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.
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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`, or `not.toThrow` when non-throwing behavior is not the contract.
- **Mock or absence only.** Only `toHaveBeenCalled`, `not.toHaveBeenCalled`, `toBeUndefined`, `not.toBe(wrongValue)`.
- **Self-referential.** The expected value comes from the code under test: `expect(f(a)).toBe(f(a))`, `expect(parsed.url).toBe(buildUrl(...))`.
…
The line at the top of this page pins the label git:20261031.336cc57, 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.
A 17 of 17 checks passed. Fixed automatic checks with no AI involved, so the same checks give the same answer every run.
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Source
GitHub
iap/pstack-hermes · 32 stars · license a license file GitHub does not identify · pushed 2026-11-04 · branch master
API
GET https://markdownregistry.com/api/v1/artifacts/art_5yo2eeafl4qfpitm
GET https://markdownregistry.com/api/v1/resolve?ref=iap/pstack-hermes/principle-test-behavior-not-implementation
GET https://markdownregistry.com/api/v1/blob/0ce60a8523ef331ee9ef0028746e2c60cac14be37004c61ba62ecc6b33a92bbc
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