VerticalSlices: Skill for Claude Code

Creating a Cratis Command

A command represents a user action that changes state. In Cratis Arc the path is:

[Command] record + Handle()  →  validator  →  dotnet build  →  TypeScript proxy  →  React .use()

The command record owns its own handler — no separate controller class required. Follow the steps in order. Jump to the reference files for deeper detail on any step.

Step 1 — Define the C# command record

A command is a record decorated with [Command] that contains its own Handle() method. No separate controller is needed.

// Accounts/OpenDebitAccount/OpenDebitAccount.cs — the slice file
namespace MyApp.Accounts.OpenDebitAccount;

using Cratis.Arc.Commands.ModelBound;
using Cratis.Chronicle.Events;

[Command]
public record OpenDebitAccount(AccountId AccountId, AccountName Name, OwnerId OwnerId)
{
    public DebitAccountOpened Handle() =>
        new(Name, OwnerId); // Arc appends the returned event; AccountId is the event source
}

/// <summary>Emitted when a debit account is opened.</summary>
[EventType]  // NO arguments — never [EventType("some-guid")]
public record DebitAccountOpened(AccountName Name, OwnerId OwnerId);

Rules:

• [Command] attribute is required — it makes the type discoverable and the analyzer will warn without it
• Handle() returns the event (or events) to append — Arc's Chronicle integration automatically appends them; never inject IEventLog to append the primary event
• [EventType] takes no arguments — the identifier is generated from the type name
• Name the command as an imperative action — OpenDebitAccount, not OpenDebitAccountCommand
• All backend artifacts for the slice live in this one file; place it in the slice folder, not an API/ or Commands/ folder (see vertical-slices.md)
• Use concept wrappers for every domain value — identity concepts derive from EventSourceId<T>, value concepts from ConceptAs<T>; never raw Guid/string