Upgrading to Ruby 4.0

published 02/14/26

Ruby 4.0 was released a couple months ago on Christmas and I had been meaning to upgrade for a while. I noticed that version 4.0.1 came out too so that's always a good sign that things are stable and ready for prime time.

Reviewing Release Notes

I started by reviewing the release post on the official Ruby blog. Nothing really stood out as all that controversial and so I figured it would be a pretty straightforward upgrade. It was.

But that isn't to say that as a fan of the Ruby language I'm not excited by what was in those release notes - there are some cool things in Ruby 4! You should search that page for "ErrorHighlight" and see what they did to improve the error messages for ArgumentError.

Installing

I still use asdf as my Ruby version manager so next was installing:

$ asdf plugin update --all
$ asdf install ruby 4.0.1
$ asdf list ruby
  3.3.10
 *3.4.8
  4.0.1

So the goal is to upgrade all my projects to Ruby 4.0.1 and then uninstall those other versions.

Setting Default Ruby

My dotfiles repo is where I set the default Ruby for my system so I made Upgrade default Ruby to set this to 4.0.1.

Upgrade All the Projects

Now the toil - go to each project and do a PR that upgrades it to Ruby 4.0.1:

• jay
• jonallured.com
• mli
• monolithium
• shrt

Mostly the process was:

• edit the .tool-versions file to switch to 4.0.1
• run bundle install to install the project's gems
• run bundle update --bundler to get the newest bundler version
• run bundle update --all to upgrade the project's gems
• run bundle exec rake to see if anything broke

Oh and then if a project has a Ruby version matrix on CI then I'd also drop the 3.2 and add 4.0. I actually forgot this on jay so I opened Update CI matrix as a follow up.

Monolithium was the project that I assumed would be the most challenging to upgrade but it Just Worked. I did notice that it needs a Rails upgrade and I have been ignoring the Tailwind upgrade as well. Those are problems for Future Jon!

Cleaning Up

This was an easy upgrade and the final step was to clean up my older Ruby installs:

$ asdf uninstall ruby 3.3.10
$ asdf uninstall ruby 3.4.8
$ asdf list ruby
 *4.0.1

I love boring software!