Session capabilities is a request/response model used by Nightwatch (and other W3C WebDriver clients) to define which browser features it requires from the WebDriver implementation or Selenium server. The WebDriver/Selenium server will also provide its supported capabilities as part of the browser session initialization process.

Session capabilities can be divided in two categories:

1. Input capabilities

In order to create a new browser session, Nightwatch has to provide the desired capabilities to the WebDriver service.

Here's an example capabilities that Nightwatch sends as part of the initial /session request:

Request POST /session  
 desiredCapabilities: {
   browserName: 'chrome',
   name: 'To-Do List End-to-End Test'

2. Browser capabilities

The WebDriver service uses the received input capabilities to create a browser session and responds with the supported capabilities.

Here's an example browser capabilities that Nightwatch receives as part of the initial /session response:

Response 200 POST /session (724ms)
 value: {
   capabilities: {
     acceptInsecureCerts: false,
     browserName: 'chrome',
     browserVersion: '102.0.5005.61',
     chrome: {
       chromedriverVersion: '101.0.4951.41 (93c720...)'
     'goog:chromeOptions': { debuggerAddress: 'localhost:52470' },
     networkConnectionEnabled: false,
     pageLoadStrategy: 'normal',
     platformName: 'mac os x',
     proxy: {},
     setWindowRect: true,
     strictFileInteractability: false,
     timeouts: { implicit: 0, pageLoad: 300000, script: 30000 },
     unhandledPromptBehavior: 'dismiss and notify',
     'webauthn:extension:credBlob': true,
     'webauthn:extension:largeBlob': true,
     'webauthn:virtualAuthenticators': true
   sessionId: '15d21f2132ff0675a97ca419bf6fbd4'

Shared capabilities

Some of the capabilities included in this set are standard and shared between all browsers, but the set may also contain browser-specific capabilities and these are always prefixed.

Capabilities can be used to require a driver that supports a certain subset of features. This can be used to require certain browser features, such as the ability to resize the window dimensions, but is also used in distributed environments to select a particular browser configuration from a matrix of choices.

The Selenium docs provide apple documentation on all the capabilities, both shared and browser specific.