fern-api/fern/custom-commands

Custom Commands for api

Overview

The api CLI supports user-authored custom commands that are compiled into the binary alongside the auto-generated API commands. Custom commands get a fully-wired SDK client that inherits the CLI's auth, retries, TLS, base URL, and global headers — zero configuration required.

Architecture

cli/api/custom.rs    ← Your command handlers (protected by .fernignore)
cli/api/sdk.rs       ← Generated bridge: client() + block_on()
cli/api/main.rs      ← Generated entrypoint (calls custom::register)
api-sdk/             ← Co-generated typed SDK crate
api-types/           ← Co-generated typed model crate

Adding a Custom Command

1. Edit cli/api/custom.rs

This file is protected by .fernignore — fern generate will never overwrite it. Register commands in the register() function:

use api_sdk::api::*;
 
pub fn register(app: CliApp) -> CliApp {
    let app = app.command(
        clap::Command::new("get-movie")
            .about("Run imdb get-movie")
            .arg(clap::Arg::new("movieId").required(true))
        ,
        |matches, ctx| {
            let movie_id = matches.get_one::<String>("movieId").unwrap();
            let client = super::sdk::client(ctx);
            let result = super::sdk::block_on(
                client.imdb.get_movie(movie_id),
            )?;
            println!("{}", serde_json::to_string_pretty(&result).unwrap());
            Ok(())
        },
    );
    app
}

Then build and test:

cargo build
api get-movie <movieId>

2. Available SDK Clients

The super::sdk::client(ctx) call returns a api_sdk::api::Client with the following sub-clients:

3. Key Patterns

Get the SDK client (execution-sharing, fully authenticated):

let client = super::sdk::client(ctx);

Run an async SDK call from a sync handler:

let result = super::sdk::block_on(
    client.some_resource.some_method(args),
)?;

Use typed models for request/response serialization:

use api_sdk::api::*;

4. Authentication

Custom commands automatically inherit the CLI's authentication. The following auth schemes are configured:

• bearer (bearer): env API_TOKEN

No manual auth wiring is needed in custom command handlers.

Regeneration Safety

After running fern generate, your custom.rs is preserved. All generated code (SDK, types, glue, main.rs) is updated to match the latest API spec. If the SDK surface changes (renamed methods, new sub-clients), update your custom.rs to match.

Build & Test

# Build the CLI (includes custom commands)
cargo build
 
# Run your custom command
api <your-command> [args]
 
# Run with verbose output for debugging
RUST_LOG=debug api <your-command> [args]