An RPC proto oneof field documents the field not used in the path as an optional request parameter

Steps you follow to reproduce the error:

rpc GetArticle (GetArticleRequest) returns (GetArticleResponse) {
    option (google.api.http) = {
        get: "/v2/articles/{slug}"
        additional_bindings: {
            get: "/v2/articles/{id.hex}"
        }
     };
}

message GetArticleRequest {
    oneof article_id {
        insider.protobuf.ObjectID id = 1;
        string slug = 2;
    }
}

With the annotations above, two "separate" endpoints are defined in the output:

/v2/articles/{slug}
/v2/articles/{id.hex}

With the non-path parameter of the oneof listed as an optional query parameter, which is incorrect:

{
    "name": "id.hex",
    "description": "xyz",
    "in": "query",
    "required": false,
    "type": "string"
 }

What did you expect to happen instead:

I expect a single GET endpoint defined with 2 path parameters listed with oneof as required (not both, and nothing listed as a request parameter).

"parameters": [
  {
    "name": "slug",
    "description": "Article slug.",
    "in": "path",
    "required": true,
    "type": "oneof"
  },
  {
    "name": "id.hex",
    "description": "Article ID",
    "in": "path",
    "required": true,
    "type": "oneof"
  }
]

Your answer here.

What's your theory on why it isn't working:

I suspect there is simply no code to deal with the protobuf oneof type.