MCP: An (Accidentally) Universal Plugin System

Posted by Stwerner 6/28/2025

MCP: An (Accidentally) Universal Plugin System(worksonmymachine.substack.com)

808 points | 350 commentspage 2

gavinray 6/28/2025|

> "The author discovers API's/JSON RPC"

I'm too young to be posting old_man_yells_at_cloud.jpg comments...

orliesaurus 6/29/2025|

That's honestly the best tl;Dr in this whole thread

moron4hire 6/28/2025||

This isn't a snide comment, I am legitimately asking. I don't understand the difference between MCP and REST. I know there are differences because I've used it a little. I mean, like, on an existential level. Why isn't it just REST? What parts do MCP give us that REST doesn't?

gunalx 6/28/2025||

I cannot really answer, but it seems you can just wrap mcp in a rest wrapper, as thqt is how open web ui seems to integrate mcp into its tooling..

moron4hire 6/29/2025||

Anthropic's example of creating an MCP server is just wrapping MCP around a REST weather forecasting service.

Maybe it's just that agentic LLMs have created a lot of interest in being interoperable, whereas efforts like Open API just didn't have any carrot to warrant the stick other than "wouldn't it be nice".

kimjune01 6/29/2025||

mcp forces a standard documentation, whereas documentation is optional for rest

afro88 6/28/2025||

The author is missing the bit that the LLM provides: automatically mapping input parameters to things the user wants to do, and responses to the way the UI displays them.

Take out the LLM and you're not that far away from existing protocols and standards. It's not plugging your app into any old MCP and it just works (like the USB-C example).

But, it is a good point that the hype is getting a lot of apps and services to offer APIs in a universal protocol. That helps.

vinkelhake 6/28/2025||

While reading this, the old ARexx (Amiga Rexx) popped into my head. It was a scripting language that in itself wasn't very noteworthy. However, it also made it easy for applications to expose functionality through an ARexx port. And again, offering up an API itself isn't noteworthy either. But it shipped by default in the system and if an application wanted to open itself up for scripting, ARexx was the natural choice. As a result, a ton of applications did have ARexx ports and there was a level of universality that was way ahead of its time.

Come to think of it - I don't know what the modern equivalent would be. AppleScript?

Hilift 6/28/2025||

IBM was big on Rexx when OS/2 2.x was released in ~1993.

"IBM also once engaged in a technology transfer with Commodore, licensing Amiga technology for OS/2 2.0 and above, in exchange for the REXX scripting language. This means that OS/2 may have some code that was not written by IBM, which can therefore prevent the OS from being re-announced as open-sourced in the future. On the other hand, IBM donated Object REXX for Windows and OS/2 to the Open Object REXX project maintained by the REXX Language Association on SourceForge."

https://en.wikipedia.org/wiki/Rexx

https://en.wikipedia.org/wiki/OS/2#Petitions_for_open_source

https://www.oorexx.org/status/index.rsp

vaxman 6/29/2025||

I’m not reading thru all that, but believe me when I say the ground truth is that IBM developed REXX on its mainframes and a genius guy from that world (poor bstrd) recreated it on Amiga as a third-party product called ARexx that was, in turn, adopted and promoted by CBM Dev Relations. One of the things the fake frenching innovation team in charge of Apple for a few years did was go down in the basement where they had an Amiga in various forms of dissection and transplanted ARexx out of it, mixed with HyperCard’s vocab (which they also didn’t invent) and announced to the World their great invention: AppleScript. But I digress. Here in the (dystopian) Future, some Apple operating systems are just now gaining the kind of power that ARexx had, because these types of systems require the cooperation of the developers and they had little incentive to “donate” functions to the system just to gain integration with Siri/Shortcuts, but they can get fired/bankrupted for not doing it to integrate with Apple Intelligence (I know, I hate that identifier too). ARexx could not be ported to MacOS back in the day because it would have had to have been championed by Apple Developer Relations (Guy Kawasaki & Co —did I just wreck the “Hackers” movie reference?) and, even/especially in the 80s, they wouldn’t have approved a tech that “competed” with AppleScript. Microsoft didn’t jump on this because it descends from DEC tech (under the direction of the Cutler) which had nothing like REXX.

billmcneale 6/28/2025|||

Microsoft introduced this in Windows in 1993, it's called COM and is still in (heavy) use today.

It basically powers all inter communication in Windows.

vaxman 6/29/2025||

Not really. COM/OLE is a different paradigm, their answer to an infamous vaporware called Taligent/OpenDoc that bankrupted many developers. Microsoft was sort of stuck with that security nightmare though

billmcneale 6/29/2025||

COM is exactly what OP was talking about.

Apps can expose endpoints that can be listed, and external processes can call these endpoints.

int_19h 6/30/2025||

"COM" by itself is a rather broad umbrella. What you're describing seems to be OLE Automation. That's the one that has type libraries (which you need for discoverability).

And then Active Scripting was supposed to be how you'd script those endpoints...

layer8 6/28/2025||

PowerShell with COM interfaces.

taytus 6/28/2025||

The author glosses over some practical realities. Just because something can be repurposed doesn't mean it should be. MCP was designed with specific assumptions about how AI models consume and process information. Using it as a general plugin system might work, but you'd likely hit limitations around things like authentication, real-time communication, or complex data flows that weren't priorities for the AI use case.

bravesoul2 6/29/2025||

This is well written and fun. Thanks OP!

Now I am excited by MCP and would be all in except security.

Security is a huge issue.

Forget AI and imagine a system where you call APIs and you get both data and JS. And that JS executes at global scope with full access to other APIs. And so do all the other MCP servers. Furthermore the MCP server may go to arbitrary Web pages and download JS. And that JS e.g. from a strangers Github issue or Web search gets executes with full API privileges.

    <cute animal interject> This isn't something MCP can fix. It is built into the dice rolling nature of LLMs. Turning predictions into privileged executions. And those dice can be loaded by any MCP server.

Or imagine surfing the Web using a 2001 browser with no protections against cross domain scripting. Then having a page where you choose what init scripts to run and then it cascades from there. You are logged into your bank at the time!

This is what worries me. It's not USBC. It's sort of USBC but where you are ordering all your peripherals from Amazon, Ali express and Temu and the house is made of tinder.

bigmattystyles 6/28/2025||

I thought MCPs just ‘figured out’ using docs how to call a program’s API. Won’t it matter that many APIs just suck?

superluserdo 6/28/2025||

Seems like we're just currently in the top-right of this comic https://xkcd.com/2044/

sgt 6/29/2025||

Makes me think of Kafka as well.

readthenotes1 6/28/2025||

Someone should write an AI tool that evaluates every top article in hacker News and provides the appropriate XKCD comic as a comment.

dexterdog 6/28/2025||

And then a few steps later it's just bots talking to bots. Then what did we read when we're on the loo?

thegreatwhale8 6/29/2025||

Offtopic: The article reads very much like it's chatgpt generated. But it's not surprising giving the subject matter. I just dislike how a computer tries to be entertaining and uses this default "voice" when writing anything. I hope there will be some way to personalize the output text, so it will be correct, but not soulless.

ls-a 6/29/2025|

I like how all the jokes/memes about what customers ask for vs. what developers produce, now apply to what developers ask for vs. what AI produces.

brap 6/28/2025|

I guess I’m finally old enough to become old-man-yelling-at-cloud.

I’m convinced that the only reason why MCP became a thing is because newcomers weren’t that familiar with OpenAPI and other existing standards, and because a protocol that is somehow tied to AI (even though it’s not, as this article shows) generates a lot of hype these days.

There’s absolutely nothing novel about MCP.

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