Feature Request: Support for MCP Apps Extension (UI capabilities)

[jaguarliuu]

Background

The MCP Apps extension was announced on January 26, 2026, bringing
interactive UI capabilities to the Model Context Protocol. This extension allows MCP servers to return rich, interactive user
interfaces (dashboards, forms, visualizations, etc.) that render directly in client applications.

According to the official blog post, MCP Apps enable
servers to deliver UIs through sandboxed iframes with bidirectional communication via JSON-RPC over postMessage.

Current Status

Currently, MCP Apps support appears to be available only in the TypeScript/JavaScript SDK
(@modelcontextprotocol/ext-apps). The Java SDK (v0.17.2 and earlier releases) does not appear to include support for the
Apps extension.

Use Case

We are building an MCP client application (MiniClaw) in Java using Spring Boot and the official Java MCP SDK (v0.17.2). We
would like to support MCP Apps to enable our users to interact with rich UIs provided by MCP servers, such as:

• Interactive data visualizations (charts, maps, 3D models)
• Multi-step forms and workflows
• Real-time dashboards and monitoring interfaces
• Document viewers and editors

Our frontend is built with Vue.js, and we're prepared to implement the client-side AppBridge integration. However, we need
the Java SDK to:

1. Detect App capabilities from server responses (when a tool returns UI metadata)
2. Parse and expose App-related data structures (UI resource declarations, iframe URLs, etc.)
3. Support the extended protocol for Apps-related messages if any backend changes are required

Questions

1. Is MCP Apps support planned for the Java SDK? If so, is there an estimated timeline or roadmap?
2. What would be required to implement this? Are there specific protocol changes, new data structures, or APIs that need
   to be added?
3. Can we contribute? If this is not currently prioritized, would the maintainers be open to community contributions?
   We'd be happy to help implement this feature.
4. Workarounds? In the meantime, are there any recommended approaches for Java-based MCP clients to handle Apps responses
   from servers?

Additional Context

• Java SDK version in use: v0.17.2
• Client architecture: Spring Boot 3.4.5 + Vue.js 3 frontend
• We've reviewed the TypeScript implementation and AppBridge documentation
• We understand the client-side UI rendering will be handled in our frontend, but need backend SDK support for protocol
  handling

References

• MCP Apps Announcement
• ext-apps Repository
• MCP Apps Specification

Thank you for maintaining this excellent SDK! We appreciate any guidance or updates on this feature request.

[jamesward]

It looks like this is currently possible:
modelcontextprotocol/ext-apps#439

But there are likely some things that could be done to make the experience nicer.