Ruby support for Bundler 1.x re-enabled until April 30th, 2025

Change effective on 21 March 2025

Ruby applications using bundler 1.x will resume being able to deploy effective immediately until April 30 2025. This update changes behavior rolled out on March 19th, 2025.

On March 19, 2025, Heroku changed the heroku/ruby buildpack’s default Ruby version which prevented the evaluation of gems with Bundler 1.x. This version of Bundler was last released in December 2018 and is not receiving support from Bundler Core. That means Bundler 1.x will not work with Ruby 3.2 or 3.3 due to an error:

remote:  !     There was an error parsing your Gemfile, we cannot continue
remote:  !     /tmp/d20250319-150-o0ictl/bundler-1.17.3/gems/bundler-1.17.3/lib/bundler/shared_helpers.rb:29:in `root': undefined method `untaint' for an instance of Pathname (NoMethodError)
remote:  !
remote:  !     Pathname.new(gemfile).untaint.expand_path.parent
remote:  !     ^^^^^^^^
remote:  !     from /tmp/d20250319-150-o0ictl/bundler-1.17.3/gems/bundler-1.17.3/lib/bundler.rb:234:in `root'

While the vast majority of applications on the platform use a modern version of bundler, it’s more likely that applications still trying to migrate off of heroku-20 are using bundler 1.x of those applications, Rails 5.x apps can easily upgrade to bundler 2.3.27 while Rails 4.x applications are hard locked to bundler 1.x. While technically that also means they’re using Unsupported Ruby versions, we felt that delaying the rollout of this internal Ruby upgrade (which is at the root of the bundler 1.x limitation) would help customers in their upgrade process.

We strongly recommend that customers upgrade their application to a non-EOL version of Ruby and Rails. For the highest levels of stability and security, we recommend staying on the latest Ruby, Rails, and Heroku stack.

Bundler 1.x will no longer work on Heroku after April 30, 2025

Customers using Bundler 1.x after April 30, 2025, will see an error message when deploying and are encouraged to upgrade to Bundler 2.3.27. To prevent this error, upgrade your bundler version as soon as possible. Run:

$ gem install bundler -v 2.3.27
$ bundle update --bundler

Verify that the correct version is listed in the Gemfile.lock:

$ cat Gemfile.lock | grep "BUNDLED WITH" -A2
BUNDLED WITH
   2.3.27

And check the results into git:

$ git add Gemfile.lock
$ git commit -m "Update bundler version to 2.3.x"

Bundler 1.17.3 was last released on December 27, 2018, and is no longer supported by bundler/rubygems core. The exception occurs because the heroku/ruby buildpack uses ruby. It invokes bundle platform --ruby to determine what Ruby version the application needs. The version of Ruby the buildpack uses is incompatible with the version of bundler requested by the application.