be applied if the application relies on set behavior for CommaSeparatedList.

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/6400

First fixed release: 5.0.1

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

When NOT to Use This Fix

This fix should not be applied if the application relies on set behavior for CommaSeparatedList.

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
5.0.1	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.