Antonym of "response"

Suppose you're on an internet forum. If post Y is a response to post X, then X is a [WHAT] to Y? What's the antonym of "response", here? The word coming to mind is "antecedent", but that's not the right word. The thesauruses I looked at (online) weren't very helpful; most of them didn't list any antonyms, and the ones that did gave words only kinda relevant, in context, like "cause".

Your question is a precursor to my answer.

precursor: one that precedes and indicates the approach of another

Merriam Webster

The word offers a little extra meaning (“and indicates the approach of ...”) to that of a mere antecedent.

By analogy with psychological and biological usage you might describe Post Y as a response to the stimulus of Post X (Britannica)