TDD by Design When an Agent Is in the Loop

When a developer is working with an agent, tests stop being only a regression safety net. They become part of the interface between the request and the implementation.

That does not mean every task needs a large test suite before any code is written. It means the expected behavior should be made executable as early as possible, especially when the implementation may be changed by an automated loop.

Start with observable behavior

A useful test describes something a user or another system can observe. “The function has three branches” is an implementation detail. “An invalid input returns a useful error and does not change state” is a behavior that can constrain an implementation.

Give the loop a stopping condition

Loop mode works when it has a reliable signal. Tests, type checks, linting, and a reproducible local environment tell the agent whether its last change moved the project forward. Without that signal, the agent can keep making plausible edits without converging.

Do not make the tests decorative

A test that only confirms the implementation’s current shape is not much of a guard rail. The valuable tests cover boundaries: invalid input, missing dependencies, authorization, empty results, retries, and state changes that must not happen.

Keep the feedback fast

The first test command should be cheap enough to run after every change. Slower integration and end-to-end checks still matter, but they should be layered behind a fast signal that catches the common mistakes before the full suite runs.

Review the gap, not just the green build

Passing tests prove that the tested behavior passed. They do not prove that the right behavior was tested. Code review still asks whether the acceptance criteria are complete, whether a failure mode is missing, and whether the test would fail if the feature were broken in the way that matters.

The practical pattern

1. Write down the expected behavior.
2. Turn the important cases into tests.
3. Let the agent implement against those tests.
4. Run focused checks first, then the broader suite.
5. Review the diff and the test coverage together.

The point is not to make an agent autonomous at any cost. The point is to make progress measurable enough that autonomy is useful.

Current as of September 2026. This describes a workplace development practice, not a prescription for every project.