A failing call answers Failure[status, <|"MessageTemplate"->detail|>], where the status is the C name usearch_status_name spells, such as shape-mismatch, io or read-only, and the detail the engine’s message:
result = DenseInsert[index, {44}, {{1.0, 2.0}}];
If[FailureQ[result], result[[1]]] (* "shape-mismatch" *)
result["MessageTemplate"] (* "The rows' width differs from the index's" *)Arguments the verb’s types do not admit fail as invalid-argument, and a call naming a released handle by its ID fails as uninitialized.