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3.0.0 • Public • Published


Generate pact contracts from the recorded mock service worker interactions.

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Check out the quick start guide.

Compatibility table

pact msw version msw version node version migration guide
^2 <=1 >=16 <=20
^3 ^2 >=18 v2 to v3

Getting started

npm install @pactflow/pact-msw-adapter --save-dev 

or yarn

yarn add -D @pactflow/pact-msw-adapter 

MSW provides a setupServer for node environments and setupWorker for browser based environment

import { setupServer } from "msw/node";
import { setupPactMswAdapter } from "@pactflow/pact-msw-adapter";

For browser based enviromnents

import { setupWorker } from "msw/browser";
import { setupPactMswAdapter } from "@pactflow/pact-msw-adapter";

See ./src/pactFromMswServer.msw.spec.ts msw mock server example (jest/msw/react)

See ./examples/react/cypress/integration/pactFromMswWorker.spec.js msw mock worker example (cypress/msw/react)

These tests will generate pacts, which can be found in the ./msw_generated_pacts folder

How to use

Let's start by listing it's methods:

  • setupPactMswAdapter: Generates an pact-msw-adapter instance. It also allows for several options on the adapter.
  • newTest: Tells the adapter a new test is about to start. This is used for validating msw calls.
  • verifyTest: Waits for all pending network calls to finish or timeout, and asserts that all these calls started and finished on the same test without unexpected errors, and that there were no calls to included urls which aren't handled by msw.
  • clear: Resets all pact-msw-adapter's internal states, same effect as generating a new pact-msw-adapter instance.
  • writeToFile: Dumps all the recorded msw calls to pact files, generating one pact file for each consumer-provider call. For browser environments, it requires a custom file writter as argument.


Parameter Required? Type Default Description
server false SetupServer server provided by msw - a server or worker must be provided
worker false SetupWorker worker provided by msw - a server or worker must be provided
timeout false number 200 Time in ms for a network request to expire, verifyTest will fail after twice this amount of time.
consumer true string name of the consumer running the tests
providers true { [string]: string[] } | ({ request: Request; requestId: string }) => string | null names and filters for each provider or function that returns name of provider for given request
pactOutDir false string ./msw_generated_pacts/ path to write pact files into
includeUrl false string[] inclusive filters for network calls
excludeUrl false string[] exclusive filters for network calls
excludeHeaders false string[] exclude generated headers from being written to request/response objects in pact file
debug false boolean false prints verbose information about pact-msw-adapter events
logger false console console logger used to print messages to console

Route filtering

By default pact-msw-adapter will try to record an interaction for every single network call, including external dependencies, fonts or static resources. This is why we’re implementing a route filtering mechanism to include only relevant paths in our pact files.

This mechanism has three layers, in order of priority:

  • excludeUrl: All paths containing any of the strings present in this array will be ignored.
  • includeUrl: All paths not containing any of the strings in this array will be ignored.
  • providers: Paths not containing any of the strings listed in the map’s values will be ignored.

The first two layers can be skipped by setting it’s value to undefined. The third layer is mandatory.

providers can be also a function that returns name of provider for given request. If no provider is matched it should return null. This allows dynamically matching providers based on url patterns.

Header filtering

By default pact-msw-adapter captures and serialises all request and response headers captured, in the generated pact file.

You may wish to exclude these on a global basis.

This mechanism currently has a layer

  • excludeHeaders: All headers containing any of the strings present in this array will be ignored.

Custom file writers

The adapter uses by default node’s filesystem to write pact files to disk. This makes it incompatible with browser environments where fs is not available. To overcome this, pact-msw-adapter allows for defining custom functions for writting files to disk.

writeToFile(writer?: (path: string, data: object) => void): Promise<void>

Writers are required to by synchronous.

The path argument contains a relative path to save the file into, already prepending pactOutDir, and including the file’s name and extension.

The data field consists of a javascript object containing a pact file (check the anatomy of a pact file).

Pact files generation

pact-msw-adapter will dump all the recorded requests into pact files when writeToFile is called.

A recorded request is a request which has started and been successfully mocked by msw since pact-msw-adapter has been instantiated or cleared. This can include duplicated requests and does not distinguishes between different test runs.

Each time writeToFile is run, it will generate one pact file for every consumer-provider pair. In practice, consumers are fixed, making it to generate one pact file per provider.

In order to do this, pact-msw-adapter uses the providers map to asociate a request with a provider. The providers map is iterated in order and each request is associated with exactly one provider.

Once this association is done, pact-msw-adapter will translate each request to a pact interaction and group these interactions on pact files by provider.

pact-msw-adapter implementation
import { setupPactMswAdapter } from '@pactflow/pact-msw-adapter';

let pactMswAdapter: any = undefined;

beforeEach(async () => {
    if (!pactMswAdapter) {
        cy.window().then(window => {
            pactMswAdapter = setupPactMswAdapter({
                worker: window.msw.worker,
                options: {
                    consumer: 'web-ea',
                    timeout: 1000,
                    providers: {
                        'edge-api-admin': [ 'edge-api-admin' ]
                    pactOutDir: './pacts',
                    excludeUrl: ['static/'],
                    // debug: true
    } else {
afterEach(async () => {
    if (!pactMswAdapter) return;
    try {
        await pactMswAdapter.verifyTest();
    } catch (err) {
        // cypress doesn't like errors on hooks...
        if (process.env.NODE_ENV !== 'production') {
            console.groupCollapsed('%cError generating pacts.', 'color:coral;font-weight:bold;');
        } else {
            // fail on pipelines
            throw err;
after(async () => {
    if (!pactMswAdapter) return;

    await pactMswAdapter.writeToFile((path: string, data: object) => cy.writeFile(path, data));

Anatomy of a Pact File

Without further do, it looks like the following:

  "consumer": { "name": "" },
  "provider": { "name": "" },
  "interactions": [
      "description": "",
      "providerState": "",
      "request": {
        "method": "GET",
        "path": "", // Ids replaced
        "query": "", // url-encoded query
        "matchingRules": { ... }
      "response": {
        "status": 200,
        "headers": { },
        "body": { ... },
        "matchingRules": { ... }
  "metadata": {
    "pactSpecification": {
      "version": "2.0.0"

Here, matchingRules represent the assertions of the expectation, while body, query and path contains it's example values.

Migrating pact-msw-adapter from v2 to v3

In October 2023 msw released new version 2 that bring significant changes to the msw interface. To reflect these changes we've released pact-msw-adapter@v3 that's compatible with msw@v2.

To migrate you'll need to update msw to >=2.0 and migrate your usage of the library (migration guide here).

Breaking changes on pact-msw-adapter side:

  • minimal required version of Node is v18
  • some exported types were renamed and extended to match msw behaviour
    • MswMatch is now MatchedRequest and is { request: Request; requestId: string; response: Response }
    • ExpiredRequest still called the same and is { request: Request; requestId: string; startTime: number; duration?: number }
    • added PendingRequest as { request: Request; requestId: string; }
  • PactMswAdapterOptions.providers function variant is now consistent with msw and has signature (event: PendingRequest) => string | null
  • PactMswAdapter.emitter events are slightly updated to match msw behaviour
    • pact-msw-adapter:expired handler must have signature (event: ExpiredRequest) => void
    • pact-msw-adapter:match handler must have signature (event: MatchedRequest) => void
  • convertMswMatchToPact is now async function



Made possible by these awesome people! You are welcome to contribute too!

Repo Contributors

Special thanks to Juan Cruz for being an early adopter and improving the experience!

Who is using pact-msw-adapter?



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