MCP Client skill

MCP Client — Consuming Model Context Protocol Services

Transports: prefer Streamable HTTP (StreamableHTTPClientTransport) per MCP spec 2025-03-26; SSE is deprecated; stdio stays for local processes.

Connect AI agents to external MCP services for web intelligence, blockchain data, document processing, and more. Production patterns for authentication, payments, and multi-tool workflows.

Overview

MCP (Model Context Protocol) lets AI agents call external tools through a standardized interface. Instead of building every capability from scratch, agents connect to MCP services that provide specialized tools — screenshots, DNS lookups, blockchain queries, OCR, and more.

This skill teaches you to:

1. Connect to MCP services via SSE or stdio transport
2. Authenticate across three tiers: free, API key, and x402 micropayments
3. Build multi-tool workflows combining several MCP tools
4. Handle errors, retries, and cost optimization
5. Configure popular AI clients (Claude Desktop, Claude Code, Cursor)

Why External MCP Services?

Build vs. consume decision matrix:

When to consume: You need web screenshots, DNS/WHOIS lookups, SSL checks, OCR, or blockchain data. These are commodity capabilities — don't rebuild them.

When to build: You need proprietary data access, custom business logic, or sub-millisecond latency.

---

Connecting to MCP Services

Transport Types

SSE (Server-Sent Events) — HTTP-based, works across networks:

Endpoint: https://mcp.skills.ws/mcp/sse
Protocol: HTTP GET (SSE stream) + HTTP POST (tool calls)

stdio — Local process, used for CLI tools:

Command: npx @mcp/some-local-server
Protocol: JSON-RPC over stdin/stdout

For remote services, SSE is the standard transport.

Health Check

Always verify a service is up before configuring:

curl -s https://mcp.skills.ws/health
# {"status":"ok","services":["screenshot","whois","blockchain"]}

---

Available Tools — mcp.skills.ws

A production MCP service providing web intelligence and blockchain tools.

Screenshot — Capture Any Webpage

# Response is JSON with base64 image — extract and decode to get the PNG
curl -s "https://mcp.skills.ws/api/screenshot?url=https://example.com" \
  | jq -r '.image' | sed 's|data:image/png;base64,||' | base64 -d > screenshot.png

Parameters:

• url (required) — URL to capture
• width — Viewport width (default: 1280)
• height — Viewport height (default: 800)
• fullPage — Capture full scrollable page (default: false)
• format — png or jpeg (default: png)

WHOIS — Domain Registration Data

curl -s "https://mcp.skills.ws/api/whois?domain=example.com"

Returns: registrar, creation/expiry dates, nameservers, registrant info (when available).

DNS — Record Lookups

curl -s "https://mcp.skills.ws/api/dns?domain=example.com&type=A"

Parameters:

• domain (required)
• type — A, AAAA, MX, NS, TXT, CNAME, SOA (default: A)

SSL — Certificate Analysis

curl -s "https://mcp.skills.ws/api/ssl?domain=example.com"

Returns: issuer, validity dates, SANs, certificate chain, protocol support.

OCR — Extract Text from Images

curl -s "https://mcp.skills.ws/api/ocr?url=https://example.com/receipt.png"

Blockchain — On-Chain Queries

# Native token balance
curl -s "https://mcp.skills.ws/api/chain/balance?address=0x...&chain=celo"

# ERC20 token balance
curl -s "https://mcp.skills.ws/api/chain/erc20?address=0x...&token=0xTOKEN_ADDRESS&chain=base"

# Transaction details
curl -s "https://mcp.skills.ws/api/chain/tx?hash=0x...&chain=ethereum"

Supported chains: Ethereum, Base, Arbitrum, Optimism, Polygon, Celo.

