How to mock gRPC failures?

[phxnsharp]

I would like to mock certain failure cases in a gRPC protocol. For example, let's say we implemented a rude greeter that would reject anyone named "Nathan":

    public override Task<HelloReply> SayHello(HelloRequest request, ServerCallContext context)
    {
        if (request.Name == "Nathan" )
        {
            throw new RpcException(new Status(StatusCode.FailedPrecondition, "No Nathans!"));
        }
        return Task.FromResult(new HelloReply
        {
            Message = $"Hello {request.Name}"
        });
   }

grpcurl -plaintext -d '{ "name": "Nathan" }' -vv -proto Protos/greet.proto localhost:5012 greet.Greeter/SayHello

Resolved method descriptor:
// Sends a greeting
rpc SayHello ( .greet.HelloRequest ) returns ( .greet.HelloReply );

Request metadata to send:
(empty)

Response headers received:
(empty)

Response trailers received:
content-length: 0
content-type: application/grpc
date: Tue, 21 Oct 2025 17:20:44 GMT
server: Kestrel
Sent 1 request and received 0 responses
ERROR:
  Code: FailedPrecondition
  Message: No Nathans!

The documentation at https://microcks.io/documentation/tutorials/first-grpc-mock/ and https://microcks.io/documentation/references/artifacts/grpc-conventions/ does not explain how to configure this in the Postman collection, and an attempt or two at doing it myself didn't seem to work.

[lbroudoux]

Hey @phxnsharp
Did you have a look at #1359? I am on the move at the moment and can not check it in details but IIRC we now have everything to do this.

[phxnsharp]

So that others don't have to go read that issue, the solution is to put the gRPC status code (0-16) into the HTTP status code field in the Postman collection.

Note:

• Documenting this would be super helpful
• [gRPC] Support mocked failure responses #1359 suggests that you can use the body as the error description. This does not work. The code as currently implemented hard codes the description to "Mocked response status code". Not sure yet if that will be an issue for me, but if it is I'll create a separate issue.

Thank you so much for the quick response!

[phxnsharp]

Note that while this works for mocking, it does not work for testing an implementation: #1800