Why Chapplin?

MCP provides the primitives (Tools, Resources, Prompts, MCP Apps), but building a production workflow around them is still a lot of work. Chapplin turns those primitives into a cohesive framework.

The Problem Without Chapplin

Single-HTML MCP Apps

MCP Apps (via @modelcontextprotocol/ext-apps) require a single HTML asset that includes your UI code and styles. Creating that asset by hand means:

• bundling JS/CSS
• inlining assets
• keeping dev and prod builds aligned

Type Safety Gap

Tool schemas and UI components often drift apart:

• output types are duplicated
• changes require manual sync
• runtime errors slip through

Boilerplate Everywhere

Registering tools/resources/prompts and wiring a dev preview takes time, and every project re-implements the same pieces.

What Chapplin Gives You

File-Based Collection

Drop files into tools/, resources/, and prompts/. Chapplin discovers and registers them automatically.

End-to-End Type Safety

Types flow from your schema to your UI component:

import { defineTool, defineApp } from "chapplin";
import z from "zod";

export const tool = defineTool({
  name: "get_todos",
  config: {
    description: "Get todos",
    inputSchema: {},
    outputSchema: {
      todos: z.array(z.object({ id: z.number(), title: z.string() })),
    },
  },
  async handler() {
    return {
      content: [{ type: "text", text: "ok" }],
      structuredContent: { todos: [{ id: 1, title: "Sample" }] },
    };
  },
});

export const app = defineApp<typeof tool>({
  config: { appInfo: { name: "todos", version: "1.0.0" } },
  ui: (props) => (
    <div>
      {props.output?.structuredContent?.todos.map((todo) => (
        <div key={todo.id}>{todo.title}</div>
      ))}
    </div>
  ),
});

Built-in Vite Integration

Chapplin ships a Vite plugin that:

• bundles MCP Apps into a single HTML file
• generates chapplin:register for MCP registration
• provides a dev server with UI preview

Summary

Chapplin removes the heavy lifting around MCP Apps and provides a clean, type-safe workflow so you can focus on your tools and UI.