MCP Model Context Protocol explained: what it is, why Anthropic built it, 6400+ servers in the registry, adoption by OpenAI and Google, security concerns, and h
In November 2024, Anthropic quietly released the Model Context Protocol — an open standard for connecting AI models to external data sources and tools. At the time, it generated modest attention. Most observers saw it as a useful but incremental improvement on the existing function calling APIs that all major LLMs already supported.
Sixteen months later, MCP is on its way to becoming the TCP/IP of AI integrations. The public server registry has crossed 6,400 servers. OpenAI and Google DeepMind have both announced MCP support, effectively making it the cross-vendor standard. 70% of large SaaS providers either offer MCP endpoints or have announced plans to do so by end of 2026. And Anthropic’s 2026 roadmap includes governance structures, server discovery infrastructure, and transport improvements that suggest MCP is being designed for the long term.
If you are building AI systems in 2026 and not thinking about MCP, you are likely creating integration debt that will be expensive to unwind.
This guide explains MCP from first principles — what it is, why it was built, how it works, where the ecosystem stands, and where it is going.
The Problem MCP Solves
To understand why MCP exists, you need to understand the problem it solves. Before MCP, connecting an AI model to an external data source or tool required custom integration code for every combination of model and tool.
Consider this scenario: you want your AI assistant to be able to query your company’s database, read and write files, search your email, look up information in your CRM, and create calendar events. Before MCP, you would need to:
- Write custom function definitions in the format required by each AI model provider (OpenAI’s format, Anthropic’s format, Google’s format are all different)
- Implement the function handlers for each tool
- Maintain this code as the AI providers update their APIs
- Rebuild everything if you switch model providers
- Repeat for every AI application you build
The M × N problem: M models times N tools means M × N integrations to build and maintain. If you have 5 AI models and 20 data sources, that is 100 custom integrations.
MCP collapses this to M + N: each model implements MCP once, each tool implements MCP once, and any model can work with any tool. The same database MCP server works with Claude, with GPT, with Gemini, with any MCP-compatible agent framework.
What MCP Is: A Technical Overview
MCP is a client-server protocol built on JSON-RPC 2.0. Here is the architecture:
The Three Primitives
Tools are executable functions. A tool takes parameters, does something, and returns a result. A database query tool takes a SQL string and returns rows. A send-email tool takes recipient, subject, and body and returns a confirmation. Tools are the most powerful primitive — they let the AI act on the world.
Resources are data endpoints. A resource provides read access to structured data — files, database records, API responses, documents. Unlike tools, resources are primarily for retrieval, not action. Think of them as the AI’s ability to read files and documents at will without needing explicit commands.
Prompts are reusable prompt templates exposed by the server. This allows servers to package common AI workflows — “summarize this document,” “extract structured data from this receipt” — as callable operations that applications can invoke.
The Connection Model
An MCP host (like Claude Desktop, or your custom agent application) connects to one or more MCP servers. Each server exposes its tools, resources, and prompts. The host discovers available capabilities through a standardized handshake, then the AI model can call any of those capabilities as part of its reasoning process.
The transport layer supports both local communication (stdio — server runs as a subprocess on your machine) and remote communication (HTTP with Server-Sent Events for streaming). The local stdio mode is particularly elegant: the MCP server runs as a background process, and the AI connects to it through standard input/output. No network configuration, no authentication complexity for local use.
What Makes MCP Different from Function Calling
Every major LLM provider has function calling (or tool use) APIs. Why is MCP different?
The key difference is server-side definition. With standard function calling, the tool definitions live in your application code — you define the functions in the API request. With MCP, the definitions live in the server, and clients discover them dynamically. This means:
- Tool definitions do not need to be maintained in application code
- Servers can update their capabilities without application code changes
- The same server works with any MCP-compatible client
- Tool capabilities can be richer — including embedded prompts, example inputs/outputs, and usage documentation that helps the AI use tools more effectively
The analogy to HTTP APIs is useful: function calling is like writing a custom API client for each service you integrate. MCP is like REST — a standard protocol that any tool can implement and any client can speak.
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