handleEmbedTags Title-cases the mimetypes: config value and looks it up with MethodByName on *Generator, then calls it with the fixed 3-argument embed-plugin signature. Any exported Generator method with a different signature is reachable from config data - e.g. mimetypes: {text/x-t: run} resolves to the real, 0-argument Run() error method, and method.Call panics with "too many input arguments". The method == reflect.ValueOf(nil) validity check was also fragile compared to the idiomatic method.IsValid(). isEmbedPluginMethod now checks IsValid() plus the exact parameter and return types before Call is ever reached, falling back to the external-plugin path for anything that doesn't match - the same behavior as when the method isn't found at all.