Typically, the tags with no questions are automatically removed by a script which runs once a day, this process is called tag pruning.

However, there is a known bug which causes that if both tags in a tag synonym have zero question, they cannot be removed by this script. See this bug-report: Auto-delete orphaned tag synonyms. This post contains even the query to find such instances, at the moment it returns one such synonym on the main site and nothing on meta.

One can even check that when searching among tags, the tag is displayed. You can even see the synonym $\to$ in the list of tag synonyms - you'll need to go back to December 2014 when it was created. (I've added both screenshots below.)

This issue was mentioned in the tagging chatroom back in 2020. Today I asked one of the moderators about this, following the recommendation from there I mentioned the issue here on meta.

A screenshot showing what I get when searching among the tags:


A screenshot from the list of the tag synonyms:


When searching for the posts which had the tag , SEDE returns only a single question, here is a direct link to the revision history. SEDE returns no deleted questions tagged cyclicity.

A related question - if somebody stumbles upon a tag synonym like this, what is the right thing to do? Is asking on meta the safest option? (Although I assume that things like this are rather rare.)

  • 4
    $\begingroup$ After consulting with other moderators, it seems that removing the link between the two tags should allow the tags to be deleted. I have, therefore, un-synonymized the two tags. They should both be removed at the next cleanup. I have also marked this post status-review, in order to bring the bug in the tag system to the attention of staff. $\endgroup$
    – Xander Henderson Mod
    Dec 5, 2023 at 15:17
  • $\begingroup$ I no longer see this tag when searching in the list of tags - I posted a screenshot in the tagging chatroom. So I suppose this could be considered as resolved. (Unless somebody see some other place where the tag appears.) Thanks for the prompt response! $\endgroup$ Dec 6, 2023 at 8:27
  • $\begingroup$ For this particular tag, the problem is resolved. However, I do think that the non-deletion of empty tags which have synonyms is a problem. I would call it a bug, and have tagged this question so that SE staff can weigh in on whether or not this is something which should be fixed. $\endgroup$
    – Xander Henderson Mod
    Dec 6, 2023 at 14:29

1 Answer 1


Unfortunately, we won't be able to prevent this from happening in the next 6-8 weeks. After some investigation, we found the impact is pretty small, only about 20 instances network-wide, and as pointed out by Xander, there are still some things we can do to prevent it.

I am marking this bug as declined, but it is on the backlog to be fixed at some point greater than 6-8 weeks.

  • 1
    $\begingroup$ Thanks for the explanation. $\endgroup$
    – Xander Henderson Mod
    Mar 9 at 14:07

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