Expose negotiated protocol version in v2

clients/web/src/App.tsx

@@ -220,6 +220,7 @@ function App() {
     capabilities,
     serverInfo,
     instructions,
+    protocolVersion,
   } = useInspectorClient(inspectorClient);
   const { tools, refresh: refreshTools } = useManagedTools(
     inspectorClient,
@@ -278,18 +279,24 @@ function App() {
   }, [inspectorClient]);
 
   // Build the InitializeResult the connected ViewHeader expects from the
-  // hook's split fields. `protocolVersion` is hard-coded for now — the
-  // useInspectorClient hook doesn't expose it. TODO(#1324): consume the
-  // negotiated value once the hook surfaces it.
+  // hook's split fields.
   const initializeResult = useMemo<InitializeResult | undefined>(() => {
-    if (connectionStatus !== "connected" || !serverInfo) return undefined;
+    if (connectionStatus !== "connected" || !serverInfo || !protocolVersion) {
+      return undefined;
+    }
     return {
-      protocolVersion: "2025-06-18",
+      protocolVersion,
       capabilities: capabilities ?? {},
       serverInfo,
       ...(instructions ? { instructions } : {}),
     };
-  }, [connectionStatus, capabilities, serverInfo, instructions]);
+  }, [
+    connectionStatus,
+    capabilities,
+    serverInfo,
+    instructions,
+    protocolVersion,
+  ]);
 
   // Derive log entries from the message log. Filters for
   // `notifications/message` (the response to `logging/setLevel`).

clients/web/src/test/core/react/useInspectorClient.test.tsx

@@ -18,12 +18,14 @@ describe("useInspectorClient", () => {
       capabilities: CAPABILITIES,
       serverInfo: SERVER_INFO,
       instructions: "hello",
+      protocolVersion: "2025-03-26",
     });
     const { result } = renderHook(() => useInspectorClient(client));
     expect(result.current.status).toBe("connected");
     expect(result.current.capabilities).toEqual(CAPABILITIES);
     expect(result.current.serverInfo).toEqual(SERVER_INFO);
     expect(result.current.instructions).toBe("hello");
+    expect(result.current.protocolVersion).toBe("2025-03-26");
     expect(result.current.appRendererClient).toBeNull();
   });
 
@@ -33,6 +35,7 @@ describe("useInspectorClient", () => {
     expect(result.current.capabilities).toBeUndefined();
     expect(result.current.serverInfo).toBeUndefined();
     expect(result.current.instructions).toBeUndefined();
+    expect(result.current.protocolVersion).toBeUndefined();
     expect(result.current.appRendererClient).toBeNull();
   });
 
@@ -50,17 +53,19 @@ describe("useInspectorClient", () => {
     expect(result.current.status).toBe("connected");
   });
 
-  it("subscribes to capabilities/serverInfo/instructions changes", () => {
+  it("subscribes to capabilities/serverInfo/instructions/protocolVersion changes", () => {
     const client = new FakeInspectorClient();
     const { result } = renderHook(() => useInspectorClient(client));
     act(() => {
       client.setCapabilities(CAPABILITIES);
       client.setServerInfo(SERVER_INFO);
       client.setInstructions("after");
+      client.setProtocolVersion("2025-06-18");
     });
     expect(result.current.capabilities).toEqual(CAPABILITIES);
     expect(result.current.serverInfo).toEqual(SERVER_INFO);
     expect(result.current.instructions).toBe("after");
+    expect(result.current.protocolVersion).toBe("2025-06-18");
   });
 
   it("connect() and disconnect() proxy to the client and update status", async () => {
@@ -97,6 +102,7 @@ describe("useInspectorClient", () => {
     rerender({ c: null });
     expect(result.current.status).toBe("disconnected");
     expect(result.current.capabilities).toBeUndefined();
+    expect(result.current.protocolVersion).toBeUndefined();
   });
 
   it("re-subscribes when the client prop changes", () => {

clients/web/src/test/integration/mcp/inspectorClient.test.ts

@@ -66,6 +66,7 @@ import type {
   ContentBlock,
 } from "@modelcontextprotocol/sdk/types.js";
 import {
+  LATEST_PROTOCOL_VERSION,
   RELATED_TASK_META_KEY,
   McpError,
   ErrorCode,
@@ -4196,6 +4197,20 @@ describe("InspectorClient", () => {
     });
 
     describe("capability detection after connect", () => {
+      it("captures the negotiated protocol version from initialize", async () => {
+        server = createTestServerHttp({
+          serverInfo: createTestServerInfo(),
+        });
+        await server.start();
+        client = new InspectorClient(
+          { type: "streamable-http", url: server.url },
+          { environment: { transport: createTransportNode } },
+        );
+        await client.connect();
+
+        expect(client.getProtocolVersion()).toBe(LATEST_PROTOCOL_VERSION);
+      });
+
       it("round-trips listChanged + subscribe flags via getCapabilities()", async () => {
         // The handler-registration arrows in InspectorClient fire during
         // connect only when the matching server capability is advertised.

