It seems that Render's Ruby version detection via the Gemfile (i.e. if there's no .ruby-version file) does not take into account file references - a feature that Bundler supports itself.

i.e. I have in my Gemfile, and it understands the syntax of ASDF's file, and reads the appropriate version out. When my services try to build, though, they mistake that filename as the version number:

Given Gemfile.lock has the computed version number, perhaps that could be referred to instead of relying on parsing the Ruby code in a Gemfile? (And ditto for gems.locked instead of gems.rb). Or even, use the .tool-versions file as a version source?

I've tweaked the internal file to cover both of these scenarios, hopefully it's useful: