API Reference

This page lists the public APIs used in typical Chapplin projects.

defineTool

import { defineTool } from "chapplin";

export const tool = defineTool({
  name: "my_tool",
  config: {
    description: "...",
    inputSchema: { ... },
    outputSchema: { ... },
    annotations: { ... },
  },
  async handler(args, extra) {
    return {
      content: [{ type: "text", text: "..." }],
      structuredContent: { ... },
      _meta: { ... },
    };
  },
});

• inputSchema / outputSchema use Zod object shapes
• _meta is UI-only data (not sent to the LLM)

defineApp

import { defineApp } from "chapplin";

export const app = defineApp<typeof tool>({
  config: {
    appInfo: { name: "my-app", version: "1.0.0" },
    capabilities: {},
    options: {},
  },
  meta: {
    prefersBorder: true,
  },
  ui: (props) => <div />,
});

• defineApp must be used as defineApp<typeof tool>
• config is passed to the MCP App constructor

defineResource

import { defineResource } from "chapplin";

export const resource = defineResource({
  name: "app-config",
  config: {
    uri: "config://app/settings",
    mimeType: "application/json",
  },
  async handler(uri) {
    return {
      contents: [{ uri: uri.href, text: "{}" }],
    };
  },
});

definePrompt

import { definePrompt } from "chapplin";

export const prompt = definePrompt({
  name: "code-review",
  config: {
    description: "...",
    argsSchema: { ... },
  },
  handler(args) {
    return {
      messages: [{ role: "user", content: { type: "text", text: "..." } }],
    };
  },
});

Vite Plugin

import { chapplin } from "chapplin/vite";

chapplin({
  entry: "./src/index.ts",
  target: "react", // react | preact | solid | hono
  toolsDir: "tools",
  resourcesDir: "resources",
  promptsDir: "prompts",
  tsconfigPath: "tsconfig.json",
});

Virtual Module: chapplin:register

import { register } from "chapplin:register";

register(server);

register(server) registers all collected tools/resources/prompts (and MCP Apps) onto the given MCP server.

Runtime Helpers

• chapplin/react
• chapplin/preact
• chapplin/solid
• chapplin/hono

useApp

It provides access to the MCP App context.

Check out the example usage in model-context-protocol/ext-apps.

// Example usage of useApp from chapplin/react
import { useApp } from "chapplin/react";

const app = useApp();
const openUrl = (url: string) => app.openLink({ url });