Like Oko says, down-stay or some other form of This Game Ends Immediately. Usually with a NOPE. Georgie will do this to my mom because she tolerates it - I do not, so she doesn't even try it with me. It's over-arousal, lack of impulse control type of stuff, or in Georgie's case she is just a pushy bitch who will do whatever she gets away with.
It seems to have coincided a lot with me wearing sleeves. He is going for my arms and not my legs/feet.
I'm not sure if it is be size he is anticipating a tug reward (and oh look! Sleeves can be tugged on!). Or if its just overstim/not level headed thinking. Or frustration when he doesn't get a command fast enough.
I had switched to food to slow him down a bit but am afraid of encouraging him to slow down too much for obvious reasons. I have been ending it if he really bites at me. Have not done a down/stay but will end the exercise and withhold reward if he bites. Today he completely ripped my shirt sleeve.
We are just doing handling exercises on jump stand. Push, out, threads foundations, wraps etc.
I just interrupt and make sure he doesn't self reward and tell him that was inappropriate then start over and try to better judge his arrousal level to promote success in the context of agility.
Away from agility I will work him up to that point and reward with a toy he can bite and playplayplay then set him up to be tempted to bite. If he chooses well, party and release to a tug. If not, calm but firm NRM and either a time out or a restart depending on the situation. Then brought it back to an agility setting.
We pretty much worked it out during heel/side work and restrained recalls, both situations where he is coming in close and we are both in motion. He figured out that biting didn't get him what he wanted: reaction and interaction. And it has translated to agility so far but we're not working extensively on sequences or anything yet.
What Shai said. You need to work on his keeping his mouth to himself outside of the times when you need it - it's a hard skill for baby dogs. I also would recommend trying to figure out the triggers so you can work specifically on those.
A conditioned relaxed settle on a mat, might be very useful as well.
Zo got me at the end of the run badly enough to poke holes in my arm. She was fine whilst we were running but was still so high after the run she would bite.
But typically I am a fan of "you bite = the game stops" Depending on the dog that can be a down stay or it can be a walk off the filed and back to your crate. (Not that Dekka bites but when she gets a 'you are crazy and not listening' she has to get off the field. Otherwise we get a behavior chain that has her doing the undesired behavior followed immediately with the offered down stay)