The built-in provider

The built-in provider is a special provider that is always available to Pulumi programs (see pulumi/pulumi:pkg/resource/deploy/builtins.go for its definition and pulumi/pulumi:pkg/resource/deploy/deployment.go#L489 for its injection into deployments as part of the provider registry). It is used to manage resources and functionality that are core to the Pulumi programming model, such as stack references and rehydrating resource references. It exposes the pulumi package and provider instances thus belong to the package pulumi:providers:pulumi.