Building Open Source Projects

Basics > Building Open Source Projects

This document provides tips and best practices for building your open source project on CircleCI in the following sections:


To support the open source community, projects that are public on GitHub or Bitbucket receive three free build containers, for a total of four containers. Multiple build containers allow you to build a single pull request (PR) faster with parallelism, or build multiple PRs at once.

These additional containers are automatically enabled, as long as the project is public and running on Linux. If you do not want to use the additional containers or do not want your CircleCI project to be public, you can change this setting. In the Advanced Settings of your project, set the Free and Open Source option to Off.

Note: If you are building an open source project on macOS, contact to enable these additional containers.


While open source can be a liberating practice, take care not to liberate sensitive information.

  • If your repository is public, your CircleCI project and its build logs are also public. Pay attention to the information you choose to print.
  • While environment variables set in the CircleCI application are hidden from the public, these variables will be shared in forked PRs unless explicitly blocked.

Features and Settings for Open Source Projects

The following features and settings are especially useful for open source projects.

Private Environment Variables

Many projects require API tokens, SSH keys, or passwords. Private environment variables allow you to safely store secrets, even if your project is public. For more information, see the Environment Variables document.

Only Build Pull Requests

By default, CircleCI builds every commit from every branch. This behavior may be too aggressive for open source projects, which often have significantly more commits than private projects. To change this setting, go to the Advanced Settings of your project and set the Only build pull requests option to On.

Note: Even if this option is enabled, CircleCI will still build all commits from your project’s default branch.

Build Pull Requests From Forked Repositories

Many open source projects accept PRs from forked repositories. Building these PRs is an effective way to catch bugs before manually reviewing changes.

By default, CircleCI does not build PRs from forked repositories. To change this setting, go to the Advanced Settings of your project and set the Build forked pull requests option to On.

Pass Secrets to Builds From Forked Pull Requests

Running an unrestricted build in a parent repository can be dangerous. Projects often contain sensitive information, and this information is freely available to anyone who can push code that triggers a build.

By default, CircleCI passes secrets to builds from forked PRs If you are uncomfortable sharing secrets with anyone who forks your project and opens a PR, you can disable this option. In the Advanced Settings of your project, set the Pass secrets to builds from forked pull requests option to Off.

When this setting is disabled, CircleCI hides four types of configuration data:

Note: If this setting is disabled, forked PR builds that require secrets will not run successfully on CircleCI.

Example Open Source Projects

Following are a few examples of projects (big and small) that build on CircleCI:

Refer to the Examples document for more public and open source project configuration links organized by CircleCI features and by programming language.