MCP vs REST API: What's the Difference?
April 2026 · 6 min read
The Model Context Protocol (MCP) launched in late 2024 and reached critical mass in 2025. It's how AI assistants like Claude, ChatGPT, and Cursor connect to external tools. If you have a REST API, should you add an MCP server? Here's the breakdown.
What is MCP?
MCP is an open protocol built on JSON-RPC 2.0 that standardizes how LLMs connect to external capabilities. It defines three primitives:
- Tools — functions the AI can invoke (like POST endpoints)
- Resources — read-only data the AI can access (like GET endpoints)
- Prompts — reusable templates for structured interactions
Key Differences
| Feature | REST API | MCP |
|---|---|---|
| Protocol | HTTP + JSON | JSON-RPC 2.0 over HTTP/SSE |
| Discovery | OpenAPI spec (optional) | Built-in tools/list endpoint |
| Auth | Any (Basic, Bearer, OAuth) | OAuth 2.1 + PKCE (standard) |
| Schema | Varies (OpenAPI, GraphQL) | JSON Schema (required) |
| Transport | HTTP request/response | stdio or Streamable HTTP |
| Use case | Any client | AI assistants specifically |
Why MCP exists
Every AI assistant used to invent its own way to call tools. Claude had "function calling", ChatGPT had "plugins", Cursor had custom integrations. Each required a separate implementation.
MCP fixes this. Build one server, every MCP client works. Like USB for AI.
When to use REST vs MCP
Use REST when:
- Building APIs for web apps, mobile apps, microservices
- You need maximum compatibility across languages/frameworks
- Your consumers are human developers, not AI
- You need fine-grained HTTP semantics (status codes, caching, etc.)
Use MCP when:
- You want AI assistants to call your tools
- You need standardized tool discovery (tools/list)
- You want built-in OAuth + schema validation
- You want it to work with Claude, ChatGPT, Cursor, etc. automatically
Best approach: both
Keep your REST API for traditional consumers. Add an MCP server as a thin wrapper that calls your REST API. IOX makes this trivial:
"Build an MCP server that wraps my REST API at https://api.mycompany.com. Tools for each endpoint in my OpenAPI spec."
IOX generates the MCP server, you deploy, connect it to Claude. Your existing API stays untouched.
Example: wrapping Stripe
Stripe has a REST API. An MCP server wrapping it might expose:
create_charge(amount, customer_id)→ POST /v1/chargeslist_customers(limit)→ GET /v1/customersrefund_charge(charge_id, amount)→ POST /v1/refunds
Same data, AI-friendly interface. Claude can now issue refunds in plain English:"Refund $50 for charge ch_abc123".