Some testing later ... I can confirm that clients are receiving the error. I can see the error in gprcurl/grpcui. Output from grpcurl:

Resolved method descriptor:
rpc find ( .Redacted.GRPC.Discovery.ServicesRequest ) returns ( .Redacted.GRPC.Discovery.ServicesReply );

Request metadata to send:
(empty)

Response headers received:
(empty)

Response trailers received:
content-length: 0
content-type: application/grpc+proto
date: Tue, 16 Dec 2025 16:32:51 GMT
server: Cowboy
Sent 1 request and received 0 responses
ERROR:
  Code: NotFound
  Message: Do you see me?

and the same in the UI:

So the error gets through in the response data, but there are no grpc trailers sent. This breaks with web clients in which grpcweb requires the grpc trailers to be sent encoded within the response data.

So this gets us most of the way there, which is great news. What's left is ensuring that the grpc trailers are also sent ... thoughts?

I see why it doesn't get into the grpcweb trailers. When an exception is thrown, then Cowboy.Handler.send_error gets called, which itself calls send_error_trailers, and send_error_trailers calls :cowboy_req.reply(status, trailers, req) directly while my branch with grpcweb trailers only changes Cowboy.Handler.info for {:stream_trailers, trailers} messages ... so the error path never makes it into the grpcweb client.

If this branch gets merged in, I will adjust the grpcweb trailers branch to work with these changes, and then we should be OK

I see why it doesn't get into the grpcweb trailers. When an exception is thrown, then Cowboy.Handler.send_error gets called, which itself calls send_error_trailers, and send_error_trailers calls :cowboy_req.reply(status, trailers, req) directly while my branch with grpcweb trailers only changes Cowboy.Handler.info for {:stream_trailers, trailers} messages ... so the error path never makes it into the grpcweb client.

Cool

If this branch gets merged in, I will adjust the grpcweb trailers branch to work with these changes, and then we should be OK

Your PR regarding the trailers has already been approved and will be added to the master as soon as I can find the best way to merge them. I'll probably try merging your first one and incorporating the changes into the other PR for the adapter, but I'm still working on it.

With a very simple (~10 lines) change to the grpcweb branch rebased onto this branch, we now see errors on both native and web clients with returning {:error, ...} tuples from map or map_error. Very nice!

I suggest merging this branch, and I can then close #486 and ammend #481

With a very simple (~10 lines) change to the grpcweb branch rebased onto this branch, we now see errors on both native and web clients with returning {:error, ...} tuples from map or map_error. Very nice!

I suggest merging this branch, and I can then close #486 and ammend #481

Done!

@aseigo Thank you for your patience and continued contribution.