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MCP Apps: Extending servers with interactive user interfaces

blog.modelcontextprotocol.io

21–30 of 124 posts

Re: MCP Apps: Extending servers with interactive user interfaces

#21
An, the dream, a cross platform App Store you can install apps into any client application that supports MCP, but is open, free and agentic.

It’s basically a “web App Store” and we side step the existing app stores (and their content guidelines, security restrictions and billing requirements) because it’s all done via a mega app (the MCP client).

How could it go wrong?

If only someone had done this before, we wouldnt be stuck in Apples, etc’s walled gardens…

Seriously though; honest question: this is literally circumventing platform requirements to use the platform app stores. How do you imagine this is going to be allowed?

Is ChatGPT really big enough they can pull the “we’re gonna do it, watcha gonna do?” to Apple?

Who’s going to curate this app store so non technical users (the explicitly stated audience) can discover these MCP apps?

It feels like MCP itself; half baked. Overly ambitious. “We’ll figure the details out later”

Re: MCP Apps: Extending servers with interactive user interfaces

#22

An, the dream, a cross platform App Store you can install apps into any client application that supports MCP, but is open, free and agentic. It’s basically a “web App Store” and we side step the existing app stores (and their content guidelines, security restrictions and billing requirements) because it’s all done via a mega app (the MCP client). How could it go wrong? If only someone had done this before, we wouldnt…

The apps are LLM agnostic, so all MCP apps will be portable. Economically, this means developers don’t have convince users to pay $20 a month, these users are already paying that. Devs just have to convince users to buy the app on the platform.

I don’t see this being the future state. We’d be talking about a world where any and all apps exist inside of fucking ChatGPT and that just sounds ridiculous.

Re: MCP Apps: Extending servers with interactive user interfaces

#23
I skimmed over this, but did I see a reference sandbox implementation? And then basically the chat UI interacts with that with postMessage (and receiving) and forwards tool calls to the MCP server. Does it also forward tool calls the MCP server doesn't handle to the host backend?

What I am imagining is something like a meta UI tool call that just creates a menu. The whole MCP server's purpose might be to add this menu creation capability to the chat user interface. But what you are selecting from isn't known ahead of time, it's the input to the UI.

When they select something I assume it would output a tool call like menuItemSelected('option B'). I suppose if you want your server to do anything specific with this then you would have to handle that in the particular server. But I guess you could also just have a tool call that just sends the inputs to the agent. This could make for what is a very slow to respond but extremely flexible overall UX.

I guess this is not the intended use, but suppose you give your agent generic MCP UI tools for showing any menu, showing any data table, showing a form, etc. So the inputSchemas would be somehow (if this is possible) quite loosely defined.

I guess the purpose is probably more about not having to go through the LLM rather than giving it the ability to dynamically put up UI elements that it has to react to individual interactions with.

But maybe one of the inputs to the dataTable are the query parameters for its data, and the table has a refresh button. Maybe another input is the URI for the details form MCP UI that slides over when you click a row.

Maybe there is an MCP UI for Layout what allows you to embed other MCP UIs in a specific structure.

This might not make sense, but I am wondering if I can use MCP Apps as an alternative to always building custom MindRoot plugins (my Python/web components agentic app framework) to provide unique web pages and UI for each client's agentic application.

I think I may have gotten the MCP Apps and MCP UI a bit conflated here so I probably need to read it again.

Re: MCP Apps: Extending servers with interactive user interfaces

#24
How reliable are the processes which these things run?

I'm processing thousands of files using Copilot, and even 20 at a time, it usually skips a couple, and sometimes, when skipping, it merges the data from one file to the next, not applying anything to the second file, other times it completely applies the data parsed from one file to the second --- not a big deal since I'm reviewing each operation manually, but the only reason the error rate is acceptable is the files are so inconsistent that normal techniques weren't working.

Is there an equivalent to "double-keying" where two different LLMs process the same input and it only moves forward if both match perfectly?

Re: MCP Apps: Extending servers with interactive user interfaces

#26

Disclosure: I work at Anthropic, have worked on MCP I also think this is pretty big. I think a problem we collectively have right now is that getting MCP closer to real user flows is pretty hard and requires a lot of handholding. Ideally, most users of MCP wouldn't even know that MCP is a thing - the same way your average user of the web has no idea about DNS/HTTP/WebSockets. They just know that the browser helps the…

I wonder how long it'll take you to figure out that you're trying to reinvent deterministic APIs.

Or just APIs in general.

MCP is incredibly vibe-coded. We know how to make APIs. We know how to make two-way communications. And yet "let's invent new terminology that makes little sense and awkward workarounds on top of unidirectional protocols and call it the best thing since sliced cheese".

Re: MCP Apps: Extending servers with interactive user interfaces

#27

Earlier quoted context omitted.

Who wants a button that has indeterministic actions?

Unless the MCP server itself has an LLM call inside of it (rare), the MCP server is pretty deterministic. It’s the AI that invokes it that’s actually indeterministic, but the user is already using that.

> pretty deterministic

This is an oxymoron.

Re: MCP Apps: Extending servers with interactive user interfaces

#28

Looks like OpenAI, Anthropic, and the MCP-UI team actually worked together on a common standard for MCP Apps: https://blog.modelcontextprotocol.io/posts/2025-11-21-mcp-ap... Honestly, I think the biggest friction for MCP adoption has been how un-userfriendly it is. It’s great for devs, but not the average users. Users don't always want to chat, sometimes they just want to click a button or adjust a slider. This feels…

You are touching on an important point. Basically OpenAI and others provide a lot of poorly integrated tools and components. You can build nice things with those but you have to deal with a lot of issues and it's a non trivial amount of work that even they aren't doing apparently. Even such a simple thing as triggering an OAuth signin to get access to models is not part of SDKs. Most developer tools require configuring API keys in some file instead. No normal user is ever going to do that,.

Things like ChatGPT are remarkably limited from a UX/UI point of view. The product can do amazing things but the UI is nothing special. The mac version currently has a bug where option+shift+1 opens a chat window but doesn't give it focus. When I do that from vs code it adds the editor window. But it's completely blind to any browser tab on which I do that. I'm sure there are good reasons for all that. But it strikes me a bit as a work in progress that a good product owner would spot.

With apps some of the more powerful capabilities (llms driving UIs directly, doing things in agentic loops, tool and API usage) are going to require much deeper integrations than are currently there. We get hints of what is possible and nice technology demos. But it's still hard to build more complicated workflows around this. Unless you build your own applications.

We've been staring at this from the point of view of automating some highly tedious stuff that we currently do in our company manually. For example, working with chat GPT seems to involve a lot of copy paste and manually doing things that it can't really do by itself. Even something as simple as working on a document it will do alright work on the text but then make a complete mess of the formatting. I spend an hour a few days ago iterating on a document where I was basically just fixing bullets and headers. Most alternatives I've tried aren't any better at this.

None of this seems particularly hard; it's just a lot of integration work that just hasn't happened yet. We have a bunch of lego bricks, not a lot of fully mature solutions. MCP isn't a full solution, it's a pretty lego brick. Mostly even OpenAI and Anthropic aren't getting around to doing much more than simplistic technology demos with their own stuff. IMHO their product teams are a lot less remarkable than their AI teams.

Re: MCP Apps: Extending servers with interactive user interfaces

#29
IMO this is not where we want to go. The future is you have a system agent that interact with all your apps via API, and websites are real-time so you can see what your agent is doing as you do it it. We don't need more rigging for this future, just better API support.

Trying to create custom agent APIs to embed apps in chat is a very "monopolist frontier lab" thing to try and do.

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