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WASI means WebAssembly System Interface. It is a developing family of APIs that lets WebAssembly programs use capabilities supplied by a host, such as filesystem access, clocks, or networking. It is not an operating system, a single runtime, or a finished, universal replacement for POSIX.
What the WebAssembly System Interface does
The WASI project describes WASI as a set of APIs being developed for eventual standardization by the WASI Subgroup of the WebAssembly Community Group. In practical terms, a program can import defined interfaces, and a compatible runtime or embedding can supply the corresponding host functionality. The interfaces make those interactions explicit rather than assuming that a WebAssembly program can access an operating system’s services directly.
WASI is an API family, not the program that runs WebAssembly. A runtime supplies the environment in which a module or component executes; which WASI interfaces it can use depends on the WASI version, target, and implementation. The project’s overview describes its scope and status at wasi.dev.
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WASI, WIT, and the Component Model are different things
- WASI is the family of APIs that define host-facing capabilities.
- WIT is an interface definition language for describing component imports and exports, including their types.
- The Component Model is the architecture and specification framework for composing WebAssembly components and their interfaces.
In WIT, an interface groups functions and types. A world describes a component’s complete set of imports and exports, and can be used to generate language bindings. This gives a useful way to reason about compatibility: a component declares the capabilities it expects and what it offers; the host or another component must provide compatible interfaces. See the Component Model documentation for WIT and its explanation of interfaces and worlds.
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How WASI versions have changed
WASI’s preview labels refer to evolving versions, not interchangeable names for one fixed API. The following status reflects the project overview and specification records available on October 10, 2026; these details can change as the project develops.
| Version | Interface approach | What distinguishes it |
|---|---|---|
| WASI 0.1 / Preview 1 | Initially described with witx | The project overview says its major influences included POSIX and CloudABI. It is an earlier API generation, not a guarantee of POSIX compatibility. |
| WASI 0.2 / Preview 2 | Modular APIs defined with WIT and based on the Component Model | The project presents this generation as more modular, with broader source-language support, a more expressive type system, and virtualizability. |
| WASI 0.3 / Preview 3 | Builds on 0.2 and uses Component Model asynchronous functionality | The project overview identifies it as the current preview. Its async direction uses future and stream types rather than the earlier explicit streams and polling interfaces. |
The Component Model feature record also notes additions for WASI 0.3.1: map<K, V> and the implements and external-id annotations, adopted on August 6, 2026. Preview status and feature details are time-sensitive; consult the official WASI overview and Component Model feature record for later changes.
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What capabilities the WASI APIs cover
The versioned WASI 0.2.12 specification lists seven WIT packages. Each package is versioned 0.2.12:
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This is the package list for that specification version, not a promise that every runtime implements every package. The package definitions are available in the WASI 0.2.12 specification.
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Why the chosen target and runtime matter
Building for “WASI” is not enough to determine which host functions a program can use. The selected target and the deployment runtime need to agree on the API generation and capabilities the program requires.
For example, the official wasi-sdk C/C++ toolchain documents separate wasm32-wasip1, wasm32-wasip2, and wasm32-wasip3 targets. Its documentation says networking is not supported by its WASIp1 targets, while its WASIp2 and WASIp3 targets support networking. That is a statement about wasi-sdk’s documented targets, not a universal rule for every toolchain or runtime.
Before choosing a target, check that the implementation you will deploy supports the specific interfaces your application imports. For version or target comparisons, focus on the interface description language and component integration, required features such as async operations, the APIs the application needs, and support across both the SDK and runtime.
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No. WASI defines interfaces through which a WebAssembly program may access host-provided functionality; it does not itself provide a complete operating system or guarantee unrestricted access to host resources. The host environment determines what is available through its implementation, and different WASI generations expose different interface sets. Nor should WASI be treated as a finished POSIX replacement: the project describes an evolving API collection, and Preview 1’s POSIX influence does not make the two interfaces equivalent.
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