protolock is a utility to help ensure no API-breaking changes are made to a set of protocol buffer definitions. This includes all native protobuf types and services + RPCs.

A list of all rules enforced by protolock include:

No Using Reserved Fields
No Removing Reserved Fields
No Removing Fields Without Reserve
No Changing Field IDs
No Changing Field Types
No Changing Field Names
No Removing RPCs
No Changing RPC Signature

Additionally, users can define their own plugins to add custom checks. protolock plugins are similar in use and creation to protoc plugins.

A more comprehensive presentation of the tool can be found here.