JSON REST client proxy generator
Usually when I see your questions there isn't much for me to say, because you usually flesh everything out really well. :) (Probably why this has been unanswered so long.)

That said, I think I do have one comment here:

If you have support to edit the HeaderAttribute or the GetAttribute, I would consider replacing the {0}, {1} (etc.) format symbols with a named format symbol.

[Get("posts/{id}")]
Task&lt;BlogPost&gt; GetAsync(int id);

[Site("https://jsonplaceholder.typicode.com")]
public interface ITypicode
{
    [Get("posts/{id}")]
    [Header("X-API-KEY: {apiKey}")]   // req - in
    [Header("Content-Type: {contentType}; charset={charset}")]  // res - out
    Task&lt;BlogPost&gt; GetAsync(
        int id, string apiKey, out string contentType, out string charset);    
}


It makes everything more meaningful.

If it's WebAPI you probably don't have access to the source for these attributes, but you can probably wrap them with a new one to add support for this feature. (Adds a little complexity, but should be really awesome to see happen.)