From 1398aaf9df8d7090070d9e4b1bca20c768b970fe Mon Sep 17 00:00:00 2001
From: Olivier Chafik <ochafik@anthropic.com>
Date: Fri, 21 Nov 2025 12:05:44 +0000
Subject: [PATCH] add AppHost

---
 src/app-host.ts | 219 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/types.ts    |   2 -
 2 files changed, 219 insertions(+), 2 deletions(-)
 create mode 100644 src/app-host.ts

diff --git a/src/app-host.ts b/src/app-host.ts
new file mode 100644
index 0000000..581d36c
--- /dev/null
+++ b/src/app-host.ts
@@ -0,0 +1,219 @@
+import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { ZodLiteral, ZodObject } from "zod";
+
+import { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
+import {
+  CallToolRequestSchema,
+  CallToolResultSchema,
+  Implementation,
+  ListPromptsRequestSchema,
+  ListPromptsResultSchema,
+  ListResourcesRequestSchema,
+  ListResourcesResultSchema,
+  ListResourceTemplatesRequestSchema,
+  ListResourceTemplatesResultSchema,
+  Notification,
+  PingRequestSchema,
+  PromptListChangedNotificationSchema,
+  ResourceListChangedNotificationSchema,
+  Result,
+  ToolListChangedNotificationSchema,
+  Request,
+} from "@modelcontextprotocol/sdk/types.js";
+import {
+  Protocol,
+  ProtocolOptions,
+  RequestOptions,
+} from "@modelcontextprotocol/sdk/shared/protocol.js";
+
+import {
+  type McpUiToolInputNotification,
+  type McpUiToolResultNotification,
+  type McpUiSandboxResourceReadyNotification,
+  type McpUiSizeChangeNotification,
+  LATEST_PROTOCOL_VERSION,
+  McpUiAppCapabilities,
+  McpUiHostCapabilities,
+  McpUiInitializedNotificationSchema,
+  McpUiInitializeRequest,
+  McpUiInitializeRequestSchema,
+  McpUiInitializeResult,
+  McpUiResourceTeardownRequest,
+  McpUiResourceTeardownResultSchema,
+} from "./types";
+export { LATEST_PROTOCOL_VERSION } from "./types";
+export { PostMessageTransport } from "./message-transport";
+
+type HostOptions = ProtocolOptions;
+
+export const SUPPORTED_PROTOCOL_VERSIONS = [LATEST_PROTOCOL_VERSION];
+
+export class AppHost extends Protocol<Request, Notification, Result> {
+  oninitialized?: () => void;
+
+  private _appCapabilities?: McpUiAppCapabilities;
+
+  constructor(
+    private _client: Client,
+    private _hostInfo: Implementation,
+    private _capabilities: McpUiHostCapabilities,
+    options?: HostOptions,
+  ) {
+    super(options);
+
+    this.setRequestHandler(McpUiInitializeRequestSchema, (request) =>
+      this._oninitialize(request),
+    );
+    this.setNotificationHandler(McpUiInitializedNotificationSchema, () =>
+      this.oninitialized?.(),
+    );
+
+    this.setRequestHandler(PingRequestSchema, (request) => {
+      console.log("Received ping:", request.params);
+      return {};
+    });
+  }
+
+  assertCapabilityForMethod(method: Request["method"]): void {
+    // TODO
+  }
+  assertRequestHandlerCapability(method: Request["method"]): void {
+    // TODO
+  }
+  assertNotificationCapability(method: Notification["method"]): void {
+    // TODO
+  }
+
+  getCapabilities(): McpUiHostCapabilities {
+    return this._capabilities;
+  }
+
+  private async _oninitialize(
+    request: McpUiInitializeRequest,
+  ): Promise<McpUiInitializeResult> {
+    const requestedVersion = request.params.protocolVersion;
+
+    this._appCapabilities = request.params.appCapabilities;
+
+    const protocolVersion = SUPPORTED_PROTOCOL_VERSIONS.includes(
+      requestedVersion,
+    )
+      ? requestedVersion
+      : LATEST_PROTOCOL_VERSION;
+
+    return {
+      protocolVersion,
+      hostCapabilities: this.getCapabilities(),
+      hostInfo: this._hostInfo,
+      hostContext: {
+        // TODO
+      },
+    };
+  }
+
+  sendSizeChange(params: McpUiSizeChangeNotification["params"]) {
