Derek Comartin • 4/15/2026

Testing Needs a Seam, Not an Interface

This article argues that creating interfaces solely for testing purposes introduces an unnecessary 'interface tax.' Using a VIN decoder example, it shows how developers often default to interfaces and mocking, but testing actually requires a seam—a point where behavior can be substituted—not necessarily an interface. The author critiques the overuse of abstraction in clean architecture, suggesting it can reduce velocity without improving testability. Aimed at software developers and architects, it offers a practical perspective on designing testable code without excessive indirection.

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#software architecture #testing #interfaces