Yes, you should change that if you want your new hierarchical tag tree and search to work reliably.
While Eleventy is smart enough to try and "fix" a comma-separated list like tags: one,two,four, it is technically interpreting that as a single string rather than a proper list (array).
Why you should change it
- Hierarchical Breaking: Your groupTagsByPath filter expects to loop through individual items. If it receives "one,two,four", it will try to find a tag named exactly one,two,four instead of three separate tags.
- Slash Confusion: Once you start using Level1/Level2, the comma-separated format becomes very hard for the computer to read without making mistakes.
---
The Recommended Change
You should switch to the YAML List format (Format A from our previous talk). It’s cleaner and prevents bugs:
YAML
title: My First Blog Post
date: 2026-02-01
tags:
- one
- two
- four
Or, if you prefer a single line, use the Square Bracket format:
YAML
tags: ["one", "two", "four"]
---
Pro Tip: Check your current tags
If you have already written many posts with the tags: one,two format and don't want to change them all manually, we could add a "cleanup" step in your .eleventy.js filter.
However, it is much better practice to fix the source data now while your blog is still young. It ensures that every plugin or feature you add in the future "just works."
Would you like me to provide a quick command-line "cheat sheet" on how to bulk-update your tags if you have a lot of files to change?