It seems that if you show a custom dialog from the Committing event of a CompletionSession, the result never makes it back to the editor. I'm not sure what happens, but it's very easy to re-produce, e.g. in your IntelliPromptCompletionCustomActions sample. Simply comment out the line that cancels the session and then make sure that the AutoCompletePreText text has a value. If the MessageBox is displayed, nothing is inserted back into the editor. If the MessageBox line is commented out, it works as expected and the AutoCompletePreText is inserted.