---

Authentication Tiers

Tier 1: Free (No Auth Required)

10 API calls per day per IP address. No signup needed.

# Just call it — no headers required
curl -s "https://mcp.skills.ws/api/dns?domain=example.com"

Rate limit headers in response:

X-RateLimit-Limit: 10
X-RateLimit-Remaining: 7

When exhausted:

{
  "error": "Daily free limit reached",
  "limit": 10,
  "upgrade": {
    "stripe": "POST /billing/checkout for unlimited API key ($9/mo)",
    "x402": {
      "price": 0.005,
      "currency": "USD",
      "receiver": "0x087ae921CE8d07a4dE6BdacAceD475e9080B2aDF",
      "networks": ["base", "celo"],
      "accepts": ["USDC", "USDT"]
    }
  }
}

Tier 2: API Key ($9/month)

Unlimited calls with a subscription API key.

Get a key:

# Create checkout session
curl -s -X POST "https://mcp.skills.ws/billing/checkout" | jq .url
# Opens Stripe checkout → pay → receive API key

Use the key:

curl -s "https://mcp.skills.ws/api/whois?domain=example.com" \
  -H "X-Api-Key: mcp_your_key_here"

Tier 3: x402 Pay-Per-Call ($0.005/call)

Pay with stablecoins per request — no subscription needed. Ideal for AI agents with crypto wallets.

Supported:

• Networks: Base, Celo
• Tokens: USDC, USDT
• Price: $0.005 per call
• Receiver: 0x087ae921CE8d07a4dE6BdacAceD475e9080B2aDF

Payment header format:

const payment = {
  network: "base",           // "base" or "celo"
  token: "USDC",             // "USDC" or "USDT"
  txHash: "0xabc123...",     // Transaction hash proving payment
  amount: "0.005",           // USD amount (must be >= 0.005)
  receiver: "0x087ae921CE8d07a4dE6BdacAceD475e9080B2aDF"
};

const header = Buffer.from(JSON.stringify(payment)).toString('base64');

Making a paid call:

# After sending $0.005 USDC to the receiver address:
PAYMENT=$(echo -n '{"network":"base","token":"USDC","txHash":"0xYOUR_TX_HASH","amount":"0.005","receiver":"0x087ae921CE8d07a4dE6BdacAceD475e9080B2aDF"}' | base64)

curl -s "https://mcp.skills.ws/api/dns?domain=example.com" \
  -H "X-Payment: $PAYMENT"

x402 flow in JavaScript:

import { encodeFunctionData, parseUnits } from 'viem';
import { base } from 'viem/chains';

const USDC_BASE = '0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913';
const RECEIVER = '0x087ae921CE8d07a4dE6BdacAceD475e9080B2aDF';
const PRICE = parseUnits('0.005', 6); // USDC has 6 decimals

// 1. Send USDC payment
const txHash = await walletClient.sendTransaction({
  to: USDC_BASE,
  data: encodeFunctionData({
    abi: [{
      name: 'transfer',
      type: 'function',
      inputs: [
        { name: 'to', type: 'address' },
        { name: 'amount', type: 'uint256' }
      ],
      outputs: [{ type: 'bool' }]
    }],
    args: [RECEIVER, PRICE]
  }),
  chain: base
});

// 2. Build payment proof
const payment = Buffer.from(JSON.stringify({
  network: 'base',
  token: 'USDC',
  txHash,
  amount: '0.005',
  receiver: RECEIVER
})).toString('base64');

// 3. Make authenticated API call
const res = await fetch('https://mcp.skills.ws/api/screenshot?url=https://example.com', {
  headers: { 'X-Payment': payment }
});

x402 flow in Python:

import base64, json, requests

payment = {
    "network": "base",
    "token": "USDC",
    "txHash": "0xYOUR_TX_HASH",
    "amount": "0.005",
    "receiver": "0x087ae921CE8d07a4dE6BdacAceD475e9080B2aDF"
}

header = base64.b64encode(json.dumps(payment).encode()).decode()

response = requests.get(
    "https://mcp.skills.ws/api/dns",
    params={"domain": "example.com"},
    headers={"X-Payment": header}
)
print(response.json())

---

Client Configuration

Claude Desktop

Edit ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "skills-ws": {
      "url": "https://mcp.skills.ws/mcp/sse",
      "headers": {
        "X-Api-Key": "mcp_your_key_here"
      }
    }
  }
}

