Getting started with developing ffx subtools

FFX Subtools are the top-level commands that the ffx cli can run. These can be either compiled directly into ffx and/or build as separate commands that can be found in the build output directory or the SDK, and they will then be invoked using the FHO tool interface.

Where to put it

First, create a directory somewhere in the fuchsia.git tree to hold your subtool. Currently subtools exist in these locations:

  • The ffx plugins tree, where built-in only and hybrid plugin/subtools go. New subtools should not generally be put here.
  • The ffx tools tree, where external-run-only subtools go. Putting it here makes it easier for the maintainers of ffx to assist with any issues or update your subtool with any changes to the interface between ffx and the tool. If you put it here and the FFX team isn't the primary maintainer of this tool, you must put an OWNERS file in the directory you put it in that adds your team's component and some individual owners so we know how to triage issues with your tool.
  • Somewhere in a project's own tree. This can make sense if the ffx tool is simply a wrapper over an existing program, but if you do this you must have your OWNERS files set up so that the FFX team can approve updates to the parts that interact with ffx. You can do this by adding file:/src/developer/ffx/OWNERS to your OWNERS file over the subdirectory the tool is in.

Other than not putting new tools in plugins, the decision of a specific location may require discussion with the tools team to decide on the best place.

What files

Once you've decided where your tool is going to go, create the source files. Best practices is to have your tool's code broken out into a library that implements things and a that simply calls into that library.

The following file set would be a normal starting point:

But of course you can break things up into more libraries if you want. Note that these examples are all based on the example echo subtool, but parts may be removed or simplified for brevity. Take a look at the files in that directory if anything here doesn't work or seems unclear.

Following is a simple example of a file for a simple subtool. Note that, if you're used to the legacy plugin interface, the ffx_tool action doesn't impose a library structure on you or do anything really complicated. It's a fairly simple wrapper around the rustc_binary action, but adds some extra targets for generating metadata, producing a host tool, and producing sdk atoms.

# Copyright 2022 The Fuchsia Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.


rustc_library("lib") {
  # This is named as such to avoid a conflict with an existing ffx echo command.
  name = "ffx_tool_echo"
  edition = "2021"
  with_unit_tests = true

  deps = [

  test_deps = [

  sources = [ "src/" ]

ffx_tool("ffx_echo") {
  edition = "2021"
  output_name = "ffx-echo"
  deps = [
  sources = [ "src/" ]

group("echo") {
  public_deps = [

group("bin") {
  public_deps = [ ":ffx_echo_versioned" ]

group("tests") {
  testonly = true
  deps = [ ":lib_test($host_toolchain)" ]

The main rust file will usually be fairly simple, simply invoking FHO with the right types to act as an entry point that ffx knows how to communicate with:

// Copyright 2022 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
use ffx_tool_echo::EchoTool;
use fho::FfxTool;

async fn main() {

This is where the main code of your tool will go. In here you will set up an argh-based struct for command arguments and derive an FfxTool and FfxMain implementation from a structure that will hold context your tool needs to run.


#[derive(ArgsInfo, FromArgs, Debug, PartialEq)]
#[argh(subcommand, name = "echo", description = "run echo test against the daemon")]
pub struct EchoCommand {
    /// text string to echo back and forth
    pub text: Option<String>,

This is the struct that defines any arguments your subtool needs after its subcommand name.

The tool structure

pub struct EchoTool {
    cmd: EchoCommand,
    echo_proxy: ffx::EchoProxy,

This is the structure that holds context your tool needs. This includes things like the argument structure defined above, any proxies to the daemon or a device you might need, or potentially other things that you can define yourself.

There must be an element in this struct that references the argument type described above, and it should have the #[command] attribute on it so that the correct associated type can be set for the FfxTool implementation.

Anything in this structure must implement the TryFromEnv or have a #[with()] annotation that points to a function that returns something that implements TryFromEnvWith. There are also several implementations of these built in to the fho library or other ffx libraries.

Also, the #[check()] annotation above the tool uses an implementation of CheckEnv to validate that the command should be run without producing an item for the struct itself. The one here, AvailabilityFlag, checks for experimental status and exits early if it's not enabled. When writing a new subcommand, it should have this declaration on it to discourage people from relying on it before it's ready for wider use.

FIDL protocols

FFX subtools can communicate with a target device using FIDL protocols through Overnet. To access FIDL protocols from your subtool:

  1. Add the FIDL Rust bindings as a dependency to the subtool's file. The following example adds bindings for the fuchsia.device FIDL library:

      deps = [
  2. Import the necessary bindings into your subtool implementation. The following example imports NameProviderProxy from fuchsia.device:

    use fidl_fuchsia_device::NameProviderProxy;
  3. Declare a member field of the tool structure. Since FIDL proxies implement the TryFromEnv trait, the FHO framework will create and initialize the field for you.

    pub struct EchoTool {
      cmd: EchoCommand,
      name_proxy: NameProviderProxy,

The FfxMain implementation

impl FfxMain for EchoTool {
    type Writer = MachineWriter<String>;
    async fn main(self, mut writer: Self::Writer) -> Result<()> {
        let text = self.cmd.text.as_deref().unwrap_or("FFX");
        let echo_out = self
            .user_message("Error returned from echo service")?;

Here you can implement the actual tool logic. You can specify a type for the Writer associated trait and that type will (through TryFromEnv) be initialized for you based on the context ffx is run in. Most new subtools should use the MachineWriter<> type, specifying a less generic type than the example String above, but what makes sense will vary by tool. In the future, it may be required that all new tools implement a machine interface.

