VerticalSlices: Skill for Claude Code

Core philosophy

Same BDD philosophy as C# specs — specs describe behaviors, not implementations. The given() helper + context class mirrors the C# Specification base class: setup is separated from the action, each it() verifies a single outcome.

---

Step 1 — Create the folder structure

for_<ClassName>/
├── given/
│   └── a_<system>.ts             ← reusable context class
├── when_<behavior>/              ← behavior with multiple outcomes
│   ├── with_<condition>.ts
│   ├── without_<condition>.ts
│   └── and_<condition>.ts
└── when_<simple_behavior>.ts     ← single outcome = single file

Example:

for_EventsCommandResponseValueHandler/
├── given/
│   └── an_events_command_response_value_handler.ts
├── when_checking_can_handle/
│   ├── with_valid_events_collection.ts
│   ├── with_null_value.ts
│   └── without_event_source_id.ts
└── when_handling/
    ├── empty_events_collection.ts
    └── multiple_events_collection.ts

---

Step 2 — Write a reusable context class

// for_AuthorService/given/an_author_service.ts
import sinon from 'sinon';
import { AuthorService } from '../../../AuthorService';

export class an_author_service {
    eventLog: sinon.StubbedInstance<IEventLog>;
    service: AuthorService;

    constructor() {
        this.eventLog = sinon.createStubInstance(EventLog);
        this.service = new AuthorService(this.eventLog);
    }
}

Properties are public (unlike C# protected fields) — tests access them via context.propertyName.

---

Step 3 — Write a spec using given()

// for_AuthorService/when_registering/with_valid_name.ts
import { an_author_service } from '../given/an_author_service';
import { given } from '../../given';   // import from package root

describe('when registering with valid name', given(an_author_service, context => {
    beforeEach(async () => {
        await context.service.register('John Doe');
    });

    it('should append an event', () => {
        context.eventLog.append.calledOnce.should.be.true;
    });

    it('should pass the author name', () => {
        const call = context.eventLog.append.firstCall;
        call.args[1].name.should.equal('John Doe');
    });
}));