My client pre-downloads videos, so I can fast forward and rewind at will.

That’s neat, it’d be identifiable in a fashion similar to missile logic. You know where ads are based on where they aren’t. Actually skipping it would be difficult but muting and doing something else for a predetermined period has been a workaround since radio.

Google’s own Shaka sdk (video playback with ads) gives ad markers in the initial video manifest so that they can be marked on the timeline, so hopefully it’ll be trivial. Usually (but not always) with SSAI, the ads are spliced into the stream just before being sent to the client. That way if a user has just recently watched an ad pod, the server can choose to ignore that marker for a better UX in hopes that they don’t bounce if ads are too frequent.

I doubt it.. They couldn’t even manage to block FFWD on their own website for the longest time. I switched to using alternative front ends long ago, but back when I just used YT directly from my phone’s browser, spam tapping +5s a few times would bypass the ad.

If skipping is blocked in the ad, the client must know it's playing an ad somehow?