WASI

Definition

WASI is the acronym for the WebAssembly System Interface, a capability-based standard API that gives WebAssembly modules portable access to system resources like files and clocks outside the browser sandbox.

Overview

WASI is most often encountered as the name developers actually type and discuss day to day — in runtime documentation, CLI flags (`wasmtime run --wasi`), and package ecosystem tooling — even though the underlying specification is formally titled the WebAssembly System Interface. In practice the two terms are used interchangeably, but WASI as a standalone reference point is useful for understanding how the specification is versioned and consumed by tooling, since runtimes, compilers, and package managers all refer to specific WASI versions rather than treating it as one monolithic standard. Compiler toolchains are a major consumer of WASI: Rust's standard library has an official `wasm32-wasi` (now increasingly `wasm32-wasip1`/`wasm32-wasip2`) compilation target that implements Rust's filesystem, networking, and I/O primitives on top of WASI's system calls, letting existing Rust programs compile to a portable Wasm binary with comparatively little code change. Similar toolchain support exists for C and C++ via the WASI SDK, and for other languages including Python (through interpreters like Pyodide or dedicated WASI ports) and Go, each mapping their language's standard library assumptions onto WASI's capability-based interfaces. WASI's versioning has practical consequences for compatibility: a module compiled against WASI Preview 1 runs on any runtime supporting that snapshot, but Preview 2's Component Model-based redesign is not binary-compatible with Preview 1 without an adapter layer, so tooling and runtimes during the transition period often ship compatibility shims that let Preview 1 modules run inside a Preview 2-oriented environment. This versioning churn is a known rough edge in the WebAssembly ecosystem as of the mid-2020s, reflecting that WASI, despite years of production use, is still formally in a preview state as its designers work through networking, threading, and component composition before finalizing a 1.0 specification.

Specification

• Acronym for the WebAssembly System Interface, used interchangeably with the full name
• Versioned specification: Preview 1 (POSIX-like baseline) and Preview 2 (Component Model-based)
• Supported as a first-class compilation target in Rust (wasm32-wasip1/wasip2), C/C++ (WASI SDK), and others
• Capability-based resource access rather than ambient filesystem/network authority
• Referenced directly in runtime CLI tooling, e.g. wasmtime run --wasi
• Compatibility shims exist to run Preview 1 modules on Preview 2-oriented runtimes
• Still formally in a preview/pre-1.0 state despite years of production adoption
• Governed through the Bytecode Alliance's open standardization process

Use Cases

Alternatives

History

WASI, the WebAssembly System Interface, is a standardized system interface that lets WebAssembly modules run securely outside the browser. Because Wasm code in a sandbox has no built-in way to talk to the operating system, WASI defines POSIX-like capabilities — files, network connections, clocks, and random numbers — behind a capability-based security model that grants access explicitly rather than by ambient authority. Mozilla announced the WASI standardization effort in March 2019, with Lin Clark communicating its vision. Stewardship soon moved to the Bytecode Alliance, an industry group announced in November 2019 with founding members Mozilla, Fastly, Intel, and Red Hat. WASI is central to using WebAssembly as a fast, portable, sandboxed runtime for server-side and edge computing.

Frequently Asked Questions

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