Guide for rolling out breaking changes

It is inevitable that we will make breaking changes to Bazel. We will have to change our designs and fix the things that do not quite work. However, we need to make sure that community and Bazel ecosystem can follow along. To that end, Bazel project has adopted a backward compatibility policy. This document describes the process for Bazel contributors to make a breaking change in Bazel to adhere to this policy.

  1. Follow the design document policy.

  2. File a GitHub issue.

  3. Implement the change.

  4. Update labels

  5. Flip the incompatible flag.

GitHub issue

File a GitHub issue in the Bazel repository. See example.

We recommend that:

  • The title starts with the name of the flag (the flag name will start with incompatible_).

  • You add the label incompatible-change. This ensures that the flag, once it’s merged, will be picked up by the bazelisk-plus-incompatible-flags pipeline.

  • The description contains a description of the change and a link to relevant design documents.

  • The description contains a migration recipe, to explain users how they should update their code. Ideally, when the change is mechanical, include a link to a migration tool.

  • The description includes the intended length of migration window.

  • The description includes an example of the error message users will get if they don’t migrate. This will make the GitHub issue more discoverable from search engines. Make sure that the error message is helpful and actionable. When possible, the error message should include the name of the incompatible flag.

For the migration tool, consider contributing to Buildifier. It is able to apply automated fixes to BUILD, WORKSPACE, and bzl files. It may also report warnings.


Create a new flag in Bazel. The default value must be false. The help text should contain the URL of the GitHub issue. As the flag name starts with incompatible_, it needs a metadata tag:

      metadataTags = {OptionMetadataTag.INCOMPATIBLE_CHANGE},

In the commit description, add a brief summary of the flag. Also add RELNOTES: in the following form: RELNOTES: --incompatible_name_of_flag has been added. See #yxz for details

The commit should also update the relevant documentation. As the documentation is versioned, we recommend updating the documentation in the same commit as the code change.


Once the commit is merged, add the label migration-ready to the GitHub issue.

Later a Bazel release manager will update the issue and replace the label with migration-xx.yy.

The label breaking-change-xx.yy communicates when we plan to flip the flag. If a migration window needs to be extended, the author updates the label on GitHub issue accordingly.

If a problem is found with the flag and users are not expected to migrate yet: remove the flags migration-xx.yy.

Updating repositories

  1. Ensure that the core repositories are migrated. On the bazelisk-plus-incompatible-flags pipeline, the flag should appear under “The following flags didn’t break any passing Bazel team owned/co-owned projects”.
  2. Notify the owners of the other repositories.
  3. Wait 14 days after the notifications to proceed, or until the flag is under “The following flags didn’t break any passing projects” in the CI.

Flipping the flag

Before flipping the default value of the flag to true, please make sure that:

  • The migration window is respected.
  • User concerns and questions have been resolved.

When changing the flag default to true, please:

  • Use RELNOTES[INC] in the commit description, with the following format: RELNOTES[INC]: --incompatible_name_of_flag is flipped to true. See #yxz for details You can include additional information in the rest of the commit description.
  • Use Fixes #xyz in the description, so that the GitHub issue gets closed when the commit is merged.
  • Review and update documentation if needed.