For free tier (no key needed):

{
  "mcpServers": {
    "skills-ws": {
      "url": "https://mcp.skills.ws/mcp/sse"
    }
  }
}

Claude Code

# Add as MCP server
claude mcp add skills-ws https://mcp.skills.ws/mcp/sse

# With API key
claude mcp add skills-ws https://mcp.skills.ws/mcp/sse --header "X-Api-Key: mcp_your_key_here"

Or in .claude/settings.json:

{
  "mcpServers": {
    "skills-ws": {
      "type": "sse",
      "url": "https://mcp.skills.ws/mcp/sse",
      "headers": {
        "X-Api-Key": "mcp_your_key_here"
      }
    }
  }
}

Cursor

In Cursor settings → MCP Servers:

{
  "skills-ws": {
    "url": "https://mcp.skills.ws/mcp/sse",
    "headers": {
      "X-Api-Key": "mcp_your_key_here"
    }
  }
}

OpenClaw

In openclaw.json:

{
  "mcp": {
    "servers": {
      "skills-ws": {
        "transport": "sse",
        "url": "https://mcp.skills.ws/mcp/sse",
        "headers": {
          "X-Api-Key": "mcp_your_key_here"
        }
      }
    }
  }
}

---

Error Handling

HTTP Status Codes

Retry Pattern (JavaScript)

async function mcpCall(url, headers = {}, maxRetries = 3) {
  for (let i = 0; i < maxRetries; i++) {
    try {
      const res = await fetch(url, { headers });
      
      if (res.status === 429) {
        const waitMs = Math.min(1000 * Math.pow(2, i), 30000);
        console.log(`Rate limited. Waiting ${waitMs}ms...`);
        await new Promise(r => setTimeout(r, waitMs));
        continue;
      }
      
      if (res.status === 402) {
        const body = await res.json();
        console.log('Payment required:', body.x402);
        throw new Error('x402 payment needed');
      }
      
      if (res.status === 401) {
        throw new Error('Invalid API key');
      }
      
      if (!res.ok) {
        throw new Error(`HTTP ${res.status}: ${await res.text()}`);
      }
      
      return await res.json();
    } catch (err) {
      if (i === maxRetries - 1) throw err;
      await new Promise(r => setTimeout(r, 1000 * Math.pow(2, i)));
    }
  }
}

// Usage
const dns = await mcpCall(
  'https://mcp.skills.ws/api/dns?domain=example.com',
  { 'X-Api-Key': 'mcp_your_key_here' }
);

Retry Pattern (Python)

import time
import requests

def mcp_call(url, headers=None, max_retries=3):
    for i in range(max_retries):
        try:
            res = requests.get(url, headers=headers or {}, timeout=30)
            
            if res.status_code == 429:
                wait = min(2 ** i, 30)
                print(f"Rate limited. Waiting {wait}s...")
                time.sleep(wait)
                continue
            
            if res.status_code == 402:
                print("Payment required:", res.json().get("x402"))
                raise Exception("x402 payment needed")
            
            if res.status_code == 401:
                raise Exception("Invalid API key")
            
            res.raise_for_status()
            return res.json()
        except requests.exceptions.RequestException as e:
            if i == max_retries - 1:
                raise
            time.sleep(2 ** i)

# Usage
dns = mcp_call(
    "https://mcp.skills.ws/api/dns?domain=example.com",
    headers={"X-Api-Key": "mcp_your_key_here"}
)

---

Multi-Tool Workflows

Website Audit (DNS + SSL + Screenshot)

async function auditWebsite(domain, apiKey) {
  const headers = { 'X-Api-Key': apiKey };
  const base = 'https://mcp.skills.ws/api';
  
  const [dns, ssl, whois] = await Promise.all([
    mcpCall(`${base}/dns?domain=${domain}&type=A`, headers),
    mcpCall(`${base}/ssl?domain=${domain}`, headers),
    mcpCall(`${base}/whois?domain=${domain}`, headers),
  ]);
  
