Lookups are a popular way to express repeated constraints in state-of-the art SNARKs. This is especially the case for zero-knowledge virtual machines (zkVMs), which produce succinct proofs of correct execution for programs expressed as bytecode according to a specific instruction set architecture (ISA). The Jolt zkVM (Arun, Setty & Thaler, Eurocrypt 2024) for RISC-V employs Lasso (Setty, Thaler & Wahby, Eurocrypt 2024), an efficient lookup argument for massive structured tables, to prove correct execution of instructions. Internally, Lasso performs multiple lookups into smaller “subtables”, then combines the results. We present an approach to formally verify Lasso-style lookup arguments against the semantics of instruction set architectures. We demonstrate our approach by formalizing and verifying all Jolt 32-bit instructions corresponding to the RISC-V base instruction set (RV32I) using the ACL2 theorem proving system. Our formal ACL2 model has undergone extensive validation against the Rust implementation of Jolt. Due to ACL2’s bitblasting, rewriting, and developer-friendly features, our formalization is highly automated. Through formalization, we also discovered optimizations to the Jolt codebase, leading to improved efficiency without impacting correctness or soundness. In particular, we removed one unnecessary lookup each for four instructions, and reduced the sizes of three subtables by 87.5%.