SecondBrain/reference/minitest-6-extracted-mock.md

summary	tags	scope	date
Minitest 6 removed Minitest::Mock from core — require 'minitest/mock' raises LoadError; the API lives in the separate minitest-mock gem, and pinning minitest < 6 double-loads constants.	type/reference tool/minitest domain/testing	global	2026-07-16

Minitest 6 extracted Minitest::Mock from core

Under minitest 6.x (observed on 6.0.6, Ruby 4.0.2), require "minitest/mock" raises LoadError: cannot load such file -- minitest/mock. The mock/stub API was extracted from the core gem into a separate minitest-mock gem. Test suites written against minitest 5.x that use Minitest::Mock or Object#stub break on load once 6.x is the activated version.

What doesn't work: pinning gem "minitest", "< 6" in a test helper. If minitest 6 is already activated (it ships as a default gem), the pin loads 5.x alongside 6.x and floods output with already initialized constant Minitest::VERSION / Runnable::SIGNALS warnings — two versions' constants coexist.

Fixes, in order of preference:

1. Add the minitest-mock gem (Gemfile or gem install minitest-mock) — restores the old require "minitest/mock" path.
2. Migrate the tests off Minitest::Mock (hand-rolled fakes/stubs — often cleaner anyway).

Discovered 2026-07-16 while diagnosing a pre-existing failure in cc-plugins' cc-architect test suite (test_runtime_verifier.rb, 6 errors).