Function Calling vs. Model Context Protocol (MCP): What You Need to Know

Integrating Large Language Models (LLMs) with external systems has transformed how businesses interact with technology. These models enable natural language inputs to control software, streamlining workflows and making operations more intuitive. However, integrating LLMs with external tools requires two key processes:

1. Translating user prompts into structured function calls (Function Calling).

2. Executing those function calls within an organized system (Model Context Protocol or MCP).

Both Function Calling and MCP play essential roles in LLM-driven automation. While Function Calling focuses on converting natural language into action-ready commands, MCP ensures those commands are executed efficiently and consistently. Let’s break down their differences and how they work together.

Before we begin, if you’re working with LLMs and need help setting up Function Calling, integrating MCP, or making your AI-driven system more efficient, I’d love to help. Whether you’re building something from scratch or improving an existing setup, having the right structure in place can save you a lot of time and effort.

If you’re looking for guidance or want to make sure everything runs smoothly, feel free to reach out at hello@fotiecodes.com, let’s chat about how we can get your AI system working exactly the way you need it to.

How LLM Integration Works in Two Phases

LLMs interact with external systems through a two-phase approach:

Phase 1: Function Calling – Translating Prompts into Actions

Function Calling enables LLMs to transform a user’s input into a structured function call. For example, if someone asks, “What’s Apple’s stock price in USD?”, the LLM generates a function call containing the necessary details (company name, currency format) for retrieving stock data.

Different LLM providers have their own way of structuring these function calls. Here’s how major models handle it:

Function Calling Examples from Leading LLMs

OpenAI:

{
  "index": 0,
  "message": {
    "role": "assistant",
    "content": null,
    "tool_calls": [
      {
        "name": "get_current_stock_price",
        "arguments": "{\n \"company\": \"AAPL\",\n \"format\": \"USD\"\n}"
      }
    ]
  },
  "finish_reason": "tool_calls"
}

Claude:

{
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "To answer this question, I will: …"
    },
    {
      "type": "tool_use",
      "id": "1xqaf90qw9g0",
      "name": "get_current_stock_price",
      "input": {"company": "AAPL", "format": "USD"}
    }
  ]
}

Gemini:

{
  "functionCall": {
    "name": "get_current_stock_price",
    "args": {
      "company": "AAPL",
      "format": "USD"
    }
  }
}

LLaMA:

{
  "role": "assistant",
  "content": null,
  "function_call": {
    "name": "get_current_stock_price",
    "arguments": {
      "company": "AAPL",
      "format": "USD"
    }
  }
}

Each model formats function calls differently, meaning there’s no universal standard yet. However, tools like LangChain help developers work with multiple LLMs by handling these variations.

Phase 2: MCP – Standardizing Execution Across Systems

Once an LLM generates a function call, that request needs to be executed by an external system. MCP provides a structured framework for handling these function calls, ensuring that tools can consistently interpret and respond to LLM-generated instructions.

MCP acts as a bridge between LLMs and software systems by managing:

• Tool discovery: Identifying the right tool for the request.

• Invocation: Executing the function call.

• Response handling: Returning results in a structured format.

Here’s what an MCP request looks like:

MCP Request Example

{
  "jsonrpc": "2.0",
  "id": 129,
  "method": "tools/call",
  "params": {
    "name": "get_current_stock_price",
    "arguments": {
      "company": "AAPL",
      "format": "USD"
    }
  }
}

In this setup, the application acts as a mediator that translates an LLM’s output into an MCP-compatible request. MCP then ensures the function call is executed correctly, sending structured results back to the LLM.

Function Calling vs. MCP: Understanding Their Roles

Though both Function Calling and MCP help LLMs interact with external systems, they serve distinct purposes.

Essentially, Function Calling is about “ordering the task,” while MCP is responsible for “executing the task.” Together, they ensure that AI-driven software automation runs efficiently.

Why This Matters for AI Powered Businesses

Well, I believe it’s crucial for companies integrating LLMs into their workflows to understand the difference between Function Calling and MCP. Here’s why:

• Scalability: MCP allows businesses to integrate LLMs across multiple applications, ensuring seamless function execution.

• Standardization: Instead of dealing with different LLM formats, MCP provides a consistent execution framework.

• Flexibility: Even as LLM vendors change their function call formats, MCP ensures continued compatibility with tools.

As AI adoption grows, businesses that leverage Function Calling + MCP together will have a more efficient and scalable AI-powered infrastructure.

Final Thoughts

In a nutshell both Function Calling and MCP play essential roles in enabling AI-driven software apps. While Function Calling translates natural language prompts into structured instructions, MCP ensures those instructions are executed consistently and reliably.

For companies looking to integrate AI into their workflows, understanding this two-phase approach will be key to maximizing efficiency and long-term scalability. As LLMs continue to evolve, having a robust Function Calling + MCP integration will be a game-changer in enterprise AI adoption.

FAQs

1. What is Function Calling in LLMs?

Function Calling allows LLMs to convert user inputs into structured API requests, enabling AI-powered automation.

2. What does MCP do?

MCP (Model Context Protocol) manages the execution of LLM-generated function calls by standardizing how software tools process these requests.

3. Why do Function Calling and MCP need each other?

Function Calling translates prompts into structured instructions, while MCP executes them, ensuring seamless AI integration.

4. Can I use Function Calling without MCP?

Yes, but without MCP, handling function execution across multiple tools becomes inconsistent and less scalable.

5. Will there be a universal standard for Function Calling?

Currently, there’s no single standard, but frameworks like LangChain help manage multiple LLM formats effectively.