Combine the cayenne and 1 tablespoon salt on a small plate. Rub the rim of a glass with the cut lime, then dip in the cayenne salt to coat.
In a martini shaker filled with ice, combine the lime juice, tequila, and agave. Shake vigorously, then strain into the prepared glass. Garnish with the lime half.
Where did you find those fabulous playing card glasses?