# Mocking

> Use when: use test doubles correctly — mock true boundaries, not the code under test.

- Skill: `kimtth/mocking` (Agent Skill)
- Install (CLI): `npx skillmds@latest add kimtth/mocking`
- Raw SKILL.md: https://api.skillmd.com/api/skills/kimtth/mocking/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: kimtth (https://skillmd.com/u/kimtth)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/kimtth/mocking

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Goal: fast, honest tests that isolate the unit without faking its behavior.

Use for:
- isolating a unit from slow or external dependencies
- deciding what to fake versus exercise for real
- fixing brittle tests coupled to implementation

Workflow:
1. Identify the true external boundary (network, clock, filesystem).
2. Mock or stub only that boundary, not internal collaborators.
3. Use the lightest double that works: stub, fake, or mock.
4. Assert on observable behavior, not on every internal call.
5. Keep doubles in sync with the real contract.
6. Verify the integration with the real dependency elsewhere.

Doubles:
- stub: returns canned data
- fake: a working lightweight implementation
- mock: asserts interactions occurred
- spy: records calls for later inspection

Rules:
- do not mock the thing you are testing
- prefer real collaborators when they are fast and deterministic
- avoid asserting on internal call sequences
- verify mocked contracts against reality in integration tests