OpenAPI (or its Swagger predecessor) or Proto (I assume by this you mean protobuf?) don't cover what MCP does. It could have layered over them instead of using JSON-RPC, but I don't see any strong reason why they would be better than JSON-RPC as the basis (Swagger has communication assumptions that don't work well with MCP's local use case; protobuf doesn't cover communication at all and would require additional consideration in the protocol layered over it.)
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.
Not much, really. A lot of LLM models need system prompts or fine-tuning to reliably use MCP; (though to be clear you don't NEED to do either of those things, it just dramatically increases the reliability of the LLM)
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.
Proto has a full associated spec (gRPC) on communication protocols and structured definitions for them. MCP could easily have built upon these and gotten a lot “for free”. Generally gRPC is better than JsonRPC (see below).
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.
Eh... No, it does not support streaming responses.
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).
That's transport and message passing. The response isn't streamed. It's delivered as a single message when the task is complete. Don't be confused by the word "Streamable". That's just there because it's using SSE to stream a series of JSON-RPC messages from the Server to the Client. But the Response to any specific Request is a single monolithic message. In the this space, an LLM that supports streaming is sending the response to a request as partials as they are generated. This allows you to present the results faster and give a lower perceived latency. MCP *does not* support this by the current specs. As I said, you can extend MCP and provide these partials in ProgresNotification messages. Then you are using a non-standard spec extension.
