.NET9 preview's OpenAPI document generation doesn't work when transcoded from gRPC

[python3js]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Description:

I am using .NET 9 Preview's built-in support for OpenAPI document generation with gRPC JSON transcoding. However, the OpenAPI document generated contains an empty paths object when the transcoding is enabled. The document generates correctly (without empty paths) when I don't use the transcoding service.

I've followed the tutorial for gRPC JSON transcoding from .NET 8 (available here) and refactored it to incorporate .NET 9 Preview's built-in OpenAPI support (as described here).

However, the paths in the OpenAPI document remain empty.

---

Generated OpenAPI Document:

{
  "openapi": "3.0.1",
  "info": {
    "title": "Transcoding | v1",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "http://localhost:5204"
    }
  ],
  "paths": {},
  "components": {}
}

Program Code:

using Transcoding.Services;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddGrpc().AddJsonTranscoding();
builder.Services.AddOpenApi();

var app = builder.Build();

app.MapOpenApi();

app.MapGrpcService<GreeterService>();

app.Run();

---

Expected Behavior

I expect the OpenAPI document to populate the paths with the routes corresponding to the gRPC methods exposed via JSON transcoding, similar to how it works without the transcoding services.

Actual Behavior

When using gRPC JSON transcoding with Microsoft.AspNetCore.Grpc.JsonTranscoding, the generated OpenAPI document has empty paths. The issue only occurs when transcoding is enabled.

Steps To Reproduce

1. Use the preview versions of Microsoft.AspNetCore.OpenApi and Microsoft.AspNetCore.Grpc.JsonTranscoding.
2. Follow the instructions from the linked articles to enable gRPC JSON transcoding and OpenAPI document generation.
3. Generate the OpenAPI document.

Exceptions (if any)

It does not throw any Exceptions. It just doesn't work as intended.

.NET Version

9.0.100-preview.7.24407.12

Anything else?

ASP.NET core 2.66.0

For reproducibility, I have created a repository that contains the setup where this issue occurs. You can find it here: https://github.com/python3js/Transcoding.

[captainsafia]

@python3js Thanks for filling this issue!

OpenAPI support for gRPC JSON transcoding is supported via the Microsoft.AspNetCore.Grpc.Swagger package as documented in this page.

The package still takes a dependency on Swashbuckle (as of .NET 9). We hope to add support for the new Microsoft.AspNetCore.OpenApi package in the future but you'll still need to use the Microsoft.AspNetCore.Grpc.Swagger package to dd support for gRPC in this way.

[python3js]

@captainsafia Thank you!