#8180 is still reproducible without --prefer-local on Bundler 4.0.18

Describe the problem

Bundler installs a regular copy of a gem even when the exact same version is already available as a Ruby default gem.

This appears to be the same underlying behavior reported in #8180. That issue was closed by #8412, but #8412 specifically fixes the behavior when --prefer-local is used. With a normal bundle install, the behavior is still reproducible with Bundler 4.0.18.

In my case this was initially exposed by Ruby LSP, whose generated lockfile referenced io-console 0.8.2. Ruby 4.0.6 already ships io-console 0.8.2 as a default gem, but Bundler downloaded, built, and installed an additional regular copy of io-console 0.8.2.

The issue is reproducible without Ruby LSP.

Environment

> ruby -v
ruby 4.0.6 (2026-07-14 revision 03b6d3f889) +PRISM [x64-mingw-ucrt]
> gem -v
4.0.18

Before the test, io-console 0.8.2 is present only as a default gem:

io-console-0.8.2 | DEFAULT | c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/io-console-0.8.2.gemspec

Minimal reproduction

Create a new directory with this Gemfile:

source "https://rubygems.org"

gem "io-console", "0.8.2"

Generate the lockfile:

bundle lock

Then run:

bundle install --verbose

Observed output:

Running `bundle install --verbose` with bundler 4.0.18
Found no changes, using resolution from the lockfile
Using bundler 4.0.18
0:  bundler (4.0.18) from c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/bundler-4.0.18.gemspec
HTTP GET https://index.rubygems.org/versions
HTTP 304 Not Modified https://index.rubygems.org/versions
Fetching gem metadata from https://rubygems.org/
Looking up gems ["io-console"]
Fetching io-console 0.8.2
Downloaded io-console in: 0.086s
Installing io-console 0.8.2 with native extensions
Temporarily enhancing PATH for MSYS/MINGW...
Building native extensions. This could take a while...
Installed io-console in: 18.301s
0:  io-console (0.8.2) from c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/io-console-0.8.2.gemspec
Bundle complete! 1 Gemfile dependency, 1 gem now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

Afterwards, both specifications exist:

io-console-0.8.2 | NORMAL  | c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/io-console-0.8.2.gemspec
io-console-0.8.2 | DEFAULT | c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/io-console-0.8.2.gemspec

So Bundler downloads, compiles, and installs a regular copy of the exact version that is already available as a default gem.

Interestingly, the verbose output then reports the default-gem specification as the one being used:

0:  io-console (0.8.2) from c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/io-console-0.8.2.gemspec

--prefer-local behaves as expected

After removing the redundant regular copy and restoring the initial state, running:

bundle install --verbose --prefer-local

does not reinstall the gem:

Running `bundle install --prefer-local --verbose` with bundler 4.0.18
Found no changes, using resolution from the lockfile
Using bundler 4.0.18
0:  bundler (4.0.18) from c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/bundler-4.0.18.gemspec
Using io-console 0.8.2
0:  io-console (0.8.2) from c:/ruby-4.0.6-x64/lib/ruby/gems/4.0.0/specifications/default/io-console-0.8.2.gemspec
Bundle complete! 1 Gemfile dependency, 1 gem now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

Expected behavior

For a normal bundle install, if the exact required name and version are already available as a default gem, I would expect Bundler to use that default gem rather than download and install an identical regular copy.

This matters in particular for default gems with native extensions: in this example the unnecessary installation recompiles io-console.

Related issues

• #8180 reported that bundle install installs default gems again even when the required versions are already provided by Ruby.
• #8412 closed #8180 by making default gems preferred when --prefer-local is used.

Since the original #8180 reproducer did not use --prefer-local, is the current behavior of plain bundle install intentional? If not, #8180 appears to remain reproducible in Bundler 4.0.18.

Start with the minimal Gemfile containing io-console 0.8.2, then compare plain bundle install --verbose with bundle install --verbose --prefer-local on the stated Ruby and Bundler versions. Trace how Bundler chooses an installed default gem during installation; done means the plain install reuses the exact default gem without downloading, compiling, or creating a regular copy.