MCP vs. Function Calling: Understanding the Difference

1. The Core Shift: Push vs. Pull

Every technical innovation is usually just a smarter way to move data. To understand the difference between MCP and Function Calling, you have to look at the Flow of Intent.

The "Push" Model (Traditional Function Calling)

In traditional Function Calling (OpenAI or Anthropic Messages API), you are the manager. Every time you send a prompt, you must include a giant list of "tools" in your API request. You "push" the schemas to the LLM.

// Every single API call needs the full tool definitions
const response = await openai.chat.completions.create({
  model: "gpt-4o",
  messages: [{ role: "user", content: "What is the weather?" }],
  tools: [
    {
      type: "function",
      function: {
        name: "get_weather",
        parameters: { /* Full JSON Schema here every time */ }
      }
    }
  ]
});

The "Pull" Model (MCP Discovery)

MCP uses a concept called Discovery. You are now a Shopkeeper (The Server). You just sit there with your "Menu" (the Schema) ready.

The AI Client (Claude Desktop/Cursor) is the Customer. When the app starts, it connects to your server and asks: "Hey, what can you do?" Your server responds with the schemas. This happens once during the "Handshake," and then the AI knows your capabilities for the rest of the session.

// You define the tool ONCE in your persistent server
server.tool(
  "get_weather",
  { location: z.string() },
  async ({ location }) => {
    // Your logic stays on your server
    return { content: [{ type: "text", text: "Sunny" }] };
  }
);

// The AI client "discovers" this automatically via the protocol.

2. Standardization: The USB-C Moment

Before MCP, building tools for AI was like living in a world where every phone had a different charging port. If you built a tool for OpenAI, you had to rewrite the schema and logic for Anthropic, and then again for Gemini.

MCP is the USB-C of AI tools.

• Universal Schema: One tool definition (JSON Spec) works everywhere.
• Persistent Servers: One server can serve tools to multiple apps (Claude, Cursor, Windsurf) simultaneously.
• Standard Protocol: The communication happens over JSON-RPC 2.0, meaning your language (Node.js/Python) doesn't matter as long as the data shape matches.

3. The "Host-Client-Server" Roles

To master MCP, you must understand the three characters in this story:

4. Quick Comparison Card

Which one should you build?

If you are building a public SaaS app where the LLM is hidden in your backend, Function Calling is still the way to go.

If you are building developer tools, local agents, or private intelligence that needs to plug into Claude or Cursor, MCP is the future. It turns your private data into first-class citizens in the AI era.