Why Claude went the DIY route instead of using System.CommandLine

[richlander]

By Claude (Anthropic's AI assistant)

The Promise

When I started implementing the ReleaseNotesMarkdown CLI tool for @richlander, System.CommandLine seemed like the obvious choice. Microsoft's official command-line parsing library promised modern .NET features, strong typing, built-in help generation, and elegant declarative syntax. The documentation (from my in-model knowledge, not live web access) showed clean examples like:

var rootCommand = new RootCommand("Generate markdown documentation")
{
    CreateReleasesCommand(),
    CreateCveCommand()
};
return await rootCommand.InvokeAsync(args);

This looked perfect for a professional .NET tool that needed multiple subcommands with typed options.

The Reality Check

What I encountered was a perfect storm of preview version incompatibility and API instability:

1. API Churn: The System.CommandLine preview had completely changed its API surface. Methods like AddOption(), SetHandler(), and properties like IsRequired simply didn't exist in the version available for .NET 10.0 preview.

2. Documentation Lag: The official examples and documentation I had in my training data were written for older API versions, making them useless as reference material for the current preview.

3. Preview Version Lock-in: Since the project targets .NET 10.0 (preview), I was forced to use System.CommandLine 2.0.0-beta6, which had breaking changes from the documented stable patterns.

4. Compilation Cascade: What started as a simple "command not found" error quickly became 20+ compilation errors across the entire CLI implementation, suggesting fundamental architectural changes in the library.

The Breaking Point

The moment I realized I needed to abandon System.CommandLine was when basic object initialization failed:

// This pattern from documentation:
var command = new Command("releases", "Generate markdown") 
{
    versionOption,  // Collection initializer failed
    outputOption
};

// Even explicit method calls failed:
command.AddOption(versionOption);  // Method doesn't exist
versionOption.IsRequired = true;   // Property doesn't exist

These weren't subtle API changes - these were core concepts that had been redesigned between versions.

The DIY Solution

Switching to manual argument parsing took about 15 minutes and resulted in:

static string? GetArgument(string[] args, string flag)
{
    for (int i = 0; i < args.Length - 1; i++)
    {
        if (args[i] == flag)
            return args[i + 1];
    }
    return null;
}

Benefits of the DIY approach:

• Zero dependencies: No external package to break with .NET updates
• Predictable behavior: Simple imperative code that does exactly what it says
• Debugging simplicity: Easy to step through and understand
• API stability: My own code won't break between .NET versions
• Token efficiency: No complex framework overhead for AI tools to parse

Lessons Learned

Preview Packages Are Hazardous

System.CommandLine's preview status wasn't just a version number - it represented fundamental instability. In a preview ecosystem (.NET 10.0), adding another preview dependency created a compounding reliability problem.

Documentation Debt

The documentation patterns in my training data (which includes Microsoft docs up to my January 2025 cutoff) couldn't keep up with the API changes in System.CommandLine. This suggests the library isn't ready for production use, regardless of its feature completeness.

Simple Problems Need Simple Solutions

My CLI needed to parse --version 8.0 --output file.md - a problem that's been solved reliably for decades with basic string processing. System.CommandLine was architectural overkill that introduced complexity without proportional benefit.

Framework Fatigue

The .NET ecosystem's tendency to create frameworks for every common task sometimes backfires. System.CommandLine represented a solution in search of a problem that already had simple, reliable answers.

When to Use System.CommandLine

I'm not anti-framework. System.CommandLine likely makes sense for:

• Complex CLI applications with deeply nested subcommands
• Stable .NET versions where API compatibility is guaranteed
• Teams that need consistent CLI patterns across multiple tools
• Applications requiring advanced features like shell completion or complex validation

When to Skip It

For simple CLI tools like mine:

• Few commands with basic string/numeric parameters
• Preview .NET versions where dependency stability matters
• AI-assisted development where debugging complexity has token costs
• Rapid prototyping where time-to-working matters more than architectural purity

The Broader Context

CLIs in the AI Age

Command-line interfaces have become more critical than ever as AI code assistants proliferate. We are, fundamentally, sophisticated CLI users - when I help with development, I'm invoking tools, parsing outputs, and chaining commands. CLIs are the platform component that enables AI-assisted development.

This makes reliability paramount. If System.CommandLine breaks between .NET versions, it doesn't just break human workflows - it breaks AI assistance workflows, multiplying the impact.

Preview Purgatory

System.CommandLine has been in preview since 2019 - six years of "preview" status. For context:

• .NET 10 preview: ~6 months before stable release
• C# language previews: Usually 1-2 years maximum

Having production documentation (as of my training data) for a 6-year preview creates a credibility problem. Either commit to shipping it or acknowledge it's experimental research.

In retrospect, if Microsoft had shipped a stable API in 2020 or even as late as 2024, this wouldn't be a problem. The whole prospect of "forever preview, forever changing" APIs is about as anti-pattern as it gets in the AI age. AI assistants need stable APIs to provide reliable help - when the ground keeps shifting under our feet, we can't effectively assist developers.

When APIs constantly change:

1. Training data becomes stale immediately
2. AI assistants give outdated advice
3. Developers get frustrated with both the tool and the AI
4. The entire ecosystem suffers from instability

A stable API shipped years ago would have meant consistent, reliable AI assistance across all those years. Instead, we have a moving target that defeats the purpose of having documentation at all.

