MCP vs REST API for AI Tool Calling: When to Use Which
REST APIs describe endpoints; the Model Context Protocol (MCP) describes tools with the metadata an LLM needs to call them safely. Both can back an agent — the choice affects discovery, auth flow, and how much per-integration glue code you write.
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What we're comparing
- REST API
HTTP-based resource protocol; requires a per-integration adapter for LLM tool calling.
- MCP
Anthropic-authored protocol standardizing how LLMs discover, call, and stream from external tools/resources.
Side-by-side
| Attribute | REST API | MCP |
|---|---|---|
| Discovery | OpenAPI/Swagger, out-of-band | Built-in tool listing over the wire |
| Transport | HTTP request/response | stdio, HTTP+SSE, or streamable HTTP |
| Streaming | SSE or WebSocket (custom) | Native streaming for long-running tools |
| Auth | OAuth/JWT/API key at edge | OAuth 2.1 with dynamic client registration (2025 spec) |
| Resource concept | CRUD on resources | First-class Resource type (readable) + Tool type (callable) |
| Prompt templates | Not part of spec | First-class Prompt type (reusable templates) |
| Client support | Universal | Claude Desktop, Cursor, Zed, VS Code (Aug 2025+), OpenAI Agents SDK |
| Best-fit workload | External-facing services, mobile/web clients | Agent tool integrations, IDE assistants, internal knowledge bases |
When to use which
- Use REST API
You already ship a public API and want a thin LLM wrapper; you need CDN edge caching; consumers are non-agent clients.
- Use MCP
You are building tools primarily consumed by LLM agents and want out-of-the-box discovery, streaming, and prompt templating.
FAQs
Can I expose the same functionality via both?
Yes, and many teams do — MCP for agent clients, REST for humans and legacy integrations. Keep them as thin wrappers over the same service layer.
Does MCP replace OpenAPI?
No. OpenAPI describes REST APIs; MCP describes tools/resources/prompts for LLM clients. They solve different problems and often coexist.
Related comparisons
Related on this site
Call this framework and its tools from your own agent via the Model Context Protocol (MCP) server. Works with Claude Desktop, Cursor, Zed, Continue, and the OpenAI Agents SDK.