Also, the result type of this function defaults to using the fho Error type, which can be used to differentiate between errors that are due to user interaction and errors that are unexpected. More information on that can be found in the errors document.

Unit tests

If you want to unit test your subtool, just follow the standard method for testing rust code on a host. The ffx_plugin() GN template generates a <target_name>_lib_test library target for unit tests when the with_unit_tests parameter is set to true.

If your contains tests, they can be invoked using fx test:

fx test ffx_example_lib_test

If fx test doesn't find your test, check that the product configuration includes your test. You can include all the ffx tests with this command:

fx set ... --with=//src/developer/ffx:tests

Using fake FIDL proxy in tests

A common patten for testing subtools is to create a fake proxy for a FIDL protocol. This allows you to return the full variety of results from calling the proxy without actually having to deal with the complexity of an integration test.

    fn setup_fake_echo_proxy() -> ffx::EchoProxy {
        let (proxy, mut stream) =
        fuchsia_async::Task::local(async move {
            while let Ok(Some(req)) = stream.try_next().await {
                match req {
                    ffx::EchoRequest::EchoString { value, responder } => {

Then use this fake proxy in a unit test

    async fn test_regular_run() {
        const ECHO: &'static str = "foo";
        let cmd = EchoCommand { text: Some(ECHO.to_owned()) };
        let echo_proxy = setup_fake_echo_proxy();
        let test_stdout = TestBuffer::default();
        let writer = MachineWriter::new_buffers(None, test_stdout.clone(), Vec::new());
        let tool = EchoTool { cmd, echo_proxy };
        assert_eq!(format!("{ECHO}\n"), test_stdout.into_string());


If this subtool is in the ffx tree, you will need to add an OWNERS file that tells us who is responsible for this code and how to route issues with it in triage. It should look something like the following:


It's better to add it as a reference (with file: or possible include) than as a direct list of people, so that it doesn't get stale due to being out of the way.

Adding to the build

To add the tool to the GN build graph as a host tool, you'll need to reference it in the main list in the ffx tools gn file, added to the public_deps of both the tools and test groups.

After this, if you fx build ffx you should be able to see your tool in the list of Workspace Commands in the output of ffx commands and you should be able to run it.

Experimental subtools and subcommands

It's recommended that subtools initially do not include an sdk_category in their These subtools without a specified category are considered “experimental”, and they will not be part of an SDK build. If users want to use the binary, they will have to be given the binary directly.

Subcommands, however, are handled differently.

Subcommands need an AvailabilityFlag attribute added to the tool (see ffx target update for an example). If users want to use a subcommand, they will need to set the associated config option in order to invoke that subcommand.

However, there are problems with this approach, such as a lack of any verification of the FIDL dependencies of the subcommand. Therefore, the mechanism for handling subcommands is currently being changed as of December, 2023.

Similar to subtools, subcommands will be able to declare their SDK category (with the default being “experimental”) to determine whether the subcommands are available. The subtool will be built with only the subcommands at or above the subtool’s category level. The FIDL dependency check will correctly verify the subcommand’s requirements.

Adding to the SDK

Once your tool has stabilized and you're ready to include it in the SDK, you'll want to add the binary to the SDK build. Note that before doing this, the tool must be considered relatively stable and well tested (as much as possible without having already included it in the SDK), and you need to make sure youhave considered compatibility issues.


There are three areas that you need to be aware of before adding your subtool to the SDK and IDK:

  1. FIDL libraries - You are required to add any FIDL libraries you are dependent on to the SDK when you add a subtool to the SDK. (For details, see Promoting an API to partner_internal.)

  2. Command line arguments - In order to test for breaking changes due to command line option changes, the ArgsInfo derive macro is used to generate a JSON representation of the command line.

    This is used in a golden file test to detect differences. Golden files. Eventually, this test will be enhanced to detect and warn of changes that break backwards compatibility.

  3. Machine friendly output - Tools and subcommands need to have machine output whenever possible, especially for tools that are used in test or build scripts. The MachineWriter object is used to facilitate encoding the output in JSON format and providing a schema that is used to detect changes to the output structure.

    Machine output must be stable along the current compatibility window. Eventually, there will be a golden check for the machine output format. The benefit of having machine writer output is that it frees you up to have unstable output in free text.

Updating the subtool

To add your subtool to the SDK, set the sdk_category in its to the appropriate category (for instance, partner). If the subtool includes subcommands that are no longer experimental, remove their AvailabilityFlag attributes so that they will no longer require a special config option to invoke.

Inclusion in the SDK

You also need to add your subtool to the host_tools molecule of the SDK GN file, for example:

sdk_molecule("host_tools") {
  visibility = [ ":*" ]

  _host_tools = [
    "//path/to/your/tool:sdk", # <-- insert this

UX and SDK review

Subtools are required to follow the CLI Guidelines.