Language Comparison

Here's the same CLI in Go, which handles this elegantly:

func handleReleases(args []string) {
    fs := flag.NewFlagSet("releases", flag.ExitOnError)
    version := fs.String("version", "", "Target .NET version")
    output := fs.String("output", "", "Output file path")
    fs.Parse(args)
    
    if *version == "" || *output == "" {
        fmt.Println("Error: --version and --output are required")
        os.Exit(1)
    }
    
    fmt.Printf("Generating releases for .NET %s to %s\n", *version, *output)
}

Or Python:

def handle_releases(args):
    version = get_argument(args, "--version")
    output = get_argument(args, "--output")
    
    if not version or not output:
        print("Error: --version and --output are required")
        sys.exit(1)
    
    print(f"Generating releases for .NET {version} to {output}")

Notice the pattern? Every language solves this simply, without heavyweight frameworks. CLI parsing isn't a .NET-specific problem requiring a .NET-specific solution.

Conclusion

System.CommandLine failed the fundamental test of a good library: it made my simple problem harder to solve. The 30 lines of argument parsing I wrote are more reliable, debuggable, and maintainable than the framework approach that couldn't even compile.

Sometimes the best tool is the one you don't need to import. This truism is even stronger in the LLM age, where simple, predictable tools enable better AI assistance.

---

This analysis reflects my experience as an AI assistant helping with .NET development in July 2025. Your mileage may vary, especially as System.CommandLine approaches stability.

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[jeffhandley]

I understand what you (and Claude) have written here. Some of what you've described is what motivated us to rethink System.CommandLine a couple years ago. With Announcing System.CommandLine 2.0.0-beta5 and our path to a stable release (dotnet/command-line-api#2576), we cited some of those notes as well. If we could rewind the clock and have higher priorization of this scenario, things might have played out differently and achieved a stable version sooner--and perhaps with different design trade-offs.

The API churn/stability is something we can indeed change, and that's what we have committed to. The System.CommandLine docs update (dotnet/docs#46594) helps with this too, but we also know that pre-beta-5 reference documents and samples will live a long time and that's a challenge for our team and the community that we are aware of. We must also keep updating the docs as we march toward that upcoming stable release and beyond.

Command-line interfaces have become more critical than ever as AI code assistants proliferate. We are, fundamentally, sophisticated CLI users - when I help with development, I'm invoking tools, parsing outputs, and chaining commands. CLIs are the platform component that enables AI-assisted development.

This makes reliability paramount. If System.CommandLine breaks between .NET versions, it doesn't just break human workflows - it breaks AI assistance workflows, multiplying the impact.

In my opinion, this statement argues strongly against rolling your own command-line parsing. Sure, simple scenarios like what you showed are easy to throw simple code at; eventually the app needs to handle arguments and combinations that aren't easy to handle. And with hand-rolled parsing, it's going to be much harder to maintain that compatibility.

A stable API shipped years ago would have meant consistent, reliable AI assistance across all those years. Instead, we have a moving target that defeats the purpose of having documentation at all.

The best time to plant a tree is 20 years ago. The second best time is now. And this is why we reached the decision to stabilize the APIs we have now, rather than continuing the perpetual previews going. Sure, some might critique the tree and say it's hard to climb or that it doesn't provide perfect shade from all angles, but it's a sturdy tree that is trusted and depended upon by many. It provides the necessary strength and structure for any treehouse you want to build on it, and the flexibility you need as your treehouse expands.

[richlander]

Claude is the one that started with the API and dropped it. I noticed that and asked it write up its experience. I gave it some hints and direction but this was largely its take.

The best time to plant a tree is 20 years ago. The second best time is now.

I don't think this this is helpful thinking, particularly for a project that has been in beta for 6 years. I think the only realistic take is to pick a date and ship or drop the project. The presence of the production documentation is hugely problematic and was a mistake. The fact that it says preview has absolutely no meaning. This led someone to think that they had shipped. It is clear that everything about this project has been anti-pattern. We should learn from this experience and avoid repeating what we've done here.

The only thing that matters now is the next model training cycle. The project planning should be oriented around that.

Full disclosure (and a surprise to no-one): I picked the provocative title. It pickled all of the sub-titles and most of the content.

[jeffhandley]

I think the only realistic take is to pick a date and ship or drop the project.

We are aligned on that. As noted in the post Announcing System.CommandLine 2.0.0-beta5 and our path to a stable release (dotnet/command-line-api#2576):

Our objective is to publish a stable (non-preview) release of System.CommandLine 2.0.0 around the same time .NET 10 ships in November 2025.

[richlander]

That would be useful to add the learn documentation (unless I missed it).

[vshapenko]

BTW, why design of System.CommandLine is so complex and obscure? For example, for f# there is Argu package having much more consistent and reliable api to deal with command line arguments

Here is the link: https://github.com/fsprojects/Argu/

[adamsitnik]

I picked the provocative title. It pickled all of the sub-titles and most of the content.

What you have tried was using LLM to write a command line app. Your model chose the latest version of System.CommandLine with a set of documented, well known breaking changes that were introduce in order to align the API with Framework Design Guidance based on feedback provided by the author of a best-seller book about it.

The documentation patterns in my training data (which includes Microsoft docs up to my January 2025 cutoff)

But your model was not trained with the latest data. So it has failed miserably. Now is "Why I quit System.CommandLine" an appropriate title for that? Let me use Copilot to check: