BigRational#format treats the entire string from #to_s as an integer, and then applies thousands separators over that directly:

require "big"

BigRational.new(100, 3).format          # => "10,0/3"
BigRational.new(1234567, 890123).format # => "12,345,67/,890,123"

This is entirely wrong. We have two options:

• Format the numerator and denominator separately, giving 100/3 and 1,234,567/890,123.
• Delegate to #to_f, giving 33.33333333333333 and 1.386962251284373.

#humanize is not affected, since it calls #format via #to_f rather than directly.