We present Schemathesis, a tool for finding semantic errors and crashes in OpenAPI or GraphQL web APIs through property-based testing. Our evaluation, thirty independent runs of eight tools against sixteen containerized open-source web services, shows that \textbf{Schemathesis wildly outperforms all previous tools}.

It is the \emph{only} tool to find defects in four targets, finds $1.4\times$ to $4.5\times$ more unique defects than the respectively second-best tool for each remaining target, and is the only tool to handle more than two-thirds of our target services without a fatal internal error.

Our full preprint on arXiv goes into considerably more detail.