  We show that for a coconnective ring spectrum satisfying regularity and
flatness assumptions, its algebraic K-theory agrees with that of its $\pi_0$.
We prove this as a consequence of a more general devissage result for stable
infinity categories. Applications of our result include giving general
conditions under which K-theory preserves pushouts, generalizations of
$\mathbb{A}^n$-invariance of K-theory, and an understanding of the K-theory of
categories of unipotent local systems.
