Le forum de XCAS

I don't understand this:

solve([a+b=6, a/b=1/2], a) returns []
solve([a+b=6, a/b=1/2], [a]) returns []
solve([a+b=6, a/b=1/2], [a,b]) returns [[2,4]] (correctly)

Is this a bug?

(Is this somehow related to solve([2*x=2,x=1.0], x) ?)

It's not a bug. In the first and second command, you are searching common solutions to 2 equations in a, b is assumed to be generic. In the last command you are searching a and b in a system.