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Can't get ajax working in custom JS for Collections column

I’m trying to pull some custom data for display in a Collections column, but the Ajax seems to be not working at all. The URL never gets called, or if it does no response is received. I have verified that the URL works. I know the script is being called, because if I make it just return a specific value, it shows up in the column. Here’s the test Collections JS function:

Collections.renderer.tagfield = function(value, metaData, record, rowIndex, colIndex, store) {
    url: "/provtag",
    success: function(response, opts) {
      return "yes";
    failure: function(response, opts) {
      return 'NO';

Any ideas?