The WebAssembly System Interface (WASI) is a developing family of APIs that lets WebAssembly programs access host-provided capabilities through defined interfaces. It is not an operating system, a single runtime, or a finished, universal replacement for POSIX. As of October 2026, the project overview identifies WASI 0.3 (Preview 3) as its current preview; the version and runtime matter when choosing APIs or deploying a program.
What does WASI mean?
WASI stands for WebAssembly System Interface. The project describes it as a set of APIs being developed for eventual standardization by the WASI Subgroup of the WebAssembly Community Group. Those APIs define ways a WebAssembly program can request capabilities supplied by its host environment.
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That distinction is important: WASI specifies interfaces, not the host services themselves. A runtime or embedding must provide compatible implementations, and the capabilities available to a program depend on the interfaces and implementation it uses. The project’s official overview describes WASI as an evolving API effort rather than a completed operating system interface.
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WASI, WIT, and the Component Model are related but different. WASI is the API family. WIT is an interface description language used to define component imports and exports. The Component Model provides the architecture for components and their interactions.
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- WIT interface: A collection of functions and types. It describes operations and the data they use.
- WIT world: A complete description of a component’s imports and exports. It can also be used to generate language bindings.
- Component: Declares the capabilities it needs from a host or other component, and the capabilities it makes available. The runtime or embedding supplies compatible imports.
The WIT documentation explains WIT’s role in describing imports, exports, and shared types; the WIT guide describes interfaces and worlds.
How WASI versions have changed
WASI’s preview labels refer to successive API generations, not interchangeable names for one fixed interface. The project overview describes the progression below; the status of previews can change as the work advances.
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| Version | Interface model and context | Notable direction |
|---|---|---|
| WASI 0.1 / Preview 1 | Initial API defined with the witx IDL; influenced by POSIX and CloudABI. | Earlier generation. Do not assume its interfaces match newer component-based APIs. |
| WASI 0.2 / Preview 2 | Modular APIs defined with WIT and based on the Component Model. | Designed for broader language support, modularity, a more expressive type system, and virtualizability. |
| WASI 0.3 / Preview 3 | Builds on 0.2 and is identified as the current preview by the project overview as of October 2026. | Uses Component Model asynchronous functionality, including future and stream types, rather than the earlier explicit streams and polling interfaces. |
The Component Model feature record also documents features adopted for WASI 0.3.1, including map<K, V> and implements/external-id annotations, adopted on August 6, 2026. These details are version-specific, not a guarantee that every implementation supports them. See the WASI overview and Component Model feature record.
What APIs does WASI include?
The versioned WASI v0.2.12 specification is a Component Model-based collection. Its listed WIT packages cover several host capabilities:
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wasi:iofor input/output abstractionswasi:randomfor random valueswasi:clocksfor clock-related interfaceswasi:socketsfor socket interfaceswasi:filesystemfor filesystem operationswasi:clifor command-line program interfaceswasi:httpfor HTTP interfaces
Each package in that specification is version 0.2.12. The package list does not mean that every WASI runtime implements every capability. See the WASI 0.2.12 specification.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should developers check before choosing a WASI target?
WASI version alone does not establish whether a program will run in a particular environment. Check the API generation and interface description language, whether the program uses core modules or Component Model components, the required host interfaces, and support in the exact SDK and runtime combination.
- Identify the interfaces your program needs. List required capabilities such as filesystem access, clocks, random values, sockets, or HTTP.
- Match the API generation. Determine whether the program targets Preview 1 or a component-based Preview 2/3 interface; their APIs and tooling are not interchangeable.
- Check the toolchain target. For example, the wasi-sdk documentation lists separate
wasm32-wasip1,wasm32-wasip2, andwasm32-wasip3targets. - Verify the deployment runtime. Confirm that it implements the required interfaces for the target you selected rather than relying on a general claim of “WASI support.”
As a specific wasi-sdk example, its documentation says networking is not supported by its WASIp1 targets but is supported by its WASIp2 and WASIp3 targets. That statement describes wasi-sdk’s documented targets; it should not be generalized to every toolchain or runtime. See the wasi-sdk documentation.
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