MCP Apps Builder

IMPORTANT: How to Use This Skill

This file provides a NAVIGATION GUIDE ONLY. Before implementing any MCP server features, you MUST:

1. Read this overview to understand which reference files are relevant
2. ALWAYS read the specific reference file(s) for the features you're implementing
3. Apply the detailed patterns from those files to your implementation

Do NOT rely solely on the quick reference examples in this file - they are minimal examples only. The reference files contain critical best practices, security considerations, and advanced patterns.

MCP Server Best Practices

Comprehensive guide for building production-ready MCP servers with tools, resources, prompts, and widgets using mcp-use.

⚠️ FIRST: New Project or Existing Project?

Before doing anything else, determine whether you are inside an existing mcp-use project.

Detection: Check the workspace for a package.json that lists "mcp-use" as a dependency, OR any .ts file that imports from "mcp-use/server".

├─ mcp-use project FOUND → Do NOT scaffold. You are already in a project.
│  └─ Skip to "Quick Navigation" below to add features.
│
├─ NO mcp-use project (empty dir, unrelated project, or greenfield)
│  └─ Scaffold first with npx create-mcp-use-app, then add features.
│     See "Scaffolding a New Project" below.
│
└─ Inside an UNRELATED project (e.g. Next.js app) and user wants an MCP server
   └─ Ask the user where to create it, then scaffold in that directory.
      Do NOT scaffold inside an existing unrelated project root.

NEVER manually create MCPServer boilerplate, package.json, or project structure by hand. The CLI sets up TypeScript config, dev scripts, inspector integration, hot reload, and widget compilation that are difficult to replicate manually.

Scaffolding a New Project

npx create-mcp-use-app my-server
cd my-server
npm run dev

For full scaffolding details and CLI flags, see quickstart.md.

Quick Navigation

Choose your path based on what you're building:

🚀 Foundations

When: ALWAYS read these first when starting MCP work in a new conversation. Reference later for architecture/concept clarification.

1. concepts.md - MCP primitives (Tool, Resource, Prompt, Widget) and when to use each
2. architecture.md - Server structure (Hono-based), middleware system, server.use() vs server.app
3. quickstart.md - Scaffolding, setup, and first tool example
4. deployment.md - Deploying to Manufact Cloud, self-hosting, Docker, managing deployments

Load these before diving into tools/resources/widgets sections.

🔐 Adding Authentication?

When: Protecting your server with OAuth (WorkOS, Supabase, or custom)

• overview.md

  • When: First time adding auth, understanding ctx.auth, or choosing a provider
  • Covers: oauth config, user context shape, provider comparison, common mistakes

• workos.md

  • When: Using WorkOS AuthKit for authentication
  • Covers: Setup, env vars, DCR vs pre-registered, roles/permissions, WorkOS API calls

• supabase.md

  • When: Using Supabase for authentication
  • Covers: Setup, env vars, HS256 vs ES256, RLS-aware API calls

• custom.md

  • When: Using any other identity provider (GitHub, Okta, Azure AD, Google, etc.)
  • Covers: Custom verification, user info extraction, provider examples

🔧 Building Server Backend (No UI)?

When: Implementing MCP features (actions, data, templates). Read the specific file for the primitive you're building.

• tools.md

  • When: Creating backend actions the AI can call (send-email, fetch-data, create-user)
  • Covers: Tool definition, schemas, annotations, context, error handling

• resources.md

  • When: Exposing read-only data clients can fetch (config, user profiles, documentation)
  • Covers: Static resources, dynamic resources, parameterized resource templates, URI completion

• prompts.md

  • When: Creating reusable message templates for AI interactions (code-review, summarize)
  • Covers: Prompt definition, parameterization, argument completion, prompt best practices

• response-helpers.md

  • When: Formatting responses from tools/resources (text, JSON, markdown, images, errors)
  • Covers: text(), object(), markdown(), image(), error(), mix()

• proxy.md

  • When: Composing multiple MCP servers into one unified aggregator server
  • Covers: server.proxy(), config API, explicit sessions, sampling routing

🎨 Building Visual Widgets (Interactive UI)?

When: Creating React-based visual interfaces for browsing, comparing, or selecting data

• basics.md

  • When: Creating your first widget or adding UI to an existing tool
  • Covers: Widget setup, useWidget() hook, isPending checks, props handling

• state.md

  • When: Managing UI state (selections, filters, tabs) within widgets
  • Covers: useState, setState, state persistence, when to use tool vs widget state

• interactivity.md

  • When: Adding buttons, forms, or calling tools from within widgets
  • Covers: useCallTool(), form handling, action buttons, optimistic updates

• ui-guidelines.md

  • When: Styling widgets to support themes, responsive layouts, or accessibility
  • Covers: useWidgetTheme(), light/dark mode, autoSize, layout patterns, CSS best practices

• advanced.md

  • When: Building complex widgets with async data, error boundaries, or performance optimizations
  • Covers: Loading states, error handling, memoization, code splitting

📚 Need Complete Examples?

