  In this article we prove that the numerical Grothendieck group of every
smooth proper dg category is invariant under primary field extensions, and also
that the mod-n algebraic K-theory of every dg category is invariant under
extensions of separably closed fields. As a byproduct, we obtain an extension
of Suslin's rigidity theorem, as well as of Yagunov-Ostvaer's equivariant
rigidity theorem, to singular varieties. Among other applications, we show that
base-change along primary field extensions yields a faithfully flat morphism
between noncommutative motivic Galois groups. Finally, along the way, we
introduce the category of n-adic noncommutative mixed motives.
