WebMCP Documentation

Dynamic Tool Registration

Conditional Tools Based on User State

import { useWebMCP } from '@mcp-b/react-webmcp';
import { useAuth } from './auth';
import { z } from 'zod';

function AdminPanel() {
  const { user } = useAuth();

  // Only register admin tools if user is admin
  useWebMCP({
    name: 'delete_user',
    description: 'Delete a user account (admin only)',
    inputSchema: {
      userId: z.string().uuid()
    },
    handler: async ({ userId }) => {
      if (!user?.isAdmin) {
        throw new Error('Unauthorized');
      }
      await api.users.delete(userId);
      return { success: true };
    },
    // Only register if user is admin
    enabled: user?.isAdmin
  });

  return <div>Admin Panel</div>;
}

Page-Specific Tools

function ProductPage({ productId }) {
  useWebMCP({
    name: 'add_to_cart',
    description: 'Add this product to cart',
    inputSchema: { quantity: z.number().min(1).default(1) },
    handler: async ({ quantity }) => {
      await addToCart(productId, quantity);
      return { success: true };
    }
  });

  return <div>Product Page</div>;
}

Context Engineering

For comprehensive context engineering patterns including URL-based scoping, progressive disclosure, role-based tools, component lifecycle scoping, and feature flags, see the dedicated Context Engineering guide.

The key principle: limit tool availability based on application state to reduce AI model confusion and improve decision quality. Use the enabled prop on useWebMCP to conditionally register tools.

State Synchronization

React State Integration

Tools can access and update React state:

function Counter() {
  const [count, setCount] = useState(0);

  useWebMCP({
    name: 'increment',
    inputSchema: { amount: z.number().default(1) },
    handler: async ({ amount }) => {
      setCount(prev => prev + amount);
      return { newValue: count + amount };
    }
  });

  return <div>Count: {count}</div>;
}

Context API Integration

Share tools across the component tree using React Context:

function AppProvider({ children }) {
  const [state, setState] = useState({});

  useWebMCP({
    name: 'update_settings',
    inputSchema: { theme: z.enum(['light', 'dark']).optional() },
    handler: async (settings) => {
      setState(prev => ({ ...prev, ...settings }));
      return { success: true };
    }
  });

  return <AppContext.Provider value={{ state }}>{children}</AppContext.Provider>;
}

Advanced Validation

Complex Input Validation

Zod supports sophisticated validation patterns:

useWebMCP({
  name: 'create_event',
  inputSchema: {
    title: z.string().min(1).max(100),
    startDate: z.string().datetime(),
    attendees: z.array(z.string().email()).min(1).max(50),
    recurrence: z.object({
      frequency: z.enum(['daily', 'weekly', 'monthly']),
      interval: z.number().min(1).max(365)
    }).optional()
  },
  handler: async (event) => await api.events.create(event)
});

Custom Validation Logic

Add business logic validation in your handler:

useWebMCP({
  name: 'transfer_funds',
  inputSchema: {
    fromAccount: z.string(),
    toAccount: z.string(),
    amount: z.number().positive()
  },
  handler: async ({ fromAccount, toAccount, amount }) => {
    const balance = await getAccountBalance(fromAccount);
    if (balance < amount) throw new Error(`Insufficient funds`);
    if (fromAccount === toAccount) throw new Error('Same account transfer');

    await api.transfer({ fromAccount, toAccount, amount });
    return { success: true };
  }
});

Error Handling

Handle errors gracefully and provide clear feedback:

useWebMCP({
  name: 'process_order',
  inputSchema: { orderId: z.string() },
  handler: async ({ orderId }) => {
    try {
      return await api.orders.process(orderId);
    } catch (error) {
      if (error.code === 'PAYMENT_FAILED') {
        return { success: false, error: 'Payment failed' };
      }
      throw error;
    }
  },
  onError: (error, input) => analytics.trackError('process_order_failed', { orderId: input.orderId })
});

