MCP vs REST API: The Difference, and When to Use Each
MCP and REST often expose the same data, but one is built for your code and the other for an AI agent. Here is the practical difference and when each wins.
TL;DR: A REST API is for your code to call against a fixed contract. An MCP server is for an AI agent to call: it advertises tools the agent discovers and invokes on its own. They frequently wrap the same data. Use REST for deterministic integrations, MCP when an agent should decide what to fetch. Most serious platforms offer both.
The core difference
With a REST API, a developer reads the docs and writes the exact request. With an MCP server, the agent reads the list of available tools and decides which to call, with what arguments, in what order. One is a contract for software; the other is a toolbox for a reasoning model.
Side by side
| REST API | MCP server | |
|---|---|---|
| Who calls it | your application code | the AI agent (Claude, etc.) |
| How it's discovered | you read documentation | the agent reads the tool manifest |
| Shape of use | fixed endpoints you wire up | composable tools the agent chains |
| Output | JSON you parse | structured results the model reasons over |
| Best for | pipelines, dashboards, deterministic jobs | agentic workflows, "ask in plain language" |
When to use REST
- You're building a dashboard, ETL job, or backend that needs the same data the same way every time.
- You want full control over caching, retries, and error handling.
- No LLM is in the loop.
When to use MCP
- You want a person to ask for something and let the agent figure out the calls.
- The task spans several lookups the agent should chain (search, then fetch, then compare).
- You're building on top of Claude or another MCP-compatible client. (New to it? See What is an MCP server?)
Why not both?
They aren't competitors. A good data platform exposes a REST API for code and an MCP server for agents, over the same sourced data. Arkolith does exactly that: REST API for programmatic access, plus an MCP server so your agent can query markets, filings, and the real-world economy directly.
Frequently asked questions
Is MCP replacing REST?
No. MCP sits above data access for agents; REST remains the workhorse for code. They coexist.
Can one server do both?
Yes, the common pattern is an MCP server that wraps an existing REST API, so agents and code share one backend.
Arkolith gives you both, over provenance-stamped data. Get a key or read the docs.