How I’m Using Agent Harnesses, TDD, and the Seam Model to Improve Development in Legacy Systems

Short summary

AI agents in production legacy systems can introduce unintended code changes and risky refactors when unconstrained. The author combines agent harnesses, TDD, and Michael Feathers' Seam Model to create controlled environments where agents safely extend functionality through isolated extension points. This approach produces smaller, highly reviewable changes with predictable safety—enabling teams to move faster without increasing instability.

• •Uncontrolled agents introduce unintended side effects in legacy systems
• •Harness + TDD + Seam Model = constrained agent behavior in production
• •Smaller, safer changes = organizational leverage and team-wide acceleration

Generated with AI, which can make mistakes.