---
summary: "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."
tags:
  - type/reference
  - tool/minitest
  - domain/testing
scope: global
date: 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).
