Help: liquid tags not resolving to partials from _includes directory

Hello! After building a new site, partials from the _includes directory are not rendering in an index.html file. I built the site using jekyll new test --blank and created a file _includes/content.html. The body of index html is as follows:

<!DOCTYPE html>
{% include content.html %}

And the body of content.html:


yet http://localhost:4000/ shows:

{% include content.html %}

Currently running jekyll version 3.4.3. I have tried reinstalling jekyll and ruby, overwriting the _includes directory in a gem-based theme, and just generally banging my head against the keyboard. I can’t seem to find this error replicated anywhere in 3.4.3, so I know that it must be something small and silly that I’ve missed. Any guidance would be appreciated.