Why does my program fail? We present a novel and general technique to automatically determine failure causes and conditions, using logical properties over input elements: “The program fails if and only if
int(⟨length⟩) > len(⟨payload⟩) holds—that is, the given
⟨length⟩ is larger than the
⟨payload⟩ length.” Our AVICENNA prototype uses modern techniques for inferring properties of passing and failing inputs and validating and refining hypotheses by having a constraint solver generate supporting test cases to obtain such diagnoses. As a result, AVICENNA produces crisp and expressive diagnoses even for complex failure conditions, considerably improving over the state of the art with diagnoses close to those of human experts.