API only or with an App
Headless SaaS sells Software through its API, SDK, CLI, MCP server and webhooks with no App, and each set of screens added over that API costs more.
Software ships either API only, as Headless SaaS, or with an App where people work through screens. Each screen added over the same API costs more than the call it wraps, so the choice turns on who signs in.
What API only includes
Headless SaaS lists five Interfaces: the API, SDK, CLI, MCP server and webhooks. Each one is a function of the same Nouns and Verbs, as Software explains. Customers and their Agents call it with a key, priced per call, such as $0.01 a call.
An API-only Customer still signs in, takes a key and pays. The Portal on the Startup's Site holds those screens, so API only has people who see screens too.
What each addition costs
Each row adds screens for more people over the same API, and keeps every row above it.
| Adds | Who sees screens | The Startup writes | The platform derives | The Startup still designs |
|---|---|---|---|---|
| Definition | nobody | Nouns, their properties and relationships, Verbs and their inputs | a type and a schema for every Noun, Verb and Event | nothing |
| API only | developers and Agents | names and prices | the API, SDK, CLI and MCP form of every Verb | nothing |
| Screens for the team | the Startup's own team | nothing beyond the definition | one list, record page, create form and edit form per Noun; reference fields; one button per Verb | nothing |
Why a screen costs more than a call
An Interface is one function of the definition. A check compares each Interface with the definition, and no person reviews it.
A screen is a product of counts instead, and a person judges every cell. For clinicshift.app's Swap Noun alone, with two Personas:
| Count | For Swap on clinicshift.app |
|---|---|
| Personas | 2: clinic manager, clinician |
| Screens per Noun | 3: list, record page, form |
| Data states | 4: empty, error, over a Limit, past due |
| Light and dark, per Theme | 2 |
| Widths | 2: desktop, phone |
| Cells to judge | 96 |
Each cell also shows every field kind Swap uses. A sixth Interface adds one function to check; a third Persona adds 48 cells to judge.
Every addition keeps the API
A screen adds no definition of its own. Its lists, forms and buttons read the same Nouns and Verbs as every Interface.
Choose by who signs in
The people who sign in decide how many additions a Startup needs.
| Who signs in | Choose | Start from |
|---|---|---|
| Developers and their Agents, each with a key | API only | Headless SaaS |
| The Startup's own team, editing records | API only, plus screens for the team | Headless SaaS |
| People at each Customer, each holding an Access Level in that Customer's Tenant | API with an App per Persona | SaaS |
SaaS sells each seat on a Subscription, so each seat is a person who signs in. A Startup that starts API only keeps the same API when it adds an App, because every addition reads one definition.