A guide to replacing real web services with fake API responses during development and testing. It covers tools such as MSW, nock, json-server, and Mirage JS.

Use it to choose an API-mocking tool, configure browser or Node request interception, keep mocks aligned with TypeScript types, and reset them between tests.

It lets you test request handling without running or depending on a real backend, while reducing errors caused by stale or leaking test data.

api-mocking — Skill for Claude Code by christopherlouet/claude-base

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add christopherlouet/claude-base --skill api-mocking

Clone the repo

git clone --depth 1 https://github.com/christopherlouet/claude-base

Made for: Claude Code.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/christopherlouet/claude-base/api-mocking/github.svg)](https://agentmods.dev/skills/christopherlouet/claude-base/api-mocking)

Your own site

<a href="https://agentmods.dev/skills/christopherlouet/claude-base/api-mocking"><img src="https://agentmods.dev/badge/skills/christopherlouet/claude-base/api-mocking/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/christopherlouet/claude-base/api-mocking"><img src="https://agentmods.dev/badge/skills/christopherlouet/claude-base/api-mocking.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 29 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 524 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00029	$0.00524
Opus 5.5	$0.00012	$0.00210
Sonnet 5.5	$0.00006	$0.00105
Haiku 4.5	$0.00003	$0.00052

Measured 21d ago against content hash a1e088775226, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

api-mocking scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 21d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.claude/skills/api-mocking/SKILL.md · 38 lines

What it actually says

API Mocking (pointer)

MSW handler API, browser/node setup, install commands and request-interception internals are canonical at vendor docs and drift on each release:

MSW — mswjs.io (recommended default; browser worker + Node server, single handler set)
nock — github.com/nock/nock (Node-only HTTP interception)
json-server — github.com/typicode/json-server (full REST fake server from a JSON file)
Mirage JS — miragejs.com (browser-only, models + factories)

Tool selection (version-agnostic)

Need to mock HTTP for tests/dev?
├── Browser AND Node from one handler set → MSW
├── Node-only unit tests, low ceremony → nock
├── Need a stateful REST fake server (CRUD) → json-server
└── Browser-only with relational fixtures (factories) → Mirage JS

Foundation discipline (keep across releases)

Type-safe mocks: derive mock payloads from the same TypeScript types as the real API (e.g. shared types/api.ts). A mock that compiles against stale types is the #1 source of mock-prod divergence.
Reset between tests: always resetHandlers() (MSW) or cleanAll() (nock) in afterEach — leaked state across tests is a debugging tax.
Realistic failure modes: simulate 5xx/timeouts/auth errors, not just happy paths. The mock is only useful if it exercises the same edge cases as prod.
Unhandled = error: in tests use onUnhandledRequest: 'error' (MSW) so a typo'd URL fails loudly instead of silently passing.

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

21d ago Changed · -2 lines a1e088775226
22d ago Changed · -6 lines 7721aac80d52
23d ago Changed · -1 lines b682e64a6f95
1mo ago First seen · 47 lines · 29 tokens per session scan A d09667c2d884

Subscribe to this mod's changes

api-mocking is a skill published in the GitHub repository christopherlouet/claude-base (5 stars, last pushed yesterday), licensed MIT. It adds 29 tokens to every session and 524 once invoked, about $0.0001 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.

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