Because while an agent can indeed call a REST API directly using some other tool ( probably curl) what LLMs are good at is calling tools, and all MCP is doing is turning an API into a tool and standardizing authorization, which if you go the API route can be a nightmare as auth is not even mentioned in OpenAPI specs, usually, so the agent has to figure out how to do auth, and it will necessarily have access to your token and maybe even credentials, so it can easily leak it to attackers your agent happens to visit by just asking for it. I can’t believe anyone who has tried to give agents access to APIs in a sane manner hasn’t concluded that something on the lines of MCP is absolutely necessary.

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