“Auto-cancel redundant workflows” on non-default branches

Build Environment
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Hello,

We would like to clarify whether the “Auto-cancel redundant workflows” feature can be extended to branches other than the default branch.

Based on the current behavior and documentation:

“With the exception of your default branch, we will automatically cancel any outstanding workflows on a branch when a newer pipeline is triggered on that branch.”

Our understanding is that only the default branch is excluded from auto-canceling previous workflows, while all other branches have their running workflows canceled when a new pipeline is triggered.

Use case:
We maintain multiple long-lived branches (e.g., 4.0, 5.0, 6.0) and would like these branches to behave like the default branch—meaning workflows should not be auto-canceled on these branches, while the feature remains active for all others.

We are aware that the default branch is defined via the vcs_info field, for example:

https://circleci.com/api/v2/project/github/<mycompany>/<myproject>

{
  ...
  "vcs_info": {
    "default_branch": "4.0"
  }
  ...
}

Additionally, we came across a workaround in the CircleCI community forum that suggests disabling the feature entirely and implementing a custom/scripted mechanism to cancel previous workflows manually:
https://discuss.circleci.com/t/workaround-auto-cancel-redundant-builds-on-the-default-branch/39468

However, given the number of workflows we maintain, this approach would be quite cumbersome and difficult to scale.

We also attempted to use pr_only_branch_overrides to define specific branches:

GET https://circleci.com/api/v2/project/github/<mycompany>/<myproject>/settings 

{
  "advanced": {
    "autocancel_builds": true,
    "build_prs_only": true,
    "pr_only_branch_overrides": [
      "4.0",
      "5.0",
      "6.0"
    ]
  }
}

However, this configuration does not appear to have any effect on the auto-cancel behavior.

Questions:

  1. Is it possible to configure multiple branches to behave like the default branch for this feature?

  2. If not, is there any workaround or alternative approach to selectively disable auto-cancel for specific branches?

Thank you for your support.

Best regards,
João