What's the highest altitude anyone has flown in a biplane?

In 1938, Mario Pezzi flew a piston-driven biplane without a payload to a height of 56,047 feet.

https://ctipft.com/records-flight-altitude/#:~:text=As%20the%20evolution%20of%20piston,an%20altitude%20of%2056%2C047%20ft.

I have to admit I was surprised by this. In my mind I had imagined an open cockpit World War I style biplane and thought the record would be under 10,000 feet because of levels of oxygen in the air.

To add a bit, at https://en.wikipedia.org/wiki/Caproni_Ca.161 you'll see the plane. Pezzi set the record twice, the first being in a pressure suit in an open cockpit, and the second time in a new version of the plane. This, the Caproni Ca.161bis, had the cockpit sealed with the pilot in a pressurised and heated suit as well. https://en.wikipedia.org/wiki/Mario_Pezzi_(aviator) In between Pezzi's records, a Brit called M.J. Adam took the record in a Bristol 138 monoplane - the record not being specifically for bi- or mono-planes.