Optional routeTemplate parameters is tagged as required in spec

config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

public class ValuesController : ApiController
{
    // GET api/<controller>
    public IEnumerable<string> Get()
    {
        return new string[] { "value1", "value2" };
    }

    // GET api/<controller>/5
    public string Get(int id)
    {
        return "value";
    }
}

The path parameter gets documented as required in swagger.json:

{
  "/api/Values": {
    "get": {
      "operationId": "Values_Get"
    }
  },
  "/api/Values/{id}": {
    "get": {
      "operationId": "Values_Get",
      "parameters": [
        {
          "name": "id",
          "in": "path",
          "required": true,
          "type": "integer",
          "format": "int32"
        }
      ]
    }
  }
}

Same result using a nullable int

public class ValuesController : ApiController
{
    public string Get(int? id)
    {
        return "value";
    }
}

Or optional string

public class ValuesController : ApiController
{
    public string Get(string id = null)
    {
        return "value";
    }
}

[moander]

I did some more testing on how things are handled by asp.net:

/{id} is required

    public string Get(int id)

    public string Get(int? id)

    public string Get(string id)

/{id} is optional (but documented as required by Swashbuckle)

    public string Get(int? id = null)

    public string Get(string id = null)

[domaindrivendev]

@moander - that’s exactly the approach I would recommend. It keeps your contract intact and minimizes code changes because the new overloads can delegate to your existing method - which should be made private