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.