Wraps gRPC server-mocking patterns for client-side tests: Go bufconn (in-memory net.Listener via google.golang.org/grpc/test/bufconn) + mockgen-generated interface mocks, Python pytest-grpc fixtures + unittest.mock patching of stubs, JVM grpc-mock library / in-process gRPC server (InProcessServerBuilder), Node @grpc/grpc-js fake server with NewServer-on-port-0. Use when writing client-side tests that need a controllable gRPC server response (success cases, error cases per grpc-status-code-mapping-reference, timeouts, and single-response error injection) without spinning up a real backend. For multi-message streaming-sequence tests (server-streaming, bidi), use grpc-streaming-test-author instead. Distinct from grpcurl-cli (ad-hoc CLI invocation against a real server) and ghz-load (perf against a real server).