Short answer:

COMMIT_INFO_BRANCH

Longer answer: Cypress uses the @cypress/commit-info package to extract git information to associate with your runs (e.g. branch, commit message, author). Under some environment setups (e.g. docker/docker-compose) if the .git directory is not available or mounted, you can pass all git related information under custom environment variables. You can read about these variables and how to debug them in our documentation here: https://docs.cypress.io/guides/continuous-integration/introduction.html#Git-information