working-with-legacy-code

Isolate dependencies and pin behavior with characterization tests for legacy codebases.

Updated Jul 31, 2026

Search Tags:#testing #refactoring #software design #legacy code #characterization tests #dependency breaking

Ready to run

npx skills add https://github.com/fsolla/teqo --skill working-with-legacy-code

💬 Prompt for ChatGPT / Claude Chat▼

Please help me install this Agent Skill.
Skill: working-with-legacy-code
Source: https://github.com/fsolla/teqo/tree/main/.agents/skills/working-with-legacy-code
Command: npx skills add https://github.com/fsolla/teqo --skill working-with-legacy-code

SYSTEM DOCUMENTATION & REQUIREMENTS

💡 This Skill includes references (resource) components.

What problem does it solve?

This skill addresses the fear and risk associated with modifying legacy codebases that lack test coverage, preventing regressions and enabling incremental improvements without requiring a full rewrite.

Core Features & Use Cases

Dependency Breaking: Provides a catalog of mechanical techniques like Extract Interface and Parameterize Constructor to isolate code for testing.
Characterization Testing: Guides the creation of golden master and snapshot tests to pin down existing behavior before making changes.
Safe Modification: Implements the cover-and-modify algorithm to ensure that behavior changes and refactoring are handled as separate, verifiable steps.

Quick Start

Use the working-with-legacy-code skill to identify seams and write characterization tests for the legacy module located in the billing directory.

Frequently Asked Questions about working-with-legacy-code

High-intent search queries and answers about installing and using this skill.

FAQPage Schema

How do I refactor legacy code without tests or breaking existing behavior?▼

To refactor legacy code without tests, apply a cover-and-modify discipline using characterization tests to pin down existing behavior, then isolate dependencies using techniques like Extract Interface to ensure structural changes do not cause regressions.

What is a characterization test and when do I need it for untested codebases?▼

A characterization test is a snapshot or golden master test that captures and verifies the current behavior of untested codebases. You need it before modifying or refactoring complex monolithic systems to ensure existing outputs remain unchanged.

How do I isolate dependencies in a monolithic system for safe modification?▼

You isolate dependencies in a monolithic system by applying mechanical dependency breaking techniques, such as Extract Interface and Parameterize Constructor, to identify seams and decouple code for targeted testing.

What's the best way to add features to complex legacy code without a full rewrite?▼

The best way to add features to legacy code without a full rewrite is through incremental test coverage. Use dependency breaking to isolate modules, pin current behavior with characterization tests, and safely add features through the cover-and-modify algorithm.

Can I safely modify untested code without writing a full test suite first?▼

Yes, you can safely modify untested code by writing targeted characterization tests only for the specific module you are changing. This incremental approach isolates dependencies and pins behavior without requiring complete test coverage upfront.

Why does modifying untested legacy code cause regressions in monolithic systems?▼

Modifying untested legacy code causes regressions in monolithic systems because tightly coupled dependencies hide behavioral interactions. Without characterization tests to pin down existing behavior, structural changes inadvertently alter downstream outputs.