I'm developing an Outlook add-in, which works fine in the OWA, but I've had trouble with the Outlook desktop client, specifically that it caches all files (js, css, as well as html). You'd think that restarting my Outlook client (or restarting the computer) would clear the cache, but no. The old version was kept. These instructions also didn't do the trick.
Following the instructions of this topic, I was finally able to clear the cache and see my new version in action.
However, I'm left with the question how the Outlook desktop client caches add-in files. Compounding the mystery is the fact that some of my colleagues experienced the same problem, while some did see the new version when I deployed it, even though they kept their Outlook client open continually.
So, what exactly is the mechanism by which the Outlook cache is refreshed? Is there some setting in the Outlook client that determines this? Surely, there must be a less cumbersome way to clear the Outlook add-in cache than using those Edge DevTools.
The caching mechanism is determined by the underlying browser, not the Outlook Desktop client. The problem you are experiencing is alleviated by the new Microsoft Edge (Chromium-based) brower. This documentation has a matrix of the Outlook + Windows configurations that support the new Chromium-based Edge browser as the underlying browser for running Outlook/Office add-ins. For a taskpane add-in, you can simply refresh the add-in by right-clicking the pane, and selecting Refresh. For an On-Send or a Ui-Less add-in, you may still have to go through the workaround of creating a taskpane to help clear the cache.