VerticalSlices: Skill for Claude Code

Implement a complete vertical slice following this EXACT order. Never skip steps or work on multiple slices in parallel.

Step 1 — Identify the slice type

Choose one of:

• State Change — a command that mutates state and records events (most common)
• State View — a query that reads from a read model
• Automation — a background reactor triggered by events
• Translation — transforms events into other events

Step 2 — Determine the namespace root

Read global.json and existing .cs files under the app source root to find the namespace root (e.g. Studio, Library). Never hard-code it.

Step 3 — Create the C# slice file

Place ALL backend artifacts in a single file in the slice folder: <Feature>/<Slice>/<Slice>.cs (under the app source root; a <Module>/ grouping above the feature is optional — there is no top-level Features/ wrapper).

File creation order within the slice:

1. Concept types (if new strongly-typed IDs are needed — see add-concept skill)
2. Command record with Handle() method and optional validation attributes
   • If a business rule depends on Chronicle event-sourced state, add the relevant read model as a parameter to Handle() — see add-business-rule skill (DCB pattern)
3. CommandValidator<T> for command-level rejection rules (see add-business-rule); ConceptValidator<T> for value invariants
4. Constraint class <Name>Constraint (if needed)
5. Event record with [EventType] (no arguments, no mutable properties)
6. Read model record with [ReadModel] and model-bound projection attributes ([FromEvent<T>], [Key], etc.)
   • Use fluent IProjectionFor<T> only when model-bound attributes don't fit

Critical rules:

• Commands are record types with a Handle() method directly on them — DO NOT create separate handler classes
• Events use [EventType] with NO arguments — never pass a GUID or string
• Projection: prefer model-bound attributes on the read model; if using IProjectionFor<T>, AutoMap is on by default — just call .From<>() directly
• Namespace mirrors the folder path under the source root: <RootNamespace>.<Module>.<Feature>.<Slice> (no Features segment — drop any level that isn't present)
• Copyright header on every file: // Copyright (c) Cratis. All rights reserved. // Licensed under the MIT license. See LICENSE file in the project root for full license information.