til/heroku/check-ruby-version-for-production-app.md

Check Ruby Version For Production App

While deploying a fresh Rails app to Heroku recently, I ran into an issue. The it block argument wasn't working despite being on Ruby 4.0. Or so I thought.

Running the following command reported the Ruby version of that Heroku server instance:

❯ heroku run -- ruby --version
Running ruby --version on ⬢ my-app... up, run.3090
ruby 3.3.9 (2025-07-24 revision f5c772fc7c) [x86_64-linux]

I was on 3.3.9 which must have been the fallback default at the time.

Though I had set the Ruby version in my .ruby-version file, I had neglected to specify it in the Gemfile as well. Once I added it to the Gemfile and redeployed, my Heroku server instance was running the expected version of Ruby.

❯ heroku run -- ruby --version
Running ruby --version on ⬢ my-app... up, run.5353
ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [x86_64-linux]

Note: because I have set HEROKU_ORGANIZATION and HEROKU_APP in my environment (.envrc) for the local copy of the app, I don't need to specify those when running the heroku run command above.

See heroku run --help for more details.