If your project has a large number of tests, it can take a long time for tests to complete running serially on one machine. Running tests in parallel across many virtual machines can save your team time and money when running tests in Continuous Integration (CI).

Cypress can run recorded tests in parallel across multiple machines since version 3.1.0. While parallel tests can also technically run on a single machine, we do not recommend it since this machine would require significant resources to run your tests efficiently.

This guide assumes you already have your project running and recording within Continuous Integration. If you have not set up your project yet, check out our Continuous Integration guide. If you are running or planning to run tests across multiple browsers (Firefox, Chrome, or Edge), we also recommend checking out our Cross Browser Testing guide for helpful CI strategies when using parallelization.

Parallelization Diagram

Splitting up your test suite

Cypress' parallelization strategy is file-based, so in order to utilize parallelization, your tests will need to be split across separate files.

Cypress will assign each spec file to an available machine based on our balance strategy. Due to this balance strategy, the run order of the spec files is not guaranteed when parallelized.

Turning on parallelization

  1. Refer to your CI provider's documentation on how to set up multiple machines to run in your CI environment.

  2. Once multiple machines are available within your CI environment, you can pass the --parallel key to cypress run to have your recorded tests parallelized.

cypress run --record --key=abc123 --parallel

CI parallelization interactions

During parallelization mode, the Cypress Dashboard Service interacts with your CI machines to orchestrate the parallelization of a test run via load-balancing of specs across available CI machines by the following process:

  1. CI machines contact the Cypress Dashboard Service to indicate which spec files to run in the project.
  2. A machine opts in to receiving a spec file to run by contacting Cypress.
  3. Upon receiving requests from a CI machine, Cypress calculates the estimated duration to test each spec file.
  4. Based on these estimations, Cypress distributes (load-balances) spec files one-by-one to each available machine in a way that minimizes overall test run time.
  5. As each CI machine finishes running its assigned spec file, more spec files are distributed to it. This process repeats until all spec files are complete.
  6. Upon completion of all spec files, Cypress waits for a configurable amount of time before considering the test run as fully complete. This is done to better support grouping of runs.

In short: each Test Runner sends a list of the spec files to the Dashboard Service, and the service sends back one spec at a time to each Test Runner to run.

Parallelization process

Parallelization Overview

Balance strategy

Cypress will automatically balance your spec files across the available machines in your CI provider. Cypress calculates which spec file to run based on the data collected from previous runs. This ensures that your spec files run as fast as possible, with no need for manual configuration.

As more and more tests are recorded to the Cypress Dashboard, Cypress can better predict how long a given spec file will take to run. To prevent irrelevant data from affecting the duration prediction, Cypress doesn't use old historical run data regarding the spec file.

Spec duration history analysis

Spec duration forecasting

With a duration estimation for each spec file of a test run, Cypress can distribute spec files to available CI resources in descending order of spec run duration. In this manner, the most time-consuming specs start first which minimizes the overall test run duration.


The examples below are from a run of our Kitchen Sink Example project. You can see the results of this run on the Cypress Dashboard.

Without parallelization

In this example, a single machine runs a job named 1x-electron, defined in the project's circle.yml file. Cypress runs all 19 spec files one by one alphabetically in this job. It takes 1:51 to complete all of the tests.

1x-electron, Machine #1
-- actions.spec.js (14s)
-- aliasing.spec.js (1s)
-- assertions.spec.js (1s)
-- connectors.spec.js (2s)
-- cookies.spec.js (2s)
-- cypress_api.spec.js (3s)
-- files.spec.js (2s)
-- local_storage.spec.js (1s)
-- location.spec.js (1s)
-- misc.spec.js (4s)
-- navigation.spec.js (3s)
-- network_requests.spec.js (3s)
-- querying.spec.js (1s)
-- spies_stubs_clocks.spec.js (1s)
-- traversal.spec.js (4s)
-- utilities.spec.js (3s)
-- viewport.spec.js (3s)
-- waiting.spec.js (5s)
-- window.spec.js (1s)

With parallelization

When we run the same tests with parallelization, Cypress uses its balance strategy to order to specs to run based on the spec's previous run history. During the same CI run as above, we ran all tests again, but this time with parallelization across 2 machines. This job was named 2x-electron in the project's circle.yml file and it finished in 59 seconds.

