Test Behavior, Not Implementation

This skill covers how a test asserts. Whether a test should exist at all, and tautology tests that copy the code under test, belong to the test-audit skill and the coding skill's Tests principle. Apply those; this skill does not restate them.

A test calls the code as its users do and asserts the exact result they see. Rewrite or delete any test that would still pass if every imported function returned undefined or a wrong value.

These shapes fail that check:

• Loose or no assertion: toBeDefined, toBeTruthy, not.toThrow, toBeGreaterThan(0).
• Call or absence only: toHaveBeenCalled, not.toHaveBeenCalled, toBeUndefined, toEqual([]).
• Restated constant, config default, table row, or prompt string.
• Assertions on data the test built, with no call to the subject.

Fix: call the subject in the test body with one concrete input and assert the literal output or effect. For a mock, assert the payload it received. For an absence, also assert the presence on another input. For a constant, test the mechanism that reads it with one input instead of restating the value.

Keep relation checks across table rows (a key in two tables, a parent that exists) and *.test-d.ts type checks.

Source

Adapted rewrite of Pstack's principle-test-behavior-not-implementation skill (cursor/plugins commit 1e5c90a), by poteto. The upstream licence is unconfirmed. Anthony approved adopting this adaptation on 2026-10-14.