[goose2] MCP Apps: translate ACP host capabilities into MCP initialization (#8623)

Signed-off-by: Andrew Harvard <aharvard@squareup.com>

ui/sdk/package.json

@@ -40,6 +40,7 @@
     "format": "prettier --write src/"
   },
   "dependencies": {
+    "@modelcontextprotocol/ext-apps": "^0.3.1",
     "zod": "^3.25.76"
   },
   "peerDependencies": {

ui/sdk/src/index.ts

@@ -2,6 +2,7 @@ export * from "./generated/types.gen.js";
 export * from "./generated/zod.gen.js";
 export { GooseClient } from "./goose-client.js";
 export { createHttpStream } from "./http-stream.js";
+export * from "./mcp-apps.js";
 
 export {
   ClientSideConnection,

ui/sdk/src/mcp-apps.ts

@@ -0,0 +1,36 @@
+import type {
+  Implementation,
+  InitializeRequest,
+} from "@agentclientprotocol/sdk";
+import { RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps";
+
+export const GOOSE_MCP_UI_EXTENSION_ID = "io.modelcontextprotocol/ui" as const;
+
+export interface GooseMcpUiExtensionSettings {
+  mimeTypes: string[];
+}
+
+export interface GooseMcpHostCapabilities {
+  extensions: Record<string, GooseMcpUiExtensionSettings>;
+}
+
+export interface GooseClientMeta {
+  goose: {
+    mcpHostCapabilities: GooseMcpHostCapabilities;
+  };
+}
+
+export type GooseInitializeRequest = InitializeRequest & {
+  clientCapabilities: NonNullable<InitializeRequest["clientCapabilities"]> & {
+    _meta: GooseClientMeta;
+  };
+  clientInfo: Implementation;
+};
+
+export const DEFAULT_GOOSE_MCP_HOST_CAPABILITIES: GooseMcpHostCapabilities = {
+  extensions: {
+    [GOOSE_MCP_UI_EXTENSION_ID]: {
+      mimeTypes: [RESOURCE_MIME_TYPE],
+    },
+  },
+};