core/mcp/__tests__/fakeInspectorClient.ts

@@ -39,6 +39,7 @@ export interface FakeInspectorClientOptions {
   capabilities?: ServerCapabilities;
   serverInfo?: Implementation;
   instructions?: string;
+  protocolVersion?: string;
 }
 
 export class FakeInspectorClient
@@ -49,6 +50,7 @@ export class FakeInspectorClient
   private capabilities: ServerCapabilities | undefined;
   private serverInfo: Implementation | undefined;
   private instructions: string | undefined;
+  private protocolVersion: string | undefined;
   private appRendererClient: AppRendererClient | null = null;
   private sessionId: string | undefined;
 
@@ -69,8 +71,7 @@ export class FakeInspectorClient
     async () => this.resourcePages.shift() ?? { resources: [] },
   );
   listResourceTemplates = vi.fn(
-    async () =>
-      this.resourceTemplatePages.shift() ?? { resourceTemplates: [] },
+    async () => this.resourceTemplatePages.shift() ?? { resourceTemplates: [] },
   );
   listRequestorTasks = vi.fn(
     async () => this.taskPages.shift() ?? { tasks: [] },
@@ -140,6 +141,7 @@ export class FakeInspectorClient
     this.capabilities = options.capabilities;
     this.serverInfo = options.serverInfo;
     this.instructions = options.instructions;
+    this.protocolVersion = options.protocolVersion;
   }
 
   getStatus(): ConnectionStatus {
@@ -158,6 +160,10 @@ export class FakeInspectorClient
     return this.instructions;
   }
 
+  getProtocolVersion(): string | undefined {
+    return this.protocolVersion;
+  }
+
   getAppRendererClient(): AppRendererClient | null {
     return this.appRendererClient;
   }
@@ -199,6 +205,11 @@ export class FakeInspectorClient
     this.dispatchTypedEvent("instructionsChange", instructions);
   }
 
+  setProtocolVersion(protocolVersion: string | undefined): void {
+    this.protocolVersion = protocolVersion;
+    this.dispatchTypedEvent("protocolVersionChange", protocolVersion);
+  }
+
   setAppRendererClient(client: AppRendererClient | null): void {
     this.appRendererClient = client;
   }
@@ -215,9 +226,7 @@ export class FakeInspectorClient
     this.promptPages.push(...pages);
   }
 
-  queueResourcePages(
-    ...pages: Array<ListResult<"resources", Resource>>
-  ): void {
+  queueResourcePages(...pages: Array<ListResult<"resources", Resource>>): void {
     this.resourcePages.push(...pages);
   }
 

core/mcp/inspectorClient.ts

@@ -147,6 +147,8 @@ export class InspectorClient extends InspectorClientEventTarget {
   private capabilities?: ServerCapabilities;
   private serverInfo?: Implementation;
   private instructions?: string;
+  private protocolVersion?: string;
+  private pendingInitializeRequestIds = new Set<JSONRPCRequest["id"]>();
   // Sampling requests
   private pendingSamples: SamplingCreateMessage[] = [];
   // Elicitation requests
@@ -310,6 +312,9 @@ export class InspectorClient extends InspectorClientEventTarget {
   private createMessageTrackingCallbacks(): MessageTrackingCallbacks {
     return {
       trackRequest: (message: JSONRPCRequest) => {
+        if (message.method === "initialize") {
+          this.pendingInitializeRequestIds.add(message.id);
+        }
         const entry: MessageEntry = {
           id: crypto.randomUUID(),
           timestamp: new Date(),
@@ -321,6 +326,7 @@ export class InspectorClient extends InspectorClientEventTarget {
       trackResponse: (
         message: JSONRPCResultResponse | JSONRPCErrorResponse,
       ) => {
+        this.captureInitializeProtocolVersion(message);
         const entry: MessageEntry = {
           id: crypto.randomUUID(),
           timestamp: new Date(),
@@ -929,10 +935,12 @@ export class InspectorClient extends InspectorClientEventTarget {
     this.capabilities = undefined;
     this.serverInfo = undefined;
     this.instructions = undefined;
+    this.protocolVersion = undefined;
     this.dispatchTypedEvent("pendingSamplesChange", this.pendingSamples);
     this.dispatchTypedEvent("capabilitiesChange", this.capabilities);
     this.dispatchTypedEvent("serverInfoChange", this.serverInfo);
     this.dispatchTypedEvent("instructionsChange", this.instructions);
+    this.dispatchTypedEvent("protocolVersionChange", this.protocolVersion);
   }
 
