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This guide covers building the Dart SDK and runtime for Fuchsia OS.

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Prerequisites

Before building for Fuchsia, you must have the Dart source tree set up using the standard build instructions. See Building from Source for details.

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Setup

1

Update your .gclient file

Configure your build to download Fuchsia dependencies by editing your .gclient file (located in the directory that contains the main dart directory):
"custom_vars": {
  "download_fuchsia_deps": True,
},
Add this to the existing custom_vars section if you already have one, or create the section if it doesn’t exist.
2

Sync dependencies

Download the Fuchsia-specific dependencies:
gclient sync
This downloads the Fuchsia SDK and related build tools.

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Building

Build the Dart SDK for Fuchsia using the --os=fuchsia flag: 
./tools/build.py --mode=release --os=fuchsia --arch=arm64 create_sdk runtime

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Build Options

  • --mode: Build mode (release or debug)
  • --os=fuchsia: Target Fuchsia OS
  • --arch: Target architecture (typically arm64 or x64)
  • create_sdk: Build the complete SDK
  • runtime: Build the Dart VM runtime

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Supported Architectures

Fuchsia builds support:
  • arm64 - 64-bit ARM (most common for Fuchsia devices)
  • x64 - 64-bit x86 (for Fuchsia emulators)

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Testing

Run tests on Fuchsia builds using the test script with the Fuchsia runtime: 
./tools/test.py -nvm-fuchsia-release-arm64 -j4 ffi

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Test Command Breakdown

  • -nvm-fuchsia-release-arm64: Named configuration for Fuchsia VM
    • vm: Use the VM runtime
    • fuchsia: Target Fuchsia OS
    • release: Release build mode
    • arm64: ARM64 architecture
  • -j4: Run 4 parallel test tasks
  • ffi: Run FFI tests (you can specify other test suites)

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Running Different Test Suites

./tools/test.py -nvm-fuchsia-release-arm64 -j4 ffi

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Build Output

The Fuchsia build artifacts are placed in: 
out/FuchsiaReleaseARM64/  # For ARM64 release builds
out/FuchsiaDebugARM64/    # For ARM64 debug builds
out/FuchsiaReleaseX64/    # For x64 release builds

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Common Build Configurations

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Development Build

For faster iteration during development: 
./tools/build.py --mode=debug --os=fuchsia --arch=arm64 runtime

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Release Build with Tests

For testing a production-like build: 
./tools/build.py --mode=release --os=fuchsia --arch=arm64 create_sdk runtime
./tools/test.py -nvm-fuchsia-release-arm64 -j4

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Multi-Architecture Build

Build for both ARM64 and x64: 
./tools/build.py --mode=release --os=fuchsia --arch=arm64 create_sdk runtime
./tools/build.py --mode=release --os=fuchsia --arch=x64 create_sdk runtime

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Troubleshooting

Ensure you’ve configured Fuchsia dependencies:
  1. Check your .gclient file contains: 
    "custom_vars": {
  "download_fuchsia_deps": True,
},
  2. Re-run gclient sync: 
    gclient sync
Verify your test configuration matches your build:
  • For ARM64 release: -nvm-fuchsia-release-arm64
  • For ARM64 debug: -nvm-fuchsia-debug-arm64
  • For x64 release: -nvm-fuchsia-release-x64
The architecture and mode in the test name must match your build.
Speed up builds by:
  1. Building only the runtime (omit create_sdk): 
    ./tools/build.py --mode=release --os=fuchsia --arch=arm64 runtime
  2. Using a local filesystem for output (see Build Performance Tips)
  3. Building in release mode (faster than debug)

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Fuchsia-Specific Considerations

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Fuchsia SDK Version

The Fuchsia SDK version is managed by the DEPS file in the Dart SDK. When you run gclient sync, it downloads the specific Fuchsia SDK version pinned in DEPS.

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Component Model

Fuchsia uses a component-based architecture. When deploying Dart applications to Fuchsia, you’ll need to package them as Fuchsia components. This build process creates the Dart runtime that can be included in such components.

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System Libraries

The Fuchsia build links against Fuchsia’s system libraries. The build system automatically handles this when you specify --os=fuchsia.

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Integration with Fuchsia Development

If you’re developing Fuchsia itself and want to integrate a custom Dart build:
  1. Build the Dart runtime for Fuchsia as shown above
  2. The build output can be integrated into Fuchsia’s build system
  3. Refer to Fuchsia’s documentation for details on integrating prebuilt components
For full Fuchsia development workflows, consult the Fuchsia documentation in addition to this guide.

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Next Steps

Build from Source

General build instructions for all platforms

Testing

Learn about testing the Dart SDK

ARM/RISC-V

Build for other architectures

Contributing

Contribute to the Dart SDK

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