Constable replaces RSpec/Minitest for Rails apps that want tests to be fast and non-flaky by construction. Every test is isolated by default, nondeterminism is a lint error rather than a CI surprise, and every escape hatch is reported until someone deals with it. Existing RSpec and Minitest suites adopt it with a one-line change per file and zero rewriting -- cold cases run verbatim through their original engine while native cases run under strict rules, side by side in one run. Ships with a case-file DSL (investigate/witness/briefing/docket), transactional isolation, flake history with rename-surviving content-hash identity, a jail and parole docket for legacy red suites, warrants for automatic flake detection, diff-based coverage, parallel workers, git-diff test selection, and a RuboCop extension. The gem is published as "constable-rails"; everything inside it -- the module, the CLI, the config directory -- is simply "constable".