  // Take screenshot after DNS resolves
  const screenshot = await fetch(
    `${base}/screenshot?url=https://${domain}&fullPage=true`,
    { headers }
  );
  
  return {
    domain,
    dns: dns.records,
    ssl: {
      issuer: ssl.certificate.issuer,
      validUntil: ssl.certificate.validUntil,
      daysRemaining: ssl.certificate.daysUntilExpiry,
    },
    whois: {
      registrar: whois.whois?.registrar,
      expires: whois.whois?.expiryDate,  // field names vary by registrar/TLD
    },
    screenshot: await screenshot.arrayBuffer(),
  };
}

Receipt Processing (Screenshot + OCR)

async function processReceipt(imageUrl, apiKey) {
  const headers = { 'X-Api-Key': apiKey };

  // Extract text from receipt image
  const ocr = await fetch(
    `https://mcp.skills.ws/api/ocr?url=${encodeURIComponent(imageUrl)}`,
    { headers }
  );

  const { text } = await ocr.json();

  // Parse extracted text for amounts, dates, vendor
  return {
    rawText: text,
    // Further parsing with regex or LLM
  };
}

Domain Portfolio Monitor

async function monitorDomains(domains, apiKey) {
  const headers = { 'X-Api-Key': apiKey };
  const base = 'https://mcp.skills.ws/api';
  const alerts = [];
  
  for (const domain of domains) {
    const [ssl, whois] = await Promise.all([
      mcpCall(`${base}/ssl?domain=${domain}`, headers),
      mcpCall(`${base}/whois?domain=${domain}`, headers),
    ]);
    
    // Alert if SSL expires within 30 days
    if (ssl.certificate.daysUntilExpiry < 30) {
      alerts.push({
        domain,
        type: 'ssl_expiry',
        message: `SSL expires in ${ssl.certificate.daysUntilExpiry} days`,
      });
    }

    // Alert if domain expires within 60 days (field names vary by registrar)
    const domainDays = Math.floor(
      (new Date(whois.whois?.expiryDate) - Date.now()) / 86400000
    );
    if (domainDays < 60) {
      alerts.push({
        domain,
        type: 'domain_expiry',
        message: `Domain expires in ${domainDays} days`,
      });
    }
  }
  
  return alerts;
}

Blockchain Wallet Dashboard

async function walletDashboard(address, chains, apiKey) {
  const headers = { 'X-Api-Key': apiKey };
  const base = 'https://mcp.skills.ws/api/chain';

  const balances = await Promise.all(
    chains.map(chain =>
      mcpCall(`${base}/balance?address=${address}&chain=${chain}`, headers)
        .then(data => ({ chain, ...data }))
    )
  );

  return {
    address,
    balances,
  };
}

---

Best Practices

Caching

Cache responses for data that doesn't change frequently:

const cache = new Map();
const CACHE_TTL = {
  dns: 300000,      // 5 minutes
  whois: 86400000,  // 24 hours
  ssl: 3600000,     // 1 hour
  screenshot: 60000, // 1 minute
};

async function cachedMcpCall(tool, params, headers) {
  const key = `${tool}:${JSON.stringify(params)}`;
  const cached = cache.get(key);
  
  if (cached && Date.now() - cached.time < (CACHE_TTL[tool] || 60000)) {
    return cached.data;
  }
  
  const query = new URLSearchParams(params).toString();
  const data = await mcpCall(
    `https://mcp.skills.ws/api/${tool}?${query}`,
    headers
  );
  
  cache.set(key, { data, time: Date.now() });
  return data;
}

Cost Optimization

1. Start with free tier — 10 calls/day is enough for development
2. Cache aggressively — DNS and WHOIS data rarely changes
3. Batch related calls — Use Promise.all() for parallel requests
4. Use API key for production — $9/mo is cheaper than x402 at 1800+ calls/month
5. Use x402 for burst traffic — Pay only for what you use, no commitment

Break-even calculation:

