MCP Apps: Support appCapabilities.tools (View-exposed tools)

[aharvard]

Description

The MCP Apps specification (2026-01-26) allows Views to expose their own tools via appCapabilities.tools in the ui/initialize request. This enables the host or model to call tools provided by the app — for example, an Excalidraw app could expose a getCanvasState tool that the model invokes to understand what the user has drawn.

Goose currently parses appCapabilities.availableDisplayModes but ignores appCapabilities.tools.

Scope

• Parse appCapabilities.tools from the View's ui/initialize message
• Register View-exposed tools so the host/model can discover and call them
• Route tool calls back to the View via the sandbox bridge

Files

• ui/desktop/src/components/McpApps/useDisplayMode.ts (parsing)
• ui/desktop/src/components/McpApps/McpAppRenderer.tsx (tool routing)
• Potentially crates/goose/src/agents/extension_manager.rs (registration with agent)