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Latest Cohesion Is the Coverage of the Agent Era

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Discover the TDD Gears model and learn how to apply TDD principles in real-world scenarios.

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Why TDD Matters More in the AI Era

The bar for "good tests" just moved. Agents made it non-optional.

  • The old metrics measured form, not concept. Files per line, average function size, cyclomatic complexity: all form. All pass silently while the concept the code was supposed to represent has scattered across five files no test crosses.
  • Agent codebases pass every structural test and fail the domain one. Three discount paths, three near-identical reducers, three subtly different names. Every function is small. Every module is decoupled. The concept of discount no longer exists anywhere.
  • The numbers are already in the record. A 2026 maintainability report clocked +81% duplication and -70% refactoring line moves in AI-assisted codebases. A 2026 debt survey found 22.7% of AI-introduced issues surviving at HEAD. The pattern is form without cohesion.
  • Cohesion is owned by the suite now. Tests are the only artifact that can cross module boundaries at the concept level and refuse to compile when the concept fractures. That is the new job.

Read the argument: Cohesion Is the Coverage of the Agent Era

The Bar Moved

Old bar

Tests exist and pass. Good enough for humans with context.

New bar

Scenario names · builders · domain types · ubiquitous language. The test suite is the interface agents operate against.

Rich TDD isn't a style preference anymore. It's infrastructure for compressed-role, AI-augmented teams.

TDD Gears

Shift gears based on context, not habit

TDD Gears model showing Low, Medium, High, and Reverse gears for test-driven development

Low Gear

New territory. Build context. Small steps. Learn the shape of the problem before solving it.

Medium Gear

Patterns emerge. Apply design principles. Let the tests guide you toward better abstractions.

High Gear

Known patterns. Follow existing architecture. Move fast because the structure is already proven.

Reverse Gear

Wrong direction. Back up. Delete the test. Try a different approach. This isn't failure — it's steering.

Learn More About TDD Gears