clap

name	clap
description	Build production Rust CLIs with Clap: subcommands, config layering, validation, exit codes, shell completions, and testable command surfaces
user-invocable	false
disable-model-invocation	true
version	1.0.0
category	toolchain
author	Claude MPM Team
license	MIT
progressive_disclosure	{"entry_point":{"summary":"Create ergonomic, testable Rust CLIs using Clap derive, subcommands, and config layering (CLI + env + config file)","when_to_use":"When building Rust CLI tools that need reliable argument parsing, good help UX, strong validation, and automated tests","quick_start":"1. Add clap (derive) 2. Define Args + Subcommand 3. Parse once in main 4. Map errors to exit codes 5. Test with assert_cmd"},"token_estimate":{"entry":120,"full":4500}}
context_limit	700
tags	["rust","cli","clap","config","testing"]
requires_tools	[]

Clap (Rust) - Production CLI Patterns

Overview

Clap provides declarative command-line parsing with strong help output, validation, and subcommand support. Use it to build CLIs with predictable UX and testable execution paths.

Quick Start

Minimal CLI

✅ Correct: derive Parser

use clap::Parser;

#[derive(Parser, Debug)]
#[command(name = "mytool", version, about = "Example CLI")]
struct Args {
    /// Enable verbose output
    #[arg(long)]
    verbose: bool,

    /// Input file path
    #[arg(value_name = "FILE")]
    input: String,
}

fn main() {
    let args = Args::parse();
    if args.verbose {
        eprintln!("verbose enabled");
    }
    println!("input={}", args.input);
}

❌ Wrong: parse multiple times

fn main() {
    let _a = Args::parse();
    let _b = Args::parse(); // duplicate parsing and inconsistent behavior
}

Subcommands (real tools)

Model multi-mode CLIs with subcommands and shared global flags.

✅ Correct: global flags + subcommands

use clap::{Parser, Subcommand, ValueEnum};

#[derive(Parser, Debug)]
  {
    
    verbose: ,

    
    config: <>,

    
    cmd: Command,
}


  {
    Serve {  port:  },
    Migrate {  mode: Mode },
}


  { Up, Down }

 () {
      = Args::();
     args.cmd {
        Command::Serve { port } => (, port),
        Command::Migrate { mode } => (, mode),
    }
}