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+---
+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).