+    return this.notification(<McpUiSizeChangeNotification>{
+      method: "ui/notifications/size-change",
+      params,
+    });
+  }
+
+  sendToolInput(params: McpUiToolInputNotification["params"]) {
+    return this.notification(<McpUiToolInputNotification>{
+      method: "ui/notifications/tool-inputs",
+      params,
+    });
+  }
+  sendToolResult(params: McpUiToolResultNotification["params"]) {
+    return this.notification(<McpUiToolResultNotification>{
+      method: "ui/notifications/tool-result",
+      params,
+    });
+  }
+
+  sendSandboxResourceReady(
+    params: McpUiSandboxResourceReadyNotification["params"],
+  ) {
+    return this.notification(<McpUiSandboxResourceReadyNotification>{
+      method: "ui/notifications/sandbox-resource-ready",
+      params,
+    });
+  }
+
+  sendResourceTeardown(
+    params: McpUiResourceTeardownRequest["params"],
+    options?: RequestOptions,
+  ) {
+    return this.request(
+      <McpUiResourceTeardownRequest>{
+        method: "ui/resource-teardown",
+        params,
+      },
+      McpUiResourceTeardownResultSchema,
+      options,
+    );
+  }
+
+  private forwardRequest<
+    Req extends ZodObject<{
+      method: ZodLiteral<string>;
+    }>,
+    Res extends ZodObject<{}>,
+  >(requestSchema: Req, resultSchema: Res) {
+    this.setRequestHandler(requestSchema, async (request, extra) => {
+      console.log(`Forwarding request ${request.method} from MCP UI client`);
+      return this._client.request(request, resultSchema, {
+        signal: extra.signal,
+      });
+    });
+  }
+  private forwardNotification<
+    N extends ZodObject<{ method: ZodLiteral<string> }>,
+  >(notificationSchema: N) {
+    this.setNotificationHandler(notificationSchema, async (notification) => {
+      console.log(
+        `Forwarding notification ${notification.method} from MCP UI client`,
+      );
+      await this._client.notification(notification);
+    });
+  }
+  async connect(transport: Transport) {
+    // Forward core available MCP features
+    const serverCapabilities = this._client.getServerCapabilities();
+    if (!serverCapabilities) {
+      throw new Error("Client server capabilities not available");
+    }
+
+    if (serverCapabilities.tools) {
+      this.forwardRequest(CallToolRequestSchema, CallToolResultSchema);
+      if (serverCapabilities.tools.listChanged) {
+        this.forwardNotification(ToolListChangedNotificationSchema);
+      }
+    }
+    if (serverCapabilities.resources) {
+      this.forwardRequest(
+        ListResourcesRequestSchema,
+        ListResourcesResultSchema,
+      );
+      this.forwardRequest(
+        ListResourceTemplatesRequestSchema,
+        ListResourceTemplatesResultSchema,
+      );
+      if (serverCapabilities.resources.listChanged) {
+        this.forwardNotification(ResourceListChangedNotificationSchema);
+      }
+    }
+    if (serverCapabilities.prompts) {
+      this.forwardRequest(ListPromptsRequestSchema, ListPromptsResultSchema);
+      if (serverCapabilities.prompts.listChanged) {
+        this.forwardNotification(PromptListChangedNotificationSchema);
+      }
+    }
+
+    // MCP-UI specific handlers are registered by the host component
+    // after the proxy is created. The standard MCP initialization
+    // (via oninitialized callback set in constructor) handles the ready signal.
+
+    return super.connect(transport);
+  }
+}
diff --git a/src/types.ts b/src/types.ts
index d216b31..a644113 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -3,10 +3,8 @@ import {
   ContentBlockSchema,
   EmptyResultSchema,
   ImplementationSchema,
-  RequestId,
   RequestIdSchema,
   RequestSchema,
-  Tool,
   ToolSchema,
 } from "@modelcontextprotocol/sdk/types.js";
 import { z } from "zod";