MCP versus REST, function calling, and plugins

Overview

• REST is an HTTP API design style for application services.
• Function calling is a model interaction mechanism for proposing structured operations.
• MCP standardizes integration discovery and invocation/context exchange; a plugin is a packaging/integration mechanism whose exact meaning depends on its host.

Why this matters

Understanding these layers prevents rebuilding a backend or expecting a protocol to orchestrate the agent.

An everyday example

A menu, ordering language, kitchen process, and restaurant app are different layers of the same service.

A technical example

Model function call → MCP client → MCP tool server → existing REST claim API.

How it works

Use model tool calling for decisions, MCP for standardized integration, and domain APIs for authoritative operations.

Architecture and flow

LLM tool proposal → host policy → MCP invocation → REST/business API → result.

The insurance assistant example

The insurer keeps existing claim APIs and adds a narrow MCP facade for approved AI hosts.

When to use it

When multiple AI hosts need consistent discovery and tool/context access.

When to avoid it

Do not replace all internal REST APIs with MCP or describe MCP as a model or agent framework.

Interview answer

“Function calling is the model's decision interface, MCP is the integration protocol, and REST can remain the business-service interface behind it.”

Benefits, trade-offs, and production considerations

Added protocol layers introduce network latency, authentication flows, and operational ownership. Keep the MCP facade narrow and independently testable.