Effect v4 sits in the middle of the starter on purpose. packages/capabilities owns the typed services for Workspaces, Audit Events, Notifications, API Tokens, and Webhook Endpoints. The web Worker, the API Worker, and the background Worker all depend on it.
What Effect buys us
Three Workers share one application backbone without each importing the other's code. Effect's services and layers compose; testing a capability does not require booting a Worker.
Schemas as contracts
Every request, response, and persisted shape is an Effect Schema. The same schema powers the OpenAPI document the API Worker serves at /openapi.json, the Effect Atom server-state layer the web Worker will adopt (ADR 0040), and the seed fixtures in packages/capabilities/src/seed-fixture.ts and scripts/seed.ts. A field rename ripples through every consumer at type-check time.
Errors as part of the type
Domain errors are Schema.TaggedErrorClass types. The HTTP mapping is centralized — AuthorizationDenied is 403, RateLimited is 429, WorkspaceNotFound is 404. The MCP endpoint skeleton in apps/api will reuse the same mapping as it grows past discovery. Adding a new error class is one declaration; every interface picks it up.
Why not factor capabilities per Worker
Splitting capabilities per Worker invites drift. The starter explicitly chooses one shared package over three convenient copies.