WebAssembly (Wasm) is a low-level binary instruction format designed for safe and high-performance execution across diverse computing environments and runtimes. As Wasm evolves with new features and proposals, testing the correctness and conformance of Wasm runtimes has become increasingly complex. Manually constructing test suites is labor-intensive and difficult to scale, especially as the specification grows in complexity. While fuzzing-based approaches offer partial automation, they often lack a principled connection to the formal specification, and adapting to evolving or restricted subsets of the specification typically requires manual intervention.

In this paper, we present WEST, a specification-based test generation framework that automatically produces Wasm test cases from mechanized specifications written in SpecTec, a Wasm-specific specification language. Given any full or subset variant of the Wasm specification as input, WEST aims to systematically generate test programs that conform to the input grammar and validation rules, and capture the runtime behavior defined by its execution semantics. The framework allows flexible integration of different test generation strategies. For instance, we demonstrate both top-down and bottom-up approaches for generating Wasm modules, but the architecture is compatible with other generation techniques as well. The framework enables the creation of customized test cases for engines that support only subsets of the Wasm specification. We evaluate WEST across multiple specification variants and engine configurations, demonstrating that it produces valid and diverse test cases. As a result, it reveals 18 bugs across five Wasm engine implementations, 10 of which are confirmed and fixed. We believe that this work provides a solid foundation for future specification-driven test generation and fuzzing.