I wish someone would write a clear, crisp explanation for why MCP is needed over simply supporting swagger or proto.
You'd still need basically the entire existing MCP spec to cover the use cases if it replaced JSON-RPC with Swagger or protobuf, plus additional material to cover the gaps and complications that that switch would involve.
I agree that swagger leaves a lot unplanned. I disagree about the local use case because (1) we could just run local HTTP servers easily and (2) I frankly assume the future of MCP is mostly remote.
Returning back to JSON-RPC, it’s a poorly executed RPC protocol. Here is an excellent HackerNews thread on it, but the TLDR is parsing JSON is expensive and complex, we have tons of tools (eg load balancers) that make modern services, and making those tools parse json is very expensive. Many people in the below thread mention alternative ways to implement J-RPC but that depends on new clients.
It's amusing to watch people refer to MCP as a set of tools, or a framework, or an SDK you can invoke, or something or other across a wide range of forums.It's just a standard. A convention. Calling it a protocol is a stretch as well. But there's no meat to it, really.
If you just used Rest API's, you'd need to create little "tools" (say, another executable) locally that the LLM can invoke that can call those API's. MCP standardizes what those tools should act like and their overall lifecycle model.
The references to it being like USB are also quite frankly absurd and delusional.
But that's the caliber of developer we're dealing with today.
If it was based on OpenAPI, servers created using transports that are not HTTP would need to implement a http server.
0: https://modelcontextprotocol.io/specification/2025-06-18/bas...
I know this because I wish it did. You can approximate streaming responses by using progress notifications. If you want something like the LLM partial response streaming, you'll have to extend MCP with custom capabilities flags. It's totally possible to extend it in this way, but then it's non standard.
Perhaps you are alluding to the fact that it's bidirectional protocol (by spec at least).