Modernizing Legacy Systems Using Agent Harnesses TDD and the Seam Model

Short summary

AI agents operating freely in legacy systems can trigger massive unintended refactors that are risky to review and deploy. The author shares a harness-based approach using TDD and Michael Feathers' Seam Model to constrain agent behavior, isolate changes, and validate against architectural constraints. This pattern reduces blast radius and creates organizational leverage for safe agent-augmented development workflows.

• •Harnesses and Seam Model constrain agent behavior to isolated, safe changes
• •Critique skills make agents review their own output against acceptance criteria and architectural boundaries
• •Shifts focus from prompt engineering to designing reliable operational environments

Generated with AI, which can make mistakes.