Explainer, written 30 October 2026
MCP vs API: what is the difference?
An API lets a developer's code talk to an app. MCP lets an AI assistant talk to an app. Most MCP servers sit on top of the app's API: MCP does not replace the API, it gives assistants a door made for them.
Side by side
| API | MCP | |
|---|---|---|
| Who uses it | A developer's code | An AI assistant, while it talks with you |
| How it is found | A developer reads the docs and writes the calls | The assistant asks the server for its tools and reads their descriptions |
| Its shape | Many endpoints, one per operation | A short list of tools shaped like tasks ("find a note", "file a ticket") |
| Sign-in | An API key or OAuth for one app | Each person signs in through their assistant, usually with OAuth |
| Who decides what runs | The code, fixed in advance | The model picks the tool for the request; you can approve each write |
| Where it runs | Anywhere your code runs | In Claude, ChatGPT, Claude Code and other MCP clients |
How they fit together
A typical MCP server is thin: each tool checks who you are and makes one or two calls into the app's own API, then returns the result in words and data the model can use. That is why the same app often has both, and why an MCP server is quick to build once an API exists.
The difference that matters most is who decides. With an API, a developer decides in advance which calls happen. With MCP, the assistant decides during the conversation, from the tool descriptions. Good tool names and descriptions, narrow permissions and approval for writes are what make that safe.
Which one do you need?
- You want your own code or another service to use an app: its API.
- You want Claude or ChatGPT to use an app for you: its MCP connector. How to add one.
- You make an app and want people's assistants to use it: build an MCP server on top of your API, with OAuth sign-in, and publish it in the MCP registry.
Questions
Is MCP replacing APIs?
No. MCP servers usually call APIs. MCP standardises how assistants discover and call tools, so one server works in many assistants instead of one plugin per assistant.
Is MCP just a REST API with a different name?
No. MCP is a protocol (JSON-RPC over HTTP) with a fixed way to list tools, describe their inputs and call them, plus sign-in and streaming. A REST API has its own shape for each app.
More: what is an MCP connector, or the connector guides.