WasmKit

WasmKit is a standalone and embeddable WebAssembly runtime (virtual machine) implementation and related tooling written in Swift. Starting with Swift 6.2, WasmKit CLI executable is included in Swift toolchains distributed at swift.org for Linux and macOS.

Usage

You can find introductory examples and API documentation on the Swift Package Index documentation page.

Command Line Tool

WasmKit provides a command line tool to run WebAssembly binaries compatible with WASI.

$ git clone https://github.com/swiftwasm/WasmKit.git
$ cd WasmKit
$ swift run wasmkit-cli run ./Examples/wasm/hello.wasm
Hello, World!

As a Library

Swift Package Manager

To use WasmKit in your package, add it as a Swift Package Manager dependency.

Run the following commands in the same directory as your Package.swift manifest to add the dependency:

swift package add-dependency https://github.com/swiftwasm/WasmKit --up-to-next-minor-from 0.2.0
swift package add-target-dependency WasmKit <your-package-target-name> --package WasmKit

You can also add the following snippet manually instead to your Package.swift file:

dependencies: [
    // ...other dependencies
    .package(url: "https://github.com/swiftwasm/WasmKit.git", .upToNextMinor(from: "0.2.0")),
],
// ...other package configuration
targets: [
    // ...other targets
    .target(
        name: "<your-package-target-name>",
        dependencies: [.product(name: "WasmKit", package: "WasmKit")],
    )
]

Features

• Reasonably fast
• Minimal dependencies
  • The core runtime engine depends only on swift-system.
  • No Foundation dependency
• Compact and embeddable
  • Debug build complete in 5 seconds¹
• Batteries included
  • WASI support, WAT (WebAssembly text format) parser/assembler, etc.

Supported Platforms

WasmKit engine works on all major platforms supported by Swift. It is continuously tested on macOS, Ubuntu, Amazon Linux 2, Android, and Windows, and should work on the following platforms:

• macOS 10.13+, iOS 12.0+, tvOS 12.0+, watchOS 6.0+
• Amazon Linux 2, Debian 12, Ubuntu 22.04+, Fedora 39+
• Android API Level 30
• Windows 10+

Implementation Status

WebAssembly MVP

Feature	Status
Parsing binary format	✅ Implemented
Parsing text format (WAT)	✅ Implemented
Execution	✅ Implemented
Validation	✅ Implemented

WebAssembly Proposals

Proposals are grouped by their phase in the standardization process. See the WebAssembly proposals repository for details.

Finished (Merged into the spec)

Proposal	Status	WasmKit version
Bulk Memory Operations	✅ Implemented	0.0.2
Fixed-width SIMD	✅ Implemented	main branch
Import/Export of Mutable Globals	✅ Implemented	0.0.2
Memory64	✅ Implemented	0.0.2
Multi-value	✅ Implemented	0.0.2
Non-trapping Float-to-Int Conversions	✅ Implemented	0.0.2
Reference Types	✅ Implemented	0.0.2
Sign-extension Operators	✅ Implemented	0.0.2
Tail Call	✅ Implemented	0.1.4
Typed Function References	🚧 Parser implemented	0.2.0
Branch Hinting	❌ Not implemented
Custom Annotation Syntax in the Text Format	❌ Not implemented
Exception Handling	❌ Not implemented
Extended Constant Expressions	❌ Not implemented
Garbage Collection	❌ Not implemented
Multiple Memories	❌ Not implemented
Relaxed SIMD	❌ Not implemented

Phase 4 - Standardize the Feature (WG)

Proposal	Status	WasmKit version
Threads and Atomics	✅ Implemented	main branch

Phase 1 - Feature Proposal (CG)

Proposal	Status	WasmKit version
Component Model	🚧 In progress	main branch

WASI

Feature	Status	WasmKit version
WASI 0.1	🚧 Majority of syscalls implemented	0.0.2
WASI Threads	❌ Not implemented

Minimum Supported Swift Version (MSSV)

Currently, the minimum supported version is Swift 6.1. The general strategy is to support last two minor versions of the Swift toolchain available at the time of WasmKit's release. At the same time, development branches of WasmKit tend to adopt newer development versions of the Swift toolchain.

Testing

To run the WasmKit test suite, you need to checkout the test suite repositories first.

# Checkout test suite repositories
$ ./Vendor/checkout-dependency
# Run tests
$ swift test

Acknowledgement

This project was originally developed by @akkyie, and is now maintained by the community.

License

WasmKit runtime modules are licensed under MIT License. See LICENSE file for license information.

GDB Remote Protocol support (GDBRemoteProtocol and WasmKitGDBHandler modules) is licensed separately under Apache License v2.0 with Runtime Library Exception, Copyright 2025 Apple Inc. and the Swift project authors.

See https://swift.org/LICENSE.txt for license information.

See https://swift.org/CONTRIBUTORS.txt for Swift project authors.

Footnotes

1. On a 2020 Mac mini (M1, 16GB RAM) with Swift 5.10. Measured by swift package resolve && swift package clean && time swift build --product PrintAdd. License ↩