Performance Optimization

Use debouncing, throttling, and caching for expensive operations:

// Debounce search requests
const debouncedSearch = useMemo(() => debounce(api.search, 300), []);

useWebMCP({
  name: 'search',
  inputSchema: { query: z.string().min(2) },
  handler: async ({ query }) => await debouncedSearch(query)
});

// Cache results
const cache = useRef(new Map());
useWebMCP({
  name: 'get_data',
  inputSchema: { id: z.string() },
  handler: async ({ id }) => {
    if (cache.current.has(id)) return cache.current.get(id);
    const data = await api.get(id);
    cache.current.set(id, data);
    return data;
  }
});

Multi-Tab Tool Collection

The MCP-B Extension collects and maintains tools from all open tabs simultaneously, making them available to agents regardless of which tab is currently active.

Key behavior: Unlike traditional tab-scoped approaches, the extension aggregates tools from every open tab, giving agents access to your entire browsing context at once.

How Tool Collection Works

When you have multiple tabs open with WebMCP servers:

Tool Routing

When an agent calls a tool:

Agent requests tool

Agent calls a tool (e.g., github_com_create_issue)

Extension identifies source

Extension determines which tab owns that tool

Extension routes call

If the tab is open, the tool is executed immediately. If the tab was closed, the extension may need to reopen it.

Results returned

Tool execution results are returned to the agent

Design Implications

Since all tools from all tabs are visible to the agent:

Tool Naming Matters

Use descriptive, namespaced tool names to avoid confusion:

// Good - clear which site/feature
useWebMCP({
  name: 'github_create_issue',
  description: 'Create a new GitHub issue',
  // ...
});

// Avoid - ambiguous
useWebMCP({
  name: 'create',
  description: 'Create something',
  // ...
});

Context in Descriptions

Since agents see all tools at once, make descriptions specific:

// Good - describes what and where
useWebMCP({
  name: 'add_to_cart',
  description: 'Add the currently viewed product from shop.example.com to your shopping cart',
  // ...
});

// Avoid - lacks context
useWebMCP({
  name: 'add_to_cart',
  description: 'Add item to cart',
  // ...
});

Tool Availability

Tools appear and disappear as you open/close tabs:

// This tool is available whenever the product page tab is open
function ProductPage() {
  useWebMCP({
    name: 'get_current_product',
    description: 'Get details about the currently viewed product',
    handler: async () => {
      return await fetchProductDetails(productId);
    }
  });

  return <div>Product Details</div>;
}

// When user closes this tab, get_current_product disappears from the toolkit

Cross-Tab Workflows

Agents can naturally compose tools from different tabs:

// Tab 1: Calendar app
useWebMCP({
  name: 'calendar_add_event',
  description: 'Add event to calendar',
  inputSchema: {
    title: z.string(),
    date: z.string().datetime()
  },
  handler: async ({ title, date }) => {
    return await calendar.addEvent({ title, date });
  }
});

// Tab 2: Email app
useWebMCP({
  name: 'email_get_unread',
  description: 'Get unread emails',
  handler: async () => {
    return await email.getUnread();
  }
});

// Agent can: "Get my unread emails and add any meeting invites to my calendar"
// It sees and can use tools from both tabs

Managing Tool Overload

When many tabs are open, agents see many tools. Use context engineering patterns to help:

// Add metadata to help agents understand when to use tools
useWebMCP({
  name: 'shop_checkout',
  description: 'Complete checkout on shop.example.com. Only use this after items have been added to cart and user has confirmed.',
  handler: async () => {
    return await processCheckout();
  }
});

// Or conditionally register tools based on state
function CartPage() {
  const { items } = useCart();

  // Only show checkout tool when cart has items
  useWebMCP({
    name: 'shop_checkout',
    description: 'Complete the checkout process',
    handler: async () => {
      return await processCheckout();
    },
    enabled: items.length > 0
  });

  return <div>Cart</div>;
}

See Context Engineering Patterns for more strategies to limit tool availability based on application state.

