# Principle Test Behavior Not Implementation

> Call the system the way its consumer does and assert meaningful observable output; avoid mocks or assertions that only mirror internal calls. Use when when writing or evaluating tests.

- Skill: `theonlyfusioncube/principle-test-behavior-not-implementation` (Agent Skill)
- Install (CLI): `npx skillmds@latest add theonlyfusioncube/principle-test-behavior-not-implementation`
- Raw SKILL.md: https://api.skillmd.com/api/skills/theonlyfusioncube/principle-test-behavior-not-implementation/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: TheOnlyFusionCube (https://skillmd.com/u/theonlyfusioncube)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/theonlyfusioncube/principle-test-behavior-not-implementation

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# Test Behavior, Not Implementation

## Trigger

When writing or evaluating tests.

## Rule

Call the system the way its consumer does and assert meaningful observable output; avoid mocks or assertions that only mirror internal calls.

## Application

1. Name the concrete decision this principle changes.
2. Apply the rule to that decision, not as a decorative citation.
3. Prefer evidence from the current repository/runtime over generic preference.
4. If the principle conflicts with an explicit user constraint, keep the constraint and state the tradeoff.