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Rails testing on autopilot: Building an agent that writes what developers won't

Mistral AI describes an autonomous Rails testing agent built on its open-source Vibe coding assistant. The agent reads Rails files, applies file-type-specific skills, generates or improves RSpec tests, and validates them with RuboCop, RSpec, and SimpleCov. In a 275-file experiment, it reached 100% passing tests, 100% average line coverage, zero RuboCop violations, and a higher LLM-as-a-judge score, while stressing that generated tests must actually run.