Generators for automorphisms of special groups

Let $G$ be a (compact) special group in the sense of Haglund and Wise. We show that $\mathrm{O}\mathrm{u}\mathrm{t}(G)$ is finitely generated, and provide a virtual generating set consisting of Dehn twists and pseudo-twists''. We exhibit instances where Dehn twists alone do not suffice and completely characterise this phenomenon: it is caused by certain abelian subgroups of $G$, called poison subgroups’', which can be removed by replacing $G$ with a finite-index subgroup. Similar results hold for coarse-median preserving automorphisms, without the pathologies: For every special group $G$, the coarse-median preserving subgroups $\mathrm{O}\mathrm{u}\mathrm{t}(G,[\mu ])\le \mathrm{O}\mathrm{u}\mathrm{t}(G)$ are virtually generated by finitely many Dehn twists with respect to splittings of $G$ over centralisers. Proofs are based on a novel, hierarchical version of Rips and Sela’s shortening argument.