Thank you for your reply. I may not have clearly conveyed my intent, so let me clarify my question. What I’d like to determine is whether the replacementText itself will be treated as marked text. In your example, even though "あ" is not marked text at the moment it is entered, I’d like to know whether that input should be treated as marked text.

Thanks for the suggestion. Unfortunately, using textInputMode to infer whether the current input will involve marked text doesn’t seem to be perfect. For Japanese input sources, this generally works in practice, except for certain cases such as emoji input. However, marked text can also appear even when the selected input source is English. For example, entering accented characters can involve marked text (e.g. Option+e produces "´", then typing "a" results in "á", where "´" is marked). In addition, the shouldChangeTextIn method is called not only during text input but also during undo operations or Writing Tools operations, where even kana characters are not marked. For these reasons, the input source alone can’t reliably indicate whether marked text is involved. That said, in the AppKit case, similar information can be obtained via NSTextView.inputContext?.selectedKeyboardInputSource. Is it correct to treat this as the AppKit equivalent of textInputMode?

I understand. I’ll implement a workaround as you suggested. Thanks again for your help.