principle-test-behavior-not-implementation from barbieri/local-openfinance is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
Apply when you write, change, or keep a test. Call the code the way its users do and assert the result they observe against a literal expected value. If the test would still pass when every imported function returns undefined, rewrite the assertion or delete the test.
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# Test Behavior, Not Implementation
A test calls the code the way its users do and asserts the result they observe against a literal expected value. A test that asserts which calls the code made, or restates a constant the code contains, does neither.
The check: before you keep a test, ask whether it would still pass if every function it imports returned `undefined`. If yes, it observes no behavior and cannot fail for a defect. Rewrite the assertion or delete the test.
**Why:** A test that cannot fail for a defect costs CI time and review attention and catches nothing. A constant pin also fails when someone edits the constant or the prompt it restates, so it prevents that edit.
**Five shapes that still pass when every imported function returns `undefined`:**
- **Weak or no assertion.** No `expect`, or only `toBeDefined`, `toBeTruthy`, `not.toThrow`, `toBeInstanceOf`, `toBeGreaterThan(0)`.
- **Mock or absence only.** Only `toHaveBeenCalled`, `not.toHaveBeenCalled`, `toBeUndefined`, `toEqual([])`, `toHaveLength(0)`, `not.toBe(wrongValue)`.
…
The line at the top of this page pins the label git:20261005.32edae5, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
GET https://markdownregistry.com/api/v1/artifacts/art_rfh6rso4rt64ronw
GET https://markdownregistry.com/api/v1/resolve?ref=barbieri/local-openfinance/principle-test-behavior-not-implementation
GET https://markdownregistry.com/api/v1/blob/87e40efe4e486f7ea639d2ed1fc0abe6c93b8fd0e89a81218086909d2470a52e
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