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

In most large Rails monoliths, organizations prioritize writing new features over writing tests for them. Over time, more and more code goes untested, forcing teams to spend more time debugging painful bugs.

We built an autonomous agent that closes that gap. It reads Rails source files, generates or improves RSpec tests, validates them against style rules and coverage targets, and runs inside a CI/CD pipeline with no human intervention. To operate on codebases at this scale, it runs in parallel: multiple instances working on different files simultaneously.

RSpec through an agent's eyes

Ruby is dynamically typed: there is no compilation step, so errors surface at runtime. For our agent, this means the only way to verify test syntax is to execute it. RSpec, the standard Rails testing framework, makes tests expressive and readable, but its domain-specific language is easy to get wrong.

When the agent reads a Ruby on Rails codebase, it reads five main file types (models, serializers, controllers, mailers, helpers), each structured differently (therefore tested in different ways). The agent needs distinct instructions for each type.

One benefit: the mapping from source file to spec file is nearly 1:1. The general convention is:

There are a few exceptions to that rule however, like that app/controllers/ are sometimes mapped to spec/requests/, or that sometimes a single source file can have multiple spec files, in which case the convention is:

This straightforward mapping makes it easy to locate the tests for any given file, or to identify files that lack tests entirely.

Where it gets harder for our agent is that to avoid duplicating code, RSpec relies heavily on shared context: factories, fixtures, database schemas...

• Factories: Reusable templates for creating test objects with predefined attributes, making it easy to generate consistent test data.

• Fixtures: Static data files that preload test database records, providing a fixed baseline for tests.