Add app-rendered elicitations to the MCP Apps draft

src/app-bridge.test.ts

@@ -151,6 +151,61 @@ describe("App <-> AppBridge integration", () => {
     });
   });
 
+  describe("app-rendered elicitation", () => {
+    it("forwards the standard request to the bound app and returns its standard result", async () => {
+      const elicitationHostCapabilities: McpUiHostCapabilities = {
+        ...testHostCapabilities,
+        elicitation: {},
+      };
+      bridge = new AppBridge(
+        createMockClient() as Client,
+        testHostInfo,
+        elicitationHostCapabilities,
+      );
+      app.onelicitation = async (params) => ({
+        action: "accept",
+        content: { choice: params.message },
+      });
+
+      await bridge.connect(bridgeTransport);
+      await app.connect(appTransport);
+
+      expect(bridge.getAppCapabilities()?.elicitation).toEqual({});
+      await expect(
+        bridge.requestElicitation({
+          message: "Choose an option",
+          requestedSchema: {
+            type: "object",
+            properties: {
+              choice: { type: "string" },
+            },
+            required: ["choice"],
+          },
+        }),
+      ).resolves.toEqual({
+        action: "accept",
+        content: { choice: "Choose an option" },
+      });
+    });
+
+    it("fails closed when the app did not advertise elicitation", async () => {
+      bridge = new AppBridge(createMockClient() as Client, testHostInfo, {
+        ...testHostCapabilities,
+        elicitation: {},
+      });
+
+      await bridge.connect(bridgeTransport);
+      await app.connect(appTransport);
+
+      expect(() =>
+        bridge.requestElicitation({
+          message: "Choose an option",
+          requestedSchema: { type: "object", properties: {} },
+        }),
+      ).toThrow("App does not support elicitation");
+    });
+  });
+
   describe("Host -> App notifications", () => {
     beforeEach(async () => {
       await bridge.connect(bridgeTransport);

src/app-bridge.ts

@@ -10,6 +10,9 @@ import {
   CreateMessageResult,
   CreateMessageResultWithTools,
   EmptyResult,
+  ElicitRequest,
+  ElicitResult,
+  ElicitResultSchema,
   Implementation,
   ListPromptsRequest,
   ListPromptsRequestSchema,
@@ -443,6 +446,36 @@ export class AppBridge extends ProtocolWithEvents<
     return this._appCapabilities;
   }
 
+  /**
+   * Forward a form-mode MCP elicitation to this exact app instance.
+   *
+   * Hosts should call this only from the core MCP client's handler for the
+   * originating `elicitation/create` request. The returned value is the
+   * standard MCP `ElicitResult` and can be returned directly to the server.
+   */
+  requestElicitation(
+    params: ElicitRequest["params"],
+    options?: RequestOptions,
+  ): Promise<ElicitResult> {
+    if (!this._initializedReceived) {
+      throw new Error("App has not completed ui/initialize");
+    }
+    if (!this._capabilities.elicitation) {
+      throw new Error("Host does not support app-rendered elicitation");
+    }
+    if (!this._appCapabilities?.elicitation) {
+      throw new Error("App does not support elicitation");
+    }
+    if (params.mode !== undefined && params.mode !== "form") {
+      throw new Error("MCP Apps only support form-mode elicitations");
+    }
+    return this.request(
+      { method: "elicitation/create", params },
+      ElicitResultSchema,
+      options,
+    );
+  }
+
   /**
    * Get the view's implementation info discovered during initialization.
    *
@@ -1408,7 +1441,14 @@ export class AppBridge extends ProtocolWithEvents<
    * @internal
    */
   assertCapabilityForMethod(method: AppRequest["method"]): void {
-    // TODO
+    if (
+      method === "elicitation/create" &&
+      !this._appCapabilities?.elicitation
+    ) {
+      throw new Error(
+        `App does not support elicitation capability (required for ${method})`,
+      );
+    }
   }
 
   /**

src/app.ts

@@ -15,6 +15,9 @@ import {
   CreateMessageResultWithTools,
   CreateMessageResultWithToolsSchema,
   EmptyResultSchema,
+  ElicitRequest,
+  ElicitRequestSchema,
+  ElicitResult,
   Implementation,
   ListResourcesRequest,
   ListResourcesResult,
@@ -1013,6 +1016,49 @@ export class App extends ProtocolWithEvents<
     );
   }
 
+  /**
+   * Handle a form-mode elicitation that the host has bound to this app.
+   *
+   * The callback receives the unchanged standard MCP `elicitation/create`
+   * parameters and returns the unchanged standard MCP `ElicitResult`.
+   * Register this handler before {@link connect `connect`}; doing so advertises
+   * the app's `elicitation` capability during `ui/initialize`.
+   */
+  private _onelicitation?: (
+    params: ElicitRequest["params"],
+    extra: RequestHandlerExtra,
+  ) => ElicitResult | Promise<ElicitResult>;
+  get onelicitation() {
+    return this._onelicitation;
+  }
+  set onelicitation(
+    callback:
+      | ((
+          params: ElicitRequest["params"],
+          extra: RequestHandlerExtra,
+        ) => ElicitResult | Promise<ElicitResult>)
+      | undefined,
+  ) {
+    this.warnIfRequestHandlerReplaced(
+      "onelicitation",
+      this._onelicitation,
+      callback,
+    );
+    this._onelicitation = callback;
+    if (callback && !this._capabilities.elicitation && !this.transport) {
+      this.registerCapabilities({ elicitation: {} });
+    }
+    this.replaceRequestHandler(ElicitRequestSchema, (request, extra) => {
+      if (!this._onelicitation) {
+        throw new Error("No onelicitation handler set");
+      }
+      if (request.params.mode !== undefined && request.params.mode !== "form") {
+        throw new Error("MCP Apps only support form-mode elicitations");
+      }
+      return this._onelicitation(request.params, extra);
+    });
+  }
+
   /**
    * Convenience handler for tool call requests from the host.
    *
@@ -1165,6 +1211,13 @@ export class App extends ProtocolWithEvents<
           );
         }
         return;
+      case "elicitation/create":
+        if (!this._capabilities.elicitation) {
+          throw new Error(
+            `App does not support elicitation capability (required for ${method})`,
+          );
+        }
+        return;
       case "ping":
       case "ui/resource-teardown":
         return;