MCP, A2A and Function Calling Explained: How AI Agents Connect to Tools and Each Other

Agents need two kinds of connection: to tools and data, and to other agents. Three approaches matter in 2026, and they solve different problems.

Function calling

Every major model supports function calling: you describe a function, the model returns a structured request to call it, and your code executes it. It is the simplest pattern and works well inside one application. Its limitation is that each integration is custom, and it lives inside your app.

Model Context Protocol (MCP)

MCP, introduced by Anthropic, is an open standard for exposing tools, data and prompts from a server to any compatible AI application. Build an MCP server for your CRM once; any MCP-capable client can use it. It is now supported by many model providers and developer tools, which makes it the strongest candidate for a shared integration layer.

Agent2Agent (A2A)

A2A, proposed by Google and developed with partners, targets communication between agents. Agents advertise what they can do and exchange tasks across vendors and organisations. Where MCP connects an agent to a tool, A2A connects one agent to another.

How they fit together

Use function calling for logic inside one application, MCP for shared access to systems, and A2A when agents from different teams or vendors must collaborate. Expect the standards to keep evolving; design so the protocol is a replaceable layer.

More on agent standards in our AI Agents resource hub, and on building MCP integrations through our agentic AI services.