Apps SDK overview

Preview. The Allternit Apps SDK packages and the allternit CLI are not published to npm yet, so the install commands on this page do not work outside the Allternit repository today. To build an MCP App now, follow the Quickstart, which uses the open MCP SDK directly. Your app will not need changes to adopt the SDK later.

The Allternit Apps SDK is a set of small packages for building MCP Apps. A MCP App has three layers:

Layer	What it is	Package
Server	MCP tools the model calls	`@allternit/apps-server`
Views	Screens shown in chat (`ui://` resources)	`@allternit/apps-ui`, `@allternit/apps-bridge`
Look pack	Design tokens (DTCG) the Views read	`@allternit/look-packs`

The allternit CLI scaffolds, runs, tests, packages and submits an app.

Built on the open standard

Everything the SDK produces is plain MCP Apps (SEP-1865): a streamable-HTTP MCP server whose tools point at ui:// resources. The SDK adds no private protocol. An app built with it runs in any host that supports MCP Apps; Allternit-only features (window.allternit) are optional and feature-detected.

The packages

apps-server

defineApp, appTool, view, a streamable-HTTP starter and the allternit.app.json manifest.

apps-bridge

A typed, dependency-free client for the standard ui/* calls, plus window.allternit and a compatibility shim.

apps-ui

Web components for Views: Card, Table, List, Form, Detail.

look-packs

DTCG token types, a validator, CSS variable output and a contrast check.

The manifest

defineApp emits one manifest, allternit.app.json, listing the tools, Views, look pack and permissions. It carries the same plugin.json and mcp.json fields the plugin package uses, so the package is built from it with no second source of truth.

allternit.app.json

{
  "schema": "allternit.app/1",
  "name": "Weather",
  "version": "0.1.0",
  "server": { "name": "weather", "transport": "streamable-http", "path": "/mcp" },
  "tools": [
    {
      "name": "show_weather",
      "title": "Show weather card",
      "description": "Display a weather card for a city. Call only when the user wants to see it.",
      "inputs": ["city"],
      "annotations": { "readOnlyHint": true, "destructiveHint": false },
      "view": "ui://weather/card.html"
    }
  ],
  "views": [
    {
      "uri": "ui://weather/card.html",
      "name": "Weather card",
      "csp": { "connectDomains": [], "resourceDomains": [] },
      "prefersBorder": true,
      "source": "kit",
      "kit": { "tag": "allternit-detail" }
    }
  ],
  "lookPack": { "id": "weather", "name": "Weather", "tokens": { "color": { "brand": { "$value": "#1f1e1d" } } } },
  "permissions": { "auth": "none", "network": [], "writes": false, "destructive": false }
}

permissions is derived, not typed by hand: writes is true when any tool is not read-only, destructive when any tool is destructive, and network is every host the Views declare in their CSP. The install prompt and directory review read it.

Get started

Follow the Quickstart.

Concepts

Get Started

Apps SDK

Build

Test & Publish

Guides

Reference

Built on the open standard

The packages

apps-server

apps-bridge

apps-ui

look-packs

The manifest

Get started

​Built on the open standard

​The packages

apps-server

apps-bridge

apps-ui

look-packs

​The manifest

​Get started

Built on the open standard

The packages

The manifest

Get started