Can you explain why my Craftsman 12.5 hp model #917255581 shuts off when I engage the blades? Engine runs fine, plenty of oil, blades aren't restricted, belts appear functional, but when the blades are engaged even slightly the engine shuts off. Is there a safety switch?

Sounds like the shift lever safety, or the seat safety may be defective, the seat switch you may guess is fine because it starts, not necessarily so though, the clutch/brake safety switch is what allowed you to start the engine.