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MCP Apps ​

MCP Apps let an MCP tool attach an interactive result view. The MCP server declares a ui:// resource, and DotCraft Desktop renders it in a sandbox through the standard AppBridge contract. Other clients keep using the tool's text fallback.

App Binding does not define a separate UI protocol. An App Binding app exposes its tools and views from its own binding-scoped Streamable HTTP MCP server.

An MCP App rendering an interactive GitHub issue form inside a DotCraft conversation

Declare a view ​

Add stable UI metadata to the MCP tool:

json
{
  "name": "ListBoardItems",
  "inputSchema": { "type": "object" },
  "_meta": {
    "ui": {
      "resourceUri": "ui://example/board.html",
      "visibility": ["model", "app"]
    }
  }
}

Use visibility to publish the tool to the model, the app, or both; omitting it publishes to both. DotCraft treats malformed metadata as unavailable rather than widening access.

Serve the resource ​

Return the resource from standard MCP resources/list and resources/read. MCP App HTML uses this MIME type:

text
text/html;profile=mcp-app

Bundle scripts and styles with the server. Desktop strips any Content-Security-Policy meta tag from the HTML and applies its own: default-src 'none', widened only to the HTTPS origins the resource declares in _meta.ui.csp. Browser permissions and a domain declared in _meta.ui are recorded as capability metadata and never granted — the view gets no camera, microphone, geolocation, or clipboard-write access.

The resource body and each result delivered to the view are both capped at 2 MB.

Return a useful fallback ​

Every call should return concise content for the model and non-visual clients. Put machine-readable result data in structuredContent; reserve result _meta for the MCP App.

json
{
  "content": [{ "type": "text", "text": "Found 4 board items." }],
  "structuredContent": { "itemIds": ["OR-12", "OR-15"] },
  "_meta": { "selectedItemId": "OR-12" }
}

The UI communicates through the official @modelcontextprotocol/ext-apps client. Use AppBridge operations such as tools/call, link opening, display-mode requests, and host-context notifications. Do not create a private postMessage protocol.

Security boundaries ​

An MCP server declares a view resource with its tool, DotCraft Desktop serves it into a sandbox with an opaque origin and only the declared CSP origins, a call the view starts goes back to the same server, and non-visual clients keep the text result

  • The iframe has an opaque origin and no host DOM or Node access.
  • The host CSP starts at default-src 'none' and opens only to the HTTPS origins the resource declares.
  • Browser permissions are never granted, whatever the resource declares.
  • App-initiated tool calls are re-checked against the live tool snapshot: same MCP server, app visibility still declared.
  • Revoking an App Binding closes its MCP session and views immediately.

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