Under Settings → Applications & Envs → Preview Environments, the new "Preview defaults" form takes custom headers – for example a Vercel protection bypass header – and a max concurrent tests cap. They are copied onto each new preview environment and refreshed on the next review if you rotate a secret.
The GitHub Action and API pick up the defaults when you don't pass customHeaders. Passing it replaces the headers for that preview, and [] clears them.
More on this: PR testing, multi-environment testing, PR and CI/CD testing and the Linear + Cursor + Vercel + QA.tech stack.