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

An autonomous agent was built to automatically write and improve tests for Rails codebases, reading source files and generating RSpec tests without human intervention. The agent operates at scale by running in parallel across multiple files and must validate tests by executing them, since Ruby has no compilation step to catch errors. The work matters because many large Rails projects prioritize writing features over tests, leading to untested code that causes debugging problems. Building the agent required context engineering through a step-by-step execution plan, specialized skills for different file types, and custom tools to validate test quality and coverage.