gRPC (https://grpc.io) is a protocol (and series of libraries) used for expressing APIs, typically powered by Protocol Buffers (https://protobuf.com). It can be considered an alternative to REST/HTTP APIs but is also frequently used to model HTTP/REST APIs in a schema-first/contract-first software development approach. Other libraries support gRPC’s protocol, such as Connect RPC (https://connectrpc.com), which provides gRPC and standard HTTP/REST endpoints from a single binary. Callers can use JSON or Protobuf messages in their API calls, whichever makes sense, and the server can transcode them on the fly.