Ensuring the reliability of smart contracts is of vital importance due to the wide adoption of smart contract programs in decentralized financial applications. However, statically checking many rich properties of smart contract programs can be challenging. On the other hand, dynamic validation approaches have shown promise for widespread adoption in practice. Nevertheless, as part of the programming environment for smart contracts, existing dynamic validation approaches have not provided programmers with a notion to clearly articulate the interface between components, especially for addresses representing opaque contract instances. We argue that the “design-by-contract” approach should complement the development of smart contract programs. Unfortunately, there is limited linguistic support for it in existing smart contract languages. In this paper, we design a Solidity language extension, ConSol, that supports behavioral contracts. ConSol provides programmers with a modular specification and monitoring system for both functional and latent address behaviors. The key capability of ConSol is to attach specifications to first-class addresses and monitor violations when invoking these addresses. We evaluate ConSol using 20 real-world cases, demonstrating its effectiveness in expressing critical conditions and preventing attacks. Additionally, we assess ConSol’s efficiency and compare gas consumption with programs fixed with manually inserted assertions, showing that our approach introduces only marginal gas overhead. By separating specifications and implementations using behavioral contracts, ConSol assists programmers in writing more robust and readable smart contracts.