2x-electron, Machine #1, 9 specs          2x-electron, Machine #2, 10 specs
--------------------------------          -----------------------------------
-- actions.spec.js (14s)                  -- waiting.spec.js (6s)
-- traversal.spec.js (4s)                 -- navigation.spec.js (3s)
-- misc.spec.js (4s)                      -- utilities.spec.js (3s)
-- cypress_api.spec.js (4s)               -- viewport.spec.js (4s)
-- cookies.spec.js (3s)                   -- network_requests.spec.js (3s)
-- files.spec.js (3s)                     -- connectors.spec.js (2s)
-- location.spec.js (2s)                  -- assertions.spec.js (1s)
-- querying.spec.js (2s)                  -- aliasing.spec.js (1s)
-- location.spec.js (1s)                  -- spies_stubs_clocks.spec.js (1s)
                                          -- window.spec.js (1s)

The difference in running times and machines used is very clear when looking at the Machines View on the Dashboard. Notice how the run parallelized across 2 machines automatically ran all specs based on their duration, while the run without parallelization did not.

Without parallelization vs parallelizing across 2 machines

Parallelizing our tests across 2 machines saved us almost 50% of the total run time, and we can further decrease the build time by adding more machines.

Grouping test runs

Multiple cypress run calls can be labeled and associated to a single run by passing in the --group <name> flag, where name is an arbitrary reference label. The group name must be unique within the associated test run.

Machines view grouping expanded

Grouping by browser

You can test your application against different browsers and view the results under a single run within the Dashboard. Below, we name our groups the same name as the browser being tested:

  • The first group can be called Windows/Chrome 69.

    cypress run --record --group Windows/Chrome-69 --browser chrome
  • The second group can be called Mac/Chrome 70.

    cypress run --record --group Mac/Chrome-70 --browser chrome
  • The third group can be called Linux/Electron. Electron is the default browser used in Cypress runs.

    cypress run --record --group Linux/Electron

Grouping to label parallelization

We also have the power of Cypress parallelization with our groups. For the sake of demonstration, let's run a group to test against Chrome with 2 machines, a group to test against Electron with 4 machines, and another group to test against Electron again, but only with one machine:

cypress run --record --group 1x-electron
cypress run --record --group 2x-chrome --browser chrome --parallel
cypress run --record --group 4x-electron --parallel

The 1x, 2x, 4x group prefix used here is an adopted convention to indicate the level of parallelism for each run, and is not required or essential.

Labeling these groups in this manner helps up later when we review our test runs in the Cypress Dashboard, as shown below:

Timeline view with grouping and parallelization

Grouping by spec context

Let's say you have an application that has a customer facing portal, guest facing portal and an administration facing portal. You could organize and test these three parts of your application within the same run:

  • One group can be called package/admin:
cypress run --record --group package/admin --spec 'cypress/integration/packages/admin/**/*'
  • Another can be called package/customer:
cypress run --record --group package/customer --spec 'cypress/integration/packages/customer/**/*'
  • The last group can be called package/guest:
cypress run --record --group package/guest --spec 'cypress/integration/packages/guest/**/*'

This pattern is especially useful for projects in a monorepo. Each segment of the monorepo can be assigned its own group, and larger segments can be parallelized to speed up their testing.

Linking CI machines for parallelization or grouping

A CI build ID is used to associate multiple CI machines to one test run. This identifier is based on environment variables that are unique to each CI build, and vary based on CI provider. Cypress has out-of-the-box support for most of the commonly-used CI providers, so you would typically not need to directly set the CI build ID via the --ci-build-id flag.

CI Machines linked by ci-build-id

CI Build ID environment variables by provider

Cypress currently uses the following CI environment variables to determine a CI build ID for a test run:

ProviderEnvironment Variable
Codeship ProCI_BUILD_ID

You can pass a different value to link agents to the same run. For example, if you are using Jenkins and think the environment variable BUILD_TAG is more unique than the environment variable BUILD_NUMBER, pass the BUILD_TAG value via CLI --ci-build-id flag.

cypress run --record --parallel --ci-build-id $BUILD_TAG

Run completion delay

During parallelization mode or when grouping runs, Cypress will wait for a specified amount of time before completing the test run in case any more relevant work remains. This is to compensate for various scenarios where CI machines could be backed-up in a queue.

This waiting period is called the run completion delay and it begins after the last known CI machine has completed as shown in the diagram below:

Test run completion delay

This delay is 60 seconds by default, but is configurable within the Dashboard project settings page.

Visualizing parallelization and groups in the Dashboard

You can see the result of each spec file that ran within the Dashboard Service in the run's Specs tab. Specs are visualized within a Timeline, Bar Chart, and Machines view.

Timeline View

The Timeline View charts your spec files as they ran relative to each other. This is especially helpful when you want to visualize how your tests ran chronologically across all available machines.

Timeline view with parallelization

Bar Chart View

The Bar Chart View visualizes the duration of your spec files relative to each other.

Bar Chart view with parallelization

Machines View

The Machines View charts spec files by the machines that executed them. This view enables you to evaluate the contribution of each machine to the overall test run.

Machines view with parallelization

Next Steps