My society uses dragons as giant flying weapons/war crafts, usually with a rider that would be half on their belly harnessed to the middle of the dragon's neck. The rider's ankles and waist would be attached to a sort of 'saddle' system with chains, and they'd have to control the head with some sort of bridle. How might this bridle be designed so the rider can still control the dragon, but when the dragon breathes fire it will still be effective? I'd imagine some sort of bit-type implement would be needed, and a loop around the snout probably wouldn't work because the dragon would need to open its mouth to breathe fire.
This fire can reach anywhere between 1112-2500 degrees Fahrenheit, so if they were to use a metal bit, it would have to be a metal that can handle that for prolonged periods without melting or softening at all.
These dragons have roughly human-level intelligence, but also run a lot on instinct and more animal-like logic.