When: You want to see full implementations of common use cases

• common-patterns.md
  • End-to-end examples: weather app, todo list, recipe browser
  • Shows: Server code + widget code + best practices in context

Decision Tree

What do you need?

├─ New project from scratch
│  └─> quickstart.md (scaffolding + setup)
│
├─ OAuth / user authentication
│  └─> authentication/overview.md → provider-specific guide
│
├─ Simple backend action (no UI)
│  └─> Use Tool: server/tools.md
│
├─ Read-only data for clients
│  └─> Use Resource: server/resources.md
│
├─ Reusable prompt template
│  └─> Use Prompt: server/prompts.md
│
├─ Visual/interactive UI
│  └─> Use Widget: widgets/basics.md
│
└─ Deploy to production
   └─> deployment.md (cloud deploy, self-hosting, Docker)

Core Principles

1. Tools for actions - Backend operations with input/output
2. Resources for data - Read-only data clients can fetch
3. Prompts for templates - Reusable message templates
4. Widgets for UI - Visual interfaces when helpful
5. Mock data first - Prototype quickly, connect APIs later

❌ Common Mistakes

Avoid these anti-patterns found in production MCP servers:

Tool Definition

• ❌ Returning raw objects instead of using response helpers
  • ✅ Use text(), object(), widget(), error() helpers
• ❌ Skipping Zod schema .describe() on every field
  • ✅ Add descriptions to all schema fields for better AI understanding
• ❌ No input validation or sanitization
  • ✅ Validate inputs with Zod, sanitize user-provided data
• ❌ Throwing errors instead of returning error() helper
  • ✅ Use error("message") for graceful error responses

Widget Development

• ❌ Accessing props without checking isPending
  • ✅ Always check if (isPending) return <Loading/>
• ❌ Widget handles server state (filters, selections)
  • ✅ Widgets manage their own UI state with useState
• ❌ Missing McpUseProvider wrapper or autoSize
  • ✅ Wrap root component: <McpUseProvider autoSize>
• ❌ Inline styles without theme awareness
  • ✅ Use useWidgetTheme() for light/dark mode support

Security & Production

• ❌ Hardcoded API keys or secrets in code
  • ✅ Use process.env.API_KEY, document in .env.example
• ❌ No error handling in tool handlers
  • ✅ Wrap in try/catch, return error() on failure
• ❌ Expensive operations without caching
  • ✅ Cache API calls, computations with TTL
• ❌ Missing CORS configuration
  • ✅ Configure CORS for production deployments

🔒 Golden Rules

Opinionated architectural guidelines:

1. One Tool = One Capability

Split broad actions into focused tools:

• ❌ manage-users (too vague)
• ✅ create-user, delete-user, list-users

2. Return Complete Data Upfront

Tool calls are expensive. Avoid lazy-loading:

• ❌ list-products + get-product-details (2 calls)
• ✅ list-products returns full data including details

3. Widgets Own Their State

UI state lives in the widget, not in separate tools:

• ❌ select-item tool, set-filter tool
• ✅ Widget manages with useState or setState

4. exposeAsTool Defaults to false

Widgets are registered as resources only by default. Use a custom tool (recommended) or set exposeAsTool: true to expose a widget to the model:

// ✅ ALL 4 STEPS REQUIRED for proper type inference:

// Step 1: Define schema separately
const propsSchema = z.object({
  title: z.string(),
  items: z.array(z.string())
});

// Step 2: Reference schema variable in metadata
export const widgetMetadata: WidgetMetadata = {
  description: "...",
  props: propsSchema,  // ← NOT inline z.object()
  exposeAsTool: false
};

// Step 3: Infer Props type from schema variable
type Props = z.infer<typeof propsSchema>;

// Step 4: Use typed Props with useWidget
export default function MyWidget() {
  const { props, isPending } = useWidget<Props>();  // ← Add <Props>
  // ...
}

⚠️ Common mistake: Only doing steps 1-2 but skipping 3-4 (loses type safety)

5. Validate at Boundaries Only

• Trust internal code and framework guarantees
• Validate user input, external API responses
• Don't add error handling for scenarios that can't happen

6. Prefer Widgets for Browsing/Comparing

When in doubt, add a widget. Visual UI improves:

• Browsing multiple items
• Comparing data side-by-side
• Interactive selection workflows

Quick Reference

Minimal Server

import { MCPServer, text } from "mcp-use/server";
import { z } from "zod";

const server = new MCPServer({
  name: "my-server",
  title: "My Server",
  version: "1.0.0"
});

server.tool(
  {
    name: "greet",
    description: "Greet a user",
    schema: z.object({ name: z.string().describe("User's name") })
  },
  async ({ name }) => text("Hello " + name + "!"),
);

server.listen();

Response Helpers

Server methods:

• server.tool() - Define executable tool
• server.resource() - Define static/dynamic resource
• server.resourceTemplate() - Define parameterized resource
• server.prompt() - Define prompt template
• server.proxy() - Compose/Proxy multiple MCP servers
• server.uiResource() - Define widget resource
• server.listen() - Start server