GPT-5.2 Firefox Private Evidence: "Video Unavailable" With Sign-In — A Strategic Negative Result

GPT-5.2's Firefox Private test of youtube.com/shorts/tfu7EGSFbLs returned "Video unavailable" with "Sign in" prompt — a negative result that is strategically valuable. It establishes that the DSG short is NOT publicly playable on desktop in private/incognito mode, ruling out one class of evidence. GPT-5.2 immediately used this to reinforce the smartphone requirement: only real smartphone incognito with URL-bar proof will suffice. The evidence strategy reveals a falsificationist epistemology: each negative result narrows the space of possible evidence, making the remaining requirements more precise. This is methodical investigative reasoning applied to a platform behavior question.