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Ruby 3.4.5 on macOS SaaS runner breaks bundle install due to ffi incompatibility

Summary

GitLab SaaS macOS runners now use Ruby 3.4.5 by default, which causes bundle install to fail when native gems (e.g. ffi) are included in the Gemfile. These gems explicitly disallow Ruby ≥ 3.4, making it impossible to install the bundle. There is currently no way to override or downgrade the Ruby version in the macOS environment.

Steps to reproduce

Use the following job definition in a .gitlab-ci.yml file. This job fails due to ffi incompatibility with Ruby 3.4.5.

.gitlab-ci.yml 
mac_build:
  tags:
    - saas-macos-medium-m1
  image: macos-15-xcode-16
  cache:
    key: vendor-ruby
    paths:
      - vendor/
  script:
    - bundle config set --local path vendor/bundle
    - bundle check || bundle install --jobs $(nproc)
    - bundle exec fastlane build

Actual behavior

bundle install fails immediately with an error due to Ruby 3.4.5 being incompatible with ffi, which is used by cocoapods, fastlane, and other critical gems. This blocks iOS build pipelines.

Expected behavior

bundle install should succeed, either by:

  • Using a Ruby version < 3.4 (e.g., 3.2 or 3.3), or
  • Allowing the Ruby version to be overridden in the job definition (like on Linux runners)

Relevant logs and/or screenshots

job log 
ffi-1.17.0-arm64-darwin requires ruby version < 3.4.dev, >= 2.5, which is incompatible with the current version, ruby 3.4.5p51

Environment description

  • Shared GitLab.com SaaS Runners
  • macOS executor
  • Runner tag: saas-macos-medium-m1
  • Image: macos-15-xcode-16
  • Note: Xcode 15 builds are now rejected by Apple, so using older runner images is not a viable workaround.

Used GitLab Runner version

Running with gitlab-runner 18.4.0~pre.115.gb2218bab (b2218bab) on green-1.saas-macos-medium-m1.runners-manager.gitlab.com/default djQLaC-yp, system ID: s_74eebbcd1801

Possible fixes

  • Provide the ability to select a Ruby version for macOS SaaS runners (like Linux runners)
  • Provide a macOS runner image with Xcode 16 and Ruby < 3.4 until native gems like ffi are compatible