McpServer: no first-class support for MCP Apps (SEP-1865 / interactive UI resources)

The effect/unstable/ai/McpServer module has no first-class implementation of SEP-1865 (MCP Apps — Interactive UIs for MCP).

What exists today

McpServer.ts pins LATEST_PROTOCOL_VERSION = "2025-06-18" (pre-1865).
McpSchema.ts ships _meta: Record<string, Json> on every relevant schema — spec-mandated passthrough.
Tool.Meta (Tool.ts:1635) is a generic Record<string, unknown> annotation; its docstring example happens to show { ui: { resourceUri: "ui://example/calculator-ui" } }, but Effect doesn't act on it.

What's missing

No handling of the ui:// URI scheme.
No HTML / widget / Skybridge resource-content variant beyond TextResourceContents / BlobResourceContents.
No McpServer.uiResource(...) (or equivalent) helper.
No typed annotations for SEP-1865 / OpenAI Apps SDK keys (openai/outputTemplate, widgetAccessible, widgetDescription, widgetPrefersBorder, etc.).
No content-type negotiation or rendering hooks.

Grep across the repo for sep-1865, mcp-app, mcpui, mcp-ui, skybridge, outputTemplate, widgetAccessible returns 0 hits; ui:// appears once, in the Tool.Meta docstring.

Ask

First-class SEP-1865 support: ui:// resource kind, typed Tool.Meta keys for the well-known fields, and bumping LATEST_PROTOCOL_VERSION once 1865 is part of a dated spec.

References

SEP-1865 spec: https://modelcontextprotocol.io/seps/1865-mcp-apps-interactive-user-interfaces-for-mcp
SEP discussion: SEP-1865: MCP Apps - Interactive User Interfaces for MCP modelcontextprotocol/modelcontextprotocol#1865
mcp-ui (reference SDK + ui:// scheme): https://mcpui.dev — https://github.com/idosal/mcp-ui
OpenAI Apps SDK (overlapping _meta keys: openai/outputTemplate, widgetAccessible, widgetDescription, widgetPrefersBorder): https://developers.openai.com/apps-sdk/build/mcp-server
MCP spec (current 2025-06-18 revision): https://modelcontextprotocol.io/specification/2025-06-18

I've actually built several mcp servers using Effect that can support Mcp App

https://github.com/lloydrichards/edu_mcp-apps

There are some prs from back in Feb/Mar that opened this up Effect-TS/effect-smol#1440 and Effect-TS/effect-smol#1653

Since Mcp App is part of the extension ecosystem for the mcp spec and with effect not have an McpClient implementation I would probably just stick o using the helpers from https://github.com/modelcontextprotocol/ext-apps to build on top of what you can expose from an Effect McpServer

https://www.lloydrichards.dev/labs/059-generative-data-viz <- and some of my thoughts and process from when I was working on this

@lloydrichards

@lloydrichards great work in Effect-TS/effect-smol#1440 and then your guide, wonderful!

I'm curious about your comment at the end of the guide, "Bundle size: the widgets are around ~1MB each, mostly from React. While this is fine for a demo, I'll explore Web Components to reduce this in the future"

In the repo I was building for the article, I used vite + react instead of lit (web components) for building up the ui:/// resource which meant that each widget came bundled with react in a sort of mini SPA. There might have been ways to cut back the bundle size a bit, but generally its a lot of minified JS thats being stringified and send over http.

It would depend a bit on what you're building but for short-lived widgets I thinks its better to use something like web components that don't rely no too much JS and can render quickly without too much code. In the edu_mcp-apps repo I was starting to explore this in the lit-lab package but hadn't figured out a good dx yet for building the components seperate from the mcp-server

This might help, but I was also using these helpers:

UiResource.ts

Which would be reused across widgets:

import { LineChartWidgetPayload } from "@repo/domain/Chart";
import { Effect } from "effect";
import {
  makeUiRenderTool,
  makeUiResource,
  readPublicAsset,
} from "../../UiResource";

const LineChartWidgetResourceUri = "ui://line-chart";

const LineChartWidgetHtml = readPublicAsset({
  importMetaDir: import.meta.dir,
  sourceSuffix: "src/widget/line-chart",
  sourcePath: "../../../public/widgets/widget-line-chart.html",
  distPath: "public/widgets/widget-line-chart.html",
});

export const LineChartWidgetResourceLayer = makeUiResource(
  LineChartWidgetResourceUri,
  {
    name: "Line Chart",
    description: "Line chart widget UI",
    html: LineChartWidgetHtml,
    meta: {
      prefersBorder: false,
    },
  },
);

export const RenderLineChartWidgetTool = makeUiRenderTool(
  LineChartWidgetResourceUri,
  {
    name: "render_line_chart_widget",
    title: "Line Chart",
    description: "Render the line chart widget UI",
    parameters: LineChartWidgetPayload,
    success: LineChartWidgetPayload,
  },
);

export const renderLineChartWidgetHandler = (
  payload: typeof LineChartWidgetPayload.Type,
) => Effect.succeed(payload);

Uaauuu Lloyd, thanks for the help!

@guillempuche

@guillempuche i finally convinced my work to open up my lab project repo: https://github.com/interactivethings/interactivethings-data-viz-mcp-lab

feel free to explore/fork the code, though its a couple months old now

uaauuu! Cool jobs Lloyd!

lloydrichards/edu_mcp-apps@5ff417d

Just did some upgrading to get this working on v4 and the new McpServer so far everything looks to be working the same as before. McpJam has greatly improved their diagnostics so its easier to prototype the components now and check compatibility with other harnesses.

I want to first get the Task expension in (#8631) to figure out some erganomics around what first-class support means, but I think if there isn't too much pushback about adding these sorts of modules then I can look into an McpApp version of it too

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McpServer: no first-class support for MCP Apps (SEP-1865 / interactive UI resources) #6353

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McpServer: no first-class support for MCP Apps (SEP-1865 / interactive UI resources) #6353

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lloydrichards commented on May 18, 2026

lloydrichards commented on May 18, 2026

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