> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://docs.cessy.ai/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://docs.cessy.ai/_mcp/server.

# Cessy Builder Docs

> Start here to build event-sourced business apps in Cessy with the Builder UI, Design MCP, runtime APIs, agents, and custom pages.

Cessy is a builder platform for configurable event-sourced business apps. You model the business once, publish it, and Cessy materializes the runtime surfaces: command APIs, projection reads, webhooks, subscriptions, MCP tools, channels, and custom pages.

These docs are written for human builders. If you are using an AI coding agent, connect it to the Design MCP early; the Design MCP exposes the current app model, reference knowledge, validation tools, workspace changesets, and page preview tools.

## First path

1. Read [Builder Journey](/start/builder-journey) for the end-to-end mental model.
2. Connect Claude, Codex, or another MCP client through [Connect Design MCP](/start/connect-design-mcp).
3. Run the [Quickstart](/start/quickstart) against a real app.
4. Use [DomainConfig](/core-concepts/domain-config) when you need to understand the generated model.
5. Use [Runtime API](/extend-the-app) when you need to integrate an external system.

## Builder surfaces

| Surface      | Use it for                                                                                                                        |
| ------------ | --------------------------------------------------------------------------------------------------------------------------------- |
| Builder UI   | Model decisions, commands, events, projections, policies, permissions, agents, adapters, credentials, pages, and publishing.      |
| Design MCP   | Let Claude, Codex, ChatGPT, or another compatible MCP client inspect and change app design with validation and reference context. |
| Runtime API  | Let external systems execute published commands and read published projections.                                                   |
| Runtime MCP  | Let app agents operate the published app through generated tools.                                                                 |
| Custom pages | Build app-specific screens on top of published runtime contracts.                                                                 |

## Core rule

Business behavior belongs in the event-sourced model. Frontends, agents, webhooks, adapters, and external clients should call the generated runtime surfaces instead of reimplementing business rules.