SwiftWasm

Compiles your Swift code to WebAssembly.

Website | Source | Docs ("the SwiftWasm book")

We use LLVM as a compiler backend to produce WebAssembly binaries. Our resulting binaries also depend on WASI, which is a modular system interface for WebAssembly. WASI is mainly required to compile Swift Standard Library.

Important note: In 2024, Apple introduced Swift Embedded. While both projects benefit from each other, it is important to understand that they are different targets at the build phase, consequentially with different sets of abilities. Embedded Swift very limited but can produce small binaries. Example.

Project Status: All compiler and standard library changes have been upstreamed to the official Swift repository, and the upstream CI is now testing WebAssembly targets.

The remaining works are: Integrating the build system with the official Swift CI.

Installation

Before installing the Swift SDK, you need to ensure the following:

Please ensure you have installed the Swift 6.1 Open Source toolchain.

Toolchain Output
❌ Xcode Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
✅ Open Source (macOS) Apple Swift version 6.1 (swift-6.1-RELEASE)
✅ Open Source (Others) Swift version 6.1 (swift-6.1-RELEASE)

Once you have installed the Open Source toolchain, you can install the Swift SDK for WebAssembly:

swift sdk install "https://github.com/swiftwasm/swift/releases/download/swift-wasm-6.1-RELEASE/swift-wasm-6.1-RELEASE-wasm32-unknown-wasi.artifactbundle.zip" --checksum "7550b4c77a55f4b637c376f5d192f297fe185607003a6212ad608276928db992"

After installing the Swift SDK, you can see 6.1-RELEASE-wasm32-unknown-wasi in the Swift SDK list:

swift sdk list

You can also find other SDKs from the GitHub Releases page.

Hello, World

First, create a new directory for your project and navigate into it:

$ mkdir hello && cd hello

Create a new Swift package:

$ swift package init --type executable

You can use the installed SDKs to cross-compile Swift packages for WebAssembly:

$ swift build --swift-sdk wasm32-unknown-wasi
...
$ file .build/wasm32-unknown-wasi/debug/hello.wasm
.build/wasm32-unknown-wasi/debug/hello.wasm: WebAssembly (wasm) binary module version 0x1 (MVP)

You can run the built WebAssembly module using wasmtime:

$ wasmtime .build/wasm32-unknown-wasi/debug/hello.wasm
Hello, world!

FAQ

How to check if I am using Open Source toolchain or Xcode toolchain?

$ swift --version | head -n1

Toolchain Output
Xcode Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Open Source (macOS) Apple Swift version 6.1 (swift-6.1-RELEASE)
Open Source (Others) Swift version 6.1 (swift-6.1-RELEASE)

:0: error: module compiled with Swift 6.1 cannot be imported by the Swift x.y.z

This error occurs when the Swift toolchain version you are using is different from the version of the Swift SDK you have installed.

To resolve this issue, you can either:

Switch the Swift toolchain to the same version as the Swift SDK you have installed. Check the official guide for how to switch the toolchain.
Install the Swift SDK for the same version as the Swift toolchain you are using. Use the following shell snippet to query compatible Swift SDK for your current toolchain version:

(
  V="$(swiftc --version | head -n1)"; \
  TAG="$(curl -sL "https://raw.githubusercontent.com/swiftwasm/swift-sdk-index/refs/heads/main/v1/tag-by-version.json" | jq -r --arg v "$V" '.[$v] | .[-1]')"; \
  curl -sL "https://raw.githubusercontent.com/swiftwasm/swift-sdk-index/refs/heads/main/v1/builds/$TAG.json" | \
  jq -r '.["swift-sdks"]["wasm32-unknown-wasi"] | "swift sdk install \"\(.url)\" --checksum \"\(.checksum)\""'
)

What is the difference between the Swift Toolchain and the Swift SDK?

The Swift toolchain is a complete package that includes the Swift compiler, standard library, and other tools.

The Swift SDK includes a subset of the Swift toolchain that includes only the necessary components for cross-compilation and some supplementary resources.

What is included in the Swift SDK for WebAssembly?

The Swift SDK for WebAssembly includes only the pre-built Swift standard libraries for WebAssembly. It does not include the Swift compiler or other tools that are part of the Swift toolchain.

6.2 release:

Tutorials

Libraries and frameworks

Developer tools

Sample projects

Resources

Awesome-SwiftWasm: A community-driven curated list of SwiftWasm projects and content

Tags: language   web assembly  

Last modified 11 October 2025