Multi-Step Workflows

Stateful Multi-Step Operations

function CheckoutFlow() {
  const [checkoutState, setCheckoutState] = useState({
    step: 'cart',
    cart: [],
    shipping: null,
    payment: null
  });

  useWebMCP({
    name: 'checkout_next_step',
    description: 'Proceed to next checkout step',
    inputSchema: {
      data: z.record(z.any())
    },
    handler: async ({ data }) => {
      const { step } = checkoutState;

      if (step === 'cart') {
        setCheckoutState(prev => ({
          ...prev,
          step: 'shipping',
          shipping: data
        }));
        return { nextStep: 'shipping' };
      }

      if (step === 'shipping') {
        setCheckoutState(prev => ({
          ...prev,
          step: 'payment',
          payment: data
        }));
        return { nextStep: 'payment' };
      }

      if (step === 'payment') {
        const order = await processOrder(checkoutState);
        setCheckoutState({ step: 'complete', orderId: order.id });
        return { complete: true, orderId: order.id };
      }
    }
  });

  useWebMCP({
    name: 'get_checkout_state',
    description: 'Get current checkout progress',
    handler: async () => checkoutState
  });

  return <div>Checkout Flow</div>;
}

Cross-Site Tool Composition

One of the most powerful features of the MCP-B Extension is the ability to compose tools from different websites. Tools from one site can use data from another site, enabling complex multi-site workflows.

Key advantage: Each site exposes its existing functionality as MCP tools, and the extension handles routing calls between them. The user maintains separate authentication contexts for each site.

How Cross-Site Calls Work

Example: Cross-Site Tool Composition

Site A exposes cart data:

useWebMCP({
  name: 'get_current_cart',
  handler: async () => ({ items: cart.items, total: cart.total })
});

Site B offers price comparison:

useWebMCP({
  name: 'compare_prices',
  inputSchema: { productName: z.string() },
  handler: async ({ productName }) => {
    const response = await fetch('/api/products/search', {
      method: 'POST',
      credentials: 'same-origin', // Uses existing session
      body: JSON.stringify({ query: productName })
    });
    return await response.json();
  }
});

How it works: Agent can call tools from both sites. The extension routes each tool call to the correct tab, preserving separate authentication contexts for each site. Tools are thin wrappers around existing APIs - no special auth needed.

Example: Content Aggregation

Agents can fetch content from one site and post to another:

// News site
useWebMCP({
  name: 'get_top_stories',
  handler: async () => ({ stories: await fetch('/api/stories/top').then(r => r.json()) })
});

// Social site
useWebMCP({
  name: 'post_to_feed',
  inputSchema: { title: z.string(), url: z.string().url() },
  handler: async ({ title, url }) => await fetch('/api/posts', {
    method: 'POST',
    credentials: 'same-origin',
    body: JSON.stringify({ title, url })
  }).then(r => r.json())
});

Agents can fetch stories from the news site and share them on social media.

Best Practices for Cross-Site Tools

Use descriptive tool names

Include the site domain or purpose in tool names to avoid collisions:

// Good
name: 'github_create_issue'
name: 'jira_create_ticket'

// Avoid
name: 'create_issue' // Which site?