• API key: $9/mo = unlimited calls
• x402: $0.005/call × 1,800 calls = $9
• If you make >1,800 calls/month → API key is cheaper
• If you make <1,800 calls/month → x402 is cheaper

Security

1. Never expose API keys in client-side code — Use server-side proxies
2. Rotate keys periodically — Use POST /admin/keys (requires admin secret) to generate a new key, then POST /admin/revoke to revoke the old one
3. Monitor usage — Track calls per key to detect abuse
4. Validate responses — Don't trust MCP responses blindly in security-critical flows

Fallback Patterns

async function resilientDnsLookup(domain) {
  try {
    // Primary: MCP service
    return await mcpCall(
      `https://mcp.skills.ws/api/dns?domain=${domain}`,
      { 'X-Api-Key': API_KEY }
    );
  } catch (err) {
    // Fallback: local dig command
    const { execSync } = await import('child_process');
    const result = execSync(`dig +short ${domain} A`).toString().trim();
    return { records: result.split('\n').filter(Boolean) };
  }
}

---

Programmatic MCP Client (SDK)

For building agents that dynamically discover and call MCP tools:

import { Client } from '@modelcontextprotocol/sdk/client/index.js';
import { SSEClientTransport } from '@modelcontextprotocol/sdk/client/sse.js';

// Connect to MCP service
const transport = new SSEClientTransport(
  new URL('https://mcp.skills.ws/mcp/sse'),
  {
    requestInit: {
      headers: { 'X-Api-Key': 'mcp_your_key_here' }
    }
  }
);

const client = new Client({ name: 'my-agent', version: '1.0.0' });
await client.connect(transport);

// Discover available tools
const { tools } = await client.listTools();
console.log('Available tools:', tools.map(t => t.name));

// Call a tool
const result = await client.callTool({
  name: 'dns',
  arguments: { domain: 'example.com', type: 'MX' }
});
console.log('DNS result:', result.content[0].text);

// Cleanup
await client.close();

Python MCP Client

from mcp import ClientSession
from mcp.client.sse import sse_client

async def main():
    headers = {"X-Api-Key": "mcp_your_key_here"}
    
    async with sse_client("https://mcp.skills.ws/mcp/sse", headers=headers) as (read, write):
        async with ClientSession(read, write) as session:
            await session.initialize()
            
            # List tools
            tools = await session.list_tools()
            for tool in tools.tools:
                print(f"  {tool.name}: {tool.description}")
            
            # Call a tool
            result = await session.call_tool("whois", {"domain": "example.com"})
            print(result)

import asyncio
asyncio.run(main())

---

Monitoring Your Usage

Check Current Status

# Free tier — check remaining calls via response headers
curl -sI "https://mcp.skills.ws/api/dns?domain=test.com" | grep RateLimit
# X-RateLimit-Limit: 10
# X-RateLimit-Remaining: 6

Admin Stats (if you have admin access)

curl -s "https://mcp.skills.ws/admin/stats" \
  -H "X-Admin-Secret: your_admin_secret" | jq .

Returns:

{
  "freeUsers": 42,
  "totalFreeRequests": 156,
  "keys": { "total": 5, "active": 4, "revoked": 1 },
  "requests": { "free": 156, "apikey": 1203, "x402": 47, "blocked": 12 },
  "freeLimit": 10,
  "x402Price": 0.005
}

---

Common Issues

"Daily free limit reached" (429)

Upgrade to API key ($9/mo) or use x402 pay-per-call.

"Invalid API key" (401)

Key may have been revoked (subscription cancelled). Generate a new one via /billing/checkout.

"Payment required" (402)

x402 payment header is malformed, amount is too low, or txHash was already used (replay prevention).

SSE connection drops

Reconnect with exponential backoff. SSE connections may timeout after extended idle periods.

Slow responses

Screenshots take 3-10s depending on page complexity. DNS/WHOIS/SSL are typically <1s. Use timeouts appropriately.

---

Quick Reference

Default service endpoint: https://mcp.skills.ws