Bundler "frozen mode, but the dependencies in your Gemfile changed"

In frozen (or --deployment) mode Bundler refuses to modify Gemfile.lock. When the Gemfile no longer matches the committed lockfile, Bundler reports the exact gems that were added, removed, or changed and stops rather than silently updating the lock.

What this error means

bundle install fails with "Your bundle is locked, but the dependencies in your Gemfile changed" (or "frozen mode"), listing the gems you added or removed. It only happens in CI because frozen mode is on there but not locally.

bundler

The dependencies in your gemfile changed, but the lockfile can't be
updated because frozen mode is set

You have added to the Gemfile:
* rspec-rails

Run `bundle install` elsewhere and add the updated Gemfile.lock to
version control.

Common causes

Gemfile edited without committing Gemfile.lock

You added or removed a gem and committed the Gemfile but not the regenerated lockfile, so CI sees a mismatch in frozen mode.

Lockfile changes not staged in the commit

bundle install updated Gemfile.lock locally but the change was never staged, so the repo carries the old lock.

How to fix it

Regenerate and commit Gemfile.lock

Run bundle install locally and commit the updated lockfile alongside the Gemfile change.

Terminal

bundle install
git add Gemfile Gemfile.lock
# commit both together

Verify the lock matches in CI

Confirm frozen/deployment mode is intentional in CI (bundle config get frozen).
Keep it on so an out-of-date lockfile fails fast instead of drifting.
Add a pre-commit or CI step that runs bundle install --frozen and fails on a lockfile diff.

How to prevent it

Always commit Gemfile and Gemfile.lock in the same change.
Keep frozen/deployment mode on in CI so lockfile drift fails fast.
Run bundle lock --check (or bundle install --frozen) in a pre-merge job.