The Caddy API supports inserting an element at an index in an existing array. However, this fails if the provided index would place the item at the end of the array (ie, append the item).

There is a range check on the index here:

The range check makes sense for GET/DELETE/PATCH, but for PUT it would be useful to allow idx to be equal to len(arr). That allows for fewer special cases when using the API.

Example use case:

I have a base config which contains

              "handle": [
                {
                  "@id": "dynamic_routes",
                  "handler": "subroute",
                  "routes": []
                },
              ],

I would like to insert dynamic routes at the start of the routes array inside @dynamic_routes. I think natural way to do this is PUT /id/dynamic_routes/routes/0. But this fails when the array starts out empty. So to always insert at the front of the array I need to first retrieve the current content, check if the array is empty, and if it is then use a POST to append the new item, but if it's not empty then use PUT to item index 0.