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Getting started with 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.

This document describes how to get started writing a new subtool for ffx. If you already have a plugin that was written before the new interface and want to migrate it to the new subtool interface, you can find more information on that in the migrating doc.

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. Unlike the legacy plugin system, subtools aren't required to be broken out into three separate rust libraries. Still, best practices are 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.


rustc_library("lib") {
  name = "ffx_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/" ]

  sdk_category = "partner"
  sdk_target_name = "ffx_echo_sdk"

group("echo") {
  deps = [

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

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:

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(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 plugin, it should have this declaration on it to discourage people from relying on it before it's ready for wider use.

The FfxMain implementation

impl FfxMain for EchoTool {
    type Writer = MachineWriter<String>;
    async fn main(self, 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 plugins 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. This maps to the way the legacy plugin interface discriminated between normal anyhow errors and errors produced by ffx_error or ffx_bail. More information on that can be found in the errors document.


Please add some tests! You can look at some existing examples of subtools to get a better idea of how to do this, but it is otherwise much like writing any other test for Fuchsia rust code.


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:


# COMPONENT: SomeComponent

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.

If your subtool is elsewhere, you will need to add the maintainers of ffx to the OWNERS file for this directory so we can review updates to the interface between ffx and your subtool:


# any other project-specific things here

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.

Adding to the SDK

Once your tool has stabilized and you're ready to include it in the SDK, you'll want to add it in to the host_tools molecule of the sdk gn file:

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

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

Note that before doing this, it should no longer have an experimental flag on it and it should be considered relatively stable and well tested (as much as possible without having already included it in the SDK, at least).