A guide to Mock Service Worker, a library that intercepts network requests and returns chosen responses during development or tests. It covers handlers for REST and GraphQL APIs, setup, matching, and error cases.

Use it when configuring MSW, creating request handlers, testing loading and failure states, mocking REST or GraphQL services, or diagnosing requests that do not match.

It lets tests and local development use predictable API responses without depending on a live backend or writing ad-hoc request fakes.

msw — Skill for Claude Code and Codex by pproenca: A guide to Mock Service Worker, a library that…

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.

agentmods

npx agentmods add skills/pproenca/dot-skills/msw

Any agent

npx skills add pproenca/dot-skills --skill msw

Clone the repo

git clone --depth 1 https://github.com/pproenca/dot-skills

Made for: Claude Code, Codex.

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/pproenca/dot-skills/msw.svg)](https://agentmods.dev/skills/pproenca/dot-skills/msw)

Your own site

<a href="https://agentmods.dev/skills/pproenca/dot-skills/msw"><img src="https://agentmods.dev/badge/skills/pproenca/dot-skills/msw.svg" alt="Measured on agentmods" height="20"></a>

Per session 66 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,253 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

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	$0.00066	$0.01253
Opus 5	$0.00033	$0.00626
Sonnet 5	$0.00013	$0.00251
Haiku 4.5	$0.00007	$0.00125

Measured 4d ago against content hash 99cdfcdcfc68, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

msw 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 4d 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.

skills/.curated/msw/SKILL.md · 121 lines

How it starts

The opening of the file, as written. The whole thing — 121 lines — stays where its author put it; the contents beside it link to each section on GitHub.

MSW Best Practices

Comprehensive API mocking guide for MSW v2 applications, designed for AI agents and LLMs. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

Setting up MSW for testing or development
Writing or organizing request handlers
Configuring test environments with MSW
Mocking REST or GraphQL APIs
Debugging handler matching issues
Testing error states and edge cases

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Setup & Initialization	CRITICAL	`setup-`
2	Handler Architecture	CRITICAL	`handler-`
3	Test Integration	HIGH	`test-`
4	Response Patterns	HIGH	`response-`
5	Request Matching	MEDIUM-HIGH	`match-`
6	GraphQL Mocking	MEDIUM	`graphql-`
7	Advanced Patterns	MEDIUM	`advanced-`
8	Debugging & Performance	LOW	`debug-`

Quick Reference

1. Setup & Initialization (CRITICAL)

setup-server-node-entrypoint - Use correct entrypoint for Node.js (msw/node)
setup-lifecycle-hooks - Configure server lifecycle in test setup
setup-worker-script-commit - Commit worker script to version control
setup-node-version - Require Node.js 18+ for MSW v2
setup-unhandled-requests - Configure unhandled request behavior
setup-typescript-config - Configure TypeScript for MSW v2

2. Handler Architecture (CRITICAL)

handler-happy-path-first - Define happy path handlers as baseline
handler-domain-grouping - Group handlers by domain
handler-absolute-urls - Use absolute URLs in handlers
handler-shared-resolvers - Extract shared response logic into resolvers
handler-v2-response-syntax - Use MSW v2 response syntax
handler-request-body-parsing - Explicitly parse request bodies
handler-resolver-argument - Destructure resolver arguments correctly
handler-reusability-environments - Share handlers across environments

Read the full file on GitHub · 121 lines

Files

What ships with it

50 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

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.

4d ago First seen · 121 lines · 66 tokens per session scan A 99cdfcdcfc68

Subscribe to this mod's changes

msw is a skill published in the GitHub repository pproenca/dot-skills (201 stars, last pushed 19d ago), licensed MIT. It adds 66 tokens to every session and 1,253 once invoked, about $0.0003 per session on Opus 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 2026-08-30.

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