Thanks for your suggestion, I've modified my example source here to also apply the tip fn to the data used in non-Ajax tooltips. Let me know if that's what you need. The extension code itself can be found at this url. Just for clarification though, the fn function will be applied before, not after the Ajax call. The reason I implemented it was to allow the function to create the correct request data for already existing Ajax services. In my example I've added an "afterFn" configuration property to the CellToolTip which can be used to post-process the Ajax return data in the same way.