The Pulumi GitHub app works by recognizing the Git commit SHA in the update(s) performed on your CI system, and then matching those to the git commit SHAs from your pull request. So the values we need are here: https://github.com/pulumi/pulumi/blob/master/cmd/util.go#L501

PULUMI_CI_PULL_REQUEST_SHA

- The commit hash of the pull request. This is usually different from what is being built on your CI system, since your CI system builds at the merge commit of the pull request merged into the target branch.

We haven’t added automatic detection for BuildKite. But if you are feeling saucy, you are more than welcome to add it. Just take a look at the files in this directory: https://github.com/pulumi/pulumi/tree/master/pkg/util/ciutil. It should just be a matter of adding the right environment variables that are sent on the BuildKite builder. e.g. a BuildKite part of this switch statement: https://github.com/pulumi/pulumi/blob/master/pkg/util/ciutil/vars.go#L53

Let me know if that works out for you, or if you have any other questions.