   /**
@@ -1164,6 +1172,13 @@ export class InspectorClient extends InspectorClientEventTarget {
     return this.instructions;
   }
 
+  /**
+   * Get the negotiated MCP protocol version from the initialize response
+   */
+  getProtocolVersion(): string | undefined {
+    return this.protocolVersion;
+  }
+
   /**
    * Set the logging level for the MCP server
    * @param level Logging level to set
@@ -1878,6 +1893,30 @@ export class InspectorClient extends InspectorClientEventTarget {
     }
   }
 
+  private captureInitializeProtocolVersion(
+    message: JSONRPCResultResponse | JSONRPCErrorResponse,
+  ): void {
+    if (message.id === undefined) {
+      return;
+    }
+    if (!this.pendingInitializeRequestIds.delete(message.id)) {
+      return;
+    }
+    if (!("result" in message)) {
+      return;
+    }
+    const result = message.result;
+    if (
+      typeof result === "object" &&
+      result !== null &&
+      "protocolVersion" in result &&
+      typeof result.protocolVersion === "string"
+    ) {
+      this.protocolVersion = result.protocolVersion;
+      this.dispatchTypedEvent("protocolVersionChange", this.protocolVersion);
+    }
+  }
+
   private dispatchStderrLog(entry: StderrLogEntry): void {
     this.dispatchTypedEvent("stderrLog", entry);
   }

core/mcp/inspectorClientEventTarget.ts

@@ -55,6 +55,7 @@ export interface InspectorClientEventMap {
   capabilitiesChange: ServerCapabilities | undefined;
   serverInfoChange: Implementation | undefined;
   instructionsChange: string | undefined;
+  protocolVersionChange: string | undefined;
   message: MessageEntry;
   stderrLog: StderrLogEntry;
   fetchRequest: FetchRequestEntry;

core/mcp/inspectorClientProtocol.ts

@@ -50,6 +50,7 @@ export interface InspectorClientProtocol extends InspectorClientEventTarget {
   getCapabilities(): ServerCapabilities | undefined;
   getServerInfo(): Implementation | undefined;
   getInstructions(): string | undefined;
+  getProtocolVersion(): string | undefined;
   getAppRendererClient(): AppRendererClient | null;
 
   // Connection control

core/react/useInspectorClient.ts

@@ -13,6 +13,7 @@ export interface UseInspectorClientResult {
   capabilities?: ServerCapabilities;
   serverInfo?: Implementation;
   instructions?: string;
+  protocolVersion?: string;
   appRendererClient: AppRendererClient | null;
   connect: () => Promise<void>;
   disconnect: () => Promise<void>;
@@ -26,7 +27,8 @@ export interface UseInspectorClientResult {
  * Note: `appRendererClient` is read lazily from the client on every render
  * and is NOT subscribed. It changes once at connect time and is not expected
  * to change again during a session, so callers will see the current value
- * on any rerender triggered by status / capabilities / serverInfo / instructions.
+ * on any rerender triggered by status / capabilities / serverInfo /
+ * instructions / protocolVersion.
  * If a future use case requires autonomous updates when the renderer attaches,
  * add an `appRendererClientChange` event to `InspectorClientEventMap` and
  * subscribe here.
@@ -46,20 +48,25 @@ export function useInspectorClient(
   const [instructions, setInstructions] = useState<string | undefined>(
     inspectorClient?.getInstructions(),
   );
+  const [protocolVersion, setProtocolVersion] = useState<string | undefined>(
+    inspectorClient?.getProtocolVersion(),
+  );
 
   useEffect(() => {
     if (!inspectorClient) {
       setStatus("disconnected");
       setCapabilities(undefined);
       setServerInfo(undefined);
       setInstructions(undefined);
+      setProtocolVersion(undefined);
       return;
     }
 
     setStatus(inspectorClient.getStatus());
     setCapabilities(inspectorClient.getCapabilities());
     setServerInfo(inspectorClient.getServerInfo());
     setInstructions(inspectorClient.getInstructions());
+    setProtocolVersion(inspectorClient.getProtocolVersion());
 
     const onStatusChange = (event: TypedEvent<"statusChange">) => {
       setStatus(event.detail);
@@ -73,6 +80,11 @@ export function useInspectorClient(
     const onInstructionsChange = (event: TypedEvent<"instructionsChange">) => {
       setInstructions(event.detail);
     };
+    const onProtocolVersionChange = (
+      event: TypedEvent<"protocolVersionChange">,
+    ) => {
+      setProtocolVersion(event.detail);
+    };
 
     inspectorClient.addEventListener("statusChange", onStatusChange);
     inspectorClient.addEventListener(
@@ -84,6 +