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MCP Apps are based on the MCP ext-apps specification—an open standard for rendering interactive UI widgets inside AI conversations. Unlike the proprietary Apps SDK, MCP Apps work across multiple AI clients.

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How MCP Apps Work

MCP Apps use a JSON-RPC postMessage bridge for communication between the widget iframe and the host application: The widget sends requests and receives responses via structured JSON-RPC messages, rather than calling methods on a global object.

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Supported Methods

The MCP ext-apps spec defines these communication methods:

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Guest-to-Host Requests

MethodDescription
ui/initializeHandshake to establish connection
ui/open-linkOpen external URLs
ui/messageSend follow-up messages to the conversation
ui/request-display-modeRequest display mode change
ui/update-model-contextUpdate widget state for the model
tools/callInvoke MCP tools

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Host-to-Guest Notifications

NotificationDescription
notifications/context-changedTheme, locale, or display mode changed
notifications/tool-inputTool arguments provided
notifications/tool-resultTool execution completed

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Client Support

MCP Apps are supported by clients implementing the ext-apps specification, these include Goose, VSCode, Postman, …
MCP Apps is an emerging specification. Client support is expanding as more AI applications adopt the standard.

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Skybridge Implementation

Skybridge implements the MCP ext-apps specification via the McpAppAdaptor. This happens automatically: you write the same hooks regardless of runtime.

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Differences from Apps SDK

FeatureApps SDKMCP Apps
Modal WindowsPortaled to hostComing soon (in-iframe)
File Upload/DownloadSupportedNot supported
Display Mode “modal”SupportedNot supported

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Handling Platform Differences

When using Apps SDK features in MCP Apps:
  • useFiles(): Not yet supported
  • useRequestModal(): Coming soon (will render in-iframe)
  • useDisplayMode("modal"): Not yet supported
  • useWidgetState(): Not yet supported (MCP spec doesn’t define widget state)
Graceful degradation planned: We’re working to ensure host-specific features never throw errors. Instead, they’ll degrade gracefully—running in a minimal but functional way so your app works everywhere.
Always check the compatibility matrix before using platform-specific features.

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Protocol Coverage

Skybridge is actively implementing the MCP ext-apps specification. Track our progress and see detailed coverage in GitHub Issue #14.

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Testing MCP Apps

Since the DevTools emulator currently uses the Apps SDK runtime, testing MCP Apps requires:
  1. Running your MCP server locally
  2. Connecting from a client that supports MCP Apps (e.g., Goose)
  3. Invoking tools that render widgets
Use DevTools for rapid development, then validate in MCP Apps clients for final testing.