Let’s say you have a app.test.yaml, app.dev.yaml and app.prod.yaml stack - is it possible to have a configuration option which affects all 3 but is only declared in one place such as pulumi.yaml?

Not via YAML. But your program itself can have the default value baked in - which is more or less equivalent to it being in Pulumi.yaml. For example

const region = config.get(“region”) || “us-west-2”.

👍