Terraform‘s plugin protocol is the contract between Terraform’s plugins and Terraform, and as such releasing a new version requires some coordination between those pieces. This document is intended to be a checklist to consult when adding a new major version of the protocol (X in X.Y) to ensure that everything that needs to be is aware of it.
The protocol is defined in protobuf files that live in the hashicorp/terraform repository. Adding a new version of the protocol involves creating a new .proto file in that directory. It is recommended that you copy the latest protocol file, and modify it accordingly.
The hashicorp/terraform-plugin-go repository serves as the foundation for Terraform's plugin ecosystem. It needs to know about the new major protocol version. Either open an issue in that repo to have the Plugin SDK team add the new package, or if you would like to contribute it yourself, open a PR. It is recommended that you copy the package for the latest protocol version and modify it accordingly.
The Terraform Registry validates the protocol versions a provider advertises support for when ingesting providers. Providers will not be able to advertise support for the new protocol version until it is added to that list.
Terraform only downloads providers that speak protocol versions it is compatible with from the Registry during terraform init. When adding support for a new protocol, you need to tell Terraform it knows that protocol version. Modify the SupportedPluginProtocols variable in hashicorp/terraform's internal/getproviders/registry_client.go file to include the new protocol.
Use the provider test framework to test a provider written with the new protocol. This end-to-end test ensures that providers written with the new protocol work correctly with the test framework, especially in communicating the protocol version between the test framework and Terraform.
Publish a provider, either to the public registry or to the staging registry, and test running terraform init and terraform apply, along with exercising any of the new functionality the protocol version introduces. This end-to-end test ensures that all the pieces needing to be updated before practitioners can use providers built with the new protocol have been updated.