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MCP Apps

open source / Overall score: 5.0(leading)

An extension to the Model Context Protocol, with its own dated specification and SDK, letting a tool declare a `ui://` resource carrying an HTML interface that the host renders in a sandboxed iframe and communicates with bidirectionally.

Openness

5 high confidence

5.0

license: Apache-2.0(OSI
source: public(the specification, the SDK packages and the examples)
self-host: yes(the specification and its SDK are the product)
core features withheld: no — nothing withheld

Open source under the MCP project's dual transition licence, with the body cited because the classifier reports NOASSERTION over it.

https://api.github.com/repos/modelcontextprotocol/ext-apps recorded 2026-09-17
Repository metadata: 2,848 stars, not archived, not a fork, last pushed 2026-09-16, description "Official repo for spec & SDK of MCP Apps protocol - standard for UIs embedded AI chatbots, served by MCP servers".
https://raw.githubusercontent.com/modelcontextprotocol/ext-apps/main/LICENSE recorded 2026-09-17
The LICENSE body, read rather than the classifier's label: the MCP project's licensing transition file, Apache-2.0 for new contributions with MIT retained where consent was not given. That shape is why the API reports NOASSERTION.

Adoption

5 high confidence

5.0

Adoption is measured by npm downloads of `@modelcontextprotocol/ext-apps`, the SDK the specification ships with and the extension's own distribution channel.

https://api.npmjs.org/downloads/point/last-month/@modelcontextprotocol/ext-apps recorded 2026-09-17
npm downloads point API: 13,385,596 downloads for `@modelcontextprotocol/ext-apps` over 2026-08-13 to 2026-09-11.

Capability

not assessed

MCP Apps implements none of the MCP client-server contract - no role, primitive or transport - because it extends the protocol instead with a UI-resource convention: a tool ships an interface that the host renders, not a client-server exchange. It publishes its own specification, stable as of 2026-01-26, and an SDK for app, host and server developers, none of which are MCP roles.

https://raw.githubusercontent.com/modelcontextprotocol/ext-apps/main/README.md recorded 2026-09-17
The README: "MCP Apps is an extension to the core MCP specification", working by "Tool definition - Your tool declares a ui:// resource containing its HTML interface" which "The host fetches ... and displays in a sandboxed iframe". Its own version table lists "2026-01-26 | Stable" and "draft | Development", and "The base MCP SDK packages are ^2.0.0 peers".

Verified 2026-09-17