Ruby applications with no specified bundler versions now receive Bundler 2.5.23

Change effective on 02 February 2026

If your Gemfile.lock doesn’t include the BUNDLED WITH key, Heroku installs a default bundler version:

Ruby’s standard version of bundler takes precedence if it’s greater than Heroku’s installed version. When there is no BUNDLED WITH in the Gemfile.lock, then bundle install uses the highest version of Bundler available.

It is strongly recommended that you have both a RUBY VERSION and BUNDLED WITH version listed in your Gemfile.lock. If you do not have those values, you can generate them and commit them to git:

$ bundle update --ruby
$ git add Gemfile.lock
$ git commit -m "Update Gemfile.lock"

Applications without these values specified in the Gemfile.lock may break unexpectedly when the defaults change.