Phase 4: Harness Engineering · 45 min · Claude Code · Python

Harness Regression Testing

A rules file, a skill, and a tool config are infrastructure. Infrastructure that isn't tested regresses silently, exactly like code does.

Real-world application: Treating CLAUDE.md and skill definitions as versioned, tested infrastructure — not just prose you edit and hope for the best — is the harness-engineering maturity this whole phase has been building toward.

What you will learn

Explain why a harness change (a rule, a skill, a tool config) can regress agent behavior the same way a code change can regress a product
Identify when a new skill silently conflicts with an existing rule for a specific task shape
Design a harness regression suite that re-runs representative scenarios after any harness change
Catch a real regression: a previously-passing scenario failing after a new skill is added

Introduction

Harness Regression Testing

Objective

Run a harness regression suite against a baseline harness config, then add a conflicting skill and confirm a previously-passing scenario now fails, catching the regression.

Unlock the full lesson

You've read the first 2 sections. The rest of this lesson covers The Problem, The Concept, Build, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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