# Abstractions URL: /products/abstractions · Id: `page:products/abstractions` > The API is the base every Product compiles to; App, UI, Agent and Service each add one small authored overlay and derive everything else. [StartupBuilder](/code/builders/startup) compiles every [Product](/business/nouns/product) from one base, the API, plus four overlays: [App](/business/nouns/app), UI, [Agent](/business/nouns/agent) and [Service](/business/nouns/service). Each overlay adds a few authored fields; the rest derives from the same [Nouns](/business/nouns/noun) and [Verbs](/business/nouns/verb). ## Five abstractions, one definition The five are not peers. The API is the base. An App is an [Interface](/business/nouns/interface) over it, the UI is design as data, and an Agent or a Service is a Product with one overlay: | Abstraction | You author | It derives | | --- | --- | --- | | API | Nouns, Verbs, and [Offers](/business/nouns/offer) | Endpoints, OpenAPI, SDK, CLI, MCP, [Events](/business/nouns/event), errors, filters | | App | [Personas](/business/nouns/persona), with their jobs and grants | Routes, screens, an inbox, a form for every Verb | | UI | [Components](/business/nouns/component) and [Theme](/business/nouns/theme) tokens | HTML for people, JSON for native clients, Markdown for agents | | Agent | A [Role](/business/nouns/role) and its eval cases | [Tools](/business/nouns/tool), context, autonomy level, an agent card | | Service | An [Outcome Contract](/business/nouns/outcome-contract) on an Offer | The work lifecycle, its Verbs, the hold on revenue, three Apps | No abstraction has a second definition. Add one Verb, and that one write gives the API an endpoint, the App a button, the Agent a Tool and the reference a row. ## The API is the base [The API](/products/platform/api) names its seven parts and its two kinds of caller. clinicslot.com is a [Headless SaaS](/products/templates/headless-saas) for physiotherapy clinics. Its definition declares an `Appointment` Noun with a `reschedule` Verb. From that one record, every Interface answers with the same name: ```bash curl -X POST https://api.sb/clinicslot.com/appointments/appt_42/reschedule \ -H "Authorization: Bearer $STARTUP_BUILDER_API_KEY" \ -H "Content-Type: application/json" \ -d '{"to":"2026-11-03T09:30:00-05:00"}' ``` The SDK method is `$.Appointment.reschedule`, the CLI command is `do Appointment.reschedule`, and the MCP call runs the same code. The Event is `Appointment.rescheduled`, and the list filter `?rescheduledBy=` comes free. Every write returns one [Action](/business/nouns/action). It carries the status, the [Mode](/business/nouns/mode), who it is `by` and `for`, and the record at its new [Version](/business/nouns/version). It reads the same whether code, a model, an Agent or a credentialed [Human](/business/nouns/human) ran the Verb. The [Function](/business/nouns/function) behind a Verb never shows. A step moves from a person to an Agent with no change to the API. The [response envelope](/code/api/guides/envelope) specifies the shape. Every Offer answers one question before any charge: can it deliver? Its `quote` Verb returns `covered` as `yes`, `with` or `no`. A `with` names what is missing, such as an [Integration](/business/nouns/integration) or a higher [Limit](/business/nouns/limit). Every Offer also declares `earns`: the Event that recognizes its revenue. Data and [Software](/business/nouns/software) earn when the Action is done, an Agent on its verdict, and a Service on acceptance. ## App: Personas over the API An App is the human Interface of a Software Product. You declare who uses it and what each Persona may do; the App compiles from that. tunedin.app is a [marketplace](/products/templates/marketplace) that books piano tuners. It declares three Personas: the household that books, the tuner who works, and the admin. Each gets routes on the API's own addresses: a home [Dashboard](/business/nouns/dashboard), an inbox, a screen per record, and a form per Verb it may call. One address serves three readers. `https://tunedin.app/bookings/bk_9` returns HTML to a browser, JSON to the native app, and Markdown to an Agent. The record and its Verbs come from the API in all three. Authorization runs before any screen renders. A tuner sees only their own bookings and only the Verbs they may call now; nothing is hidden with CSS. ## UI: design as data A screen is a list of Components with props, filled from records and styled by a Theme. One catalog serves every [Site](/business/nouns/site), App and Dashboard, and three renderers draw it for people, devices and Agents. [UI](/code/ui) covers the catalog and its renderers. ## Agent: a Role and its evals An Agent has no API of its own. You author a Role, with its [Permissions](/business/nouns/permission), [Goals](/business/nouns/goal), Limits and [Skills](/business/nouns/skill), and the eval cases that define good work. Everything else derives: - Its Tools are the Verbs the Role may call, described in the same words as these docs. - Its first context is the Role's Dashboard, rendered as Markdown. - Its autonomy is earned per Verb, from [evals](/agents/evals). Autonomy climbs three levels: assist, supervised and full. In assist a person approves each result, in supervised a person reviews exceptions, and in full a sample. Promotion needs at least 95% agreement from an [Evaluator](/business/nouns/evaluator) over the Role's sample, and a regression demotes at once. The doer is never the checker: an Evaluator sharing the Agent's model is refused. listsmith.ai sells an Agent that writes product listings for online stores. It bills per listing that passes its Evaluator. The Offer's `earns` is the verdict Event, never the Agent's own claim of done. ## Service: an outcome, warranted A Service is an Offer with an Outcome Contract. Five fields make the promise checkable: done, accept, assurance, remedy and earns. The [Startup](/business/nouns/startup) holds the work until done and answers for acceptance. The contract compiles into a work record with its own lifecycle: - Ordered → Intake; Intake → Working; Working → Done; Done → Accepted; Accepted → Earned; Done → Disputed; Disputed → Remedied; Remedied → Earned Done is a claim; acceptance is a separate Event. Revenue stays on hold from the [Order](/business/nouns/order) until the `earns` Event fires. A certified translation Service shows the mix. An Agent drafts each document, and a credentialed translator certifies it as a human Function. Each step is a Verb on the work record, so a step moves between people and Agents without touching the [Customer](/business/nouns/customer)'s contract. The same compile produces three Apps: one for the buyer, a worklist for the translators that works offline, and one for the operator. [Outcome contracts](/products/service/outcome-contracts) covers the five terms in depth. ## Next - The [project structure](/code/project-structure): where each authored field lives in a repository. - [Interfaces](/code/builders/software/interfaces): the API, MCP, CLI and SDK in depth. - [Agents](/products/agent/agents): selling an Agent, with quotes and billing on verdicts.