Interoperability between deductive program verification tools is a well-recognized long-standing challenge. In this paper we propose a solution for a well-delineated aspect of this challenge, namely the exchange of abstract contracts for possibly stateful interfaces that represent modularity boundaries. Interoperability across tools, specification paradigms, and programming languages is achieved by focusing on abstract implementation-independent behavioral models. The approach, called Contract-LIB in reminiscence of the widely-successful SMT-LIB format, aims to standardize the language over which such contracts are formulated and provides clear guidance on its integration with established methods to connect high-level specifications with code-level data structures. We demonstrate the ideas with examples, define syntax and semantics, and discuss the rationale behind key design decisions.