Return structured data

Make it easy for tools on other sites to consume your data:

// Good - structured, predictable
return {
  items: [...],
  total: 99.99,
  currency: 'USD'
};

// Avoid - unstructured text
return "You have 3 items totaling $99.99";

Keep tools focused

Each tool should do one thing well, making them easier to compose:

// Good - focused tools
useWebMCP({ name: 'get_cart', ... });
useWebMCP({ name: 'add_to_cart', ... });
useWebMCP({ name: 'checkout', ... });

// Avoid - one tool that does everything
useWebMCP({ name: 'cart_manager', ... });

Document data formats

Use clear descriptions and schemas so other sites know what to expect:

useWebMCP({
  name: 'get_cart',
  description: 'Get cart contents. Returns {items: Array<{name, price, sku, quantity}>, total: number}',
  handler: async () => { ... }
});

Security Considerations

When building tools that will be composed with other sites:

Never expose sensitive data in tool responses
Validate all inputs from external sources
Don’t trust data from other sites without validation
Be aware that AI may pass data between sites
Review security best practices for multi-site scenarios

Read-Only Context Tools

Exposing Application State

import { useWebMCPContext } from '@mcp-b/react-webmcp';

function AppLayout() {
  const { pathname } = useLocation();
  const { user } = useAuth();

  // Lightweight read-only context
  useWebMCPContext(
    'context_current_route',
    'Get the current page route and user info',
    () => ({
      path: pathname,
      user: user ? {
        id: user.id,
        name: user.name,
        role: user.role
      } : null,
      timestamp: new Date().toISOString()
    })
  );

  return <div>App Layout</div>;
}

Security Best Practices

Always sanitize inputs, implement rate limiting, and validate permissions. See the Security Guide for comprehensive patterns including:

Input sanitization with DOMPurify
Rate limiting implementations
Permission checks and auth validation
Handling sensitive data securely

Next Steps

Package Documentation

Deep dive into package APIs

Examples

See complete working examples

Troubleshooting

Common issues and solutions

Discord Community

Get help from the community

Start Here

Advanced Patterns

Dynamic Tool Registration

Conditional Tools Based on User State

Page-Specific Tools

Context Engineering

State Synchronization

React State Integration

Context API Integration

Advanced Validation

Complex Input Validation

Custom Validation Logic

Error Handling

Performance Optimization

Multi-Tab Tool Collection

How Tool Collection Works

Tool Routing

Design Implications

Managing Tool Overload

Multi-Step Workflows

Stateful Multi-Step Operations

Cross-Site Tool Composition

How Cross-Site Calls Work

Example: Cross-Site Tool Composition

Example: Content Aggregation

Best Practices for Cross-Site Tools

Security Considerations

Read-Only Context Tools

Exposing Application State

Security Best Practices

Next Steps

Package Documentation

Examples

Troubleshooting

Discord Community

​Dynamic Tool Registration

​Conditional Tools Based on User State

​Page-Specific Tools

​Context Engineering

​State Synchronization

​React State Integration

​Context API Integration

​Advanced Validation

​Complex Input Validation

​Custom Validation Logic

​Error Handling

​Performance Optimization

​Multi-Tab Tool Collection

​How Tool Collection Works

​Tool Routing

​Design Implications

​Managing Tool Overload

​Multi-Step Workflows

​Stateful Multi-Step Operations

​Cross-Site Tool Composition

​How Cross-Site Calls Work

​Example: Cross-Site Tool Composition

​Example: Content Aggregation

​Best Practices for Cross-Site Tools

​Security Considerations

​Read-Only Context Tools

​Exposing Application State

​Security Best Practices

​Next Steps

Package Documentation

Examples

Troubleshooting

Discord Community

Dynamic Tool Registration

Conditional Tools Based on User State

Page-Specific Tools

Context Engineering

State Synchronization

React State Integration

Context API Integration

Advanced Validation

Complex Input Validation

Custom Validation Logic

Error Handling

Performance Optimization

Multi-Tab Tool Collection

How Tool Collection Works

Tool Routing

Design Implications

Managing Tool Overload

Multi-Step Workflows

Stateful Multi-Step Operations

Cross-Site Tool Composition

How Cross-Site Calls Work

Example: Cross-Site Tool Composition

Example: Content Aggregation

Best Practices for Cross-Site Tools

Security Considerations

Read-Only Context Tools

Exposing Application State

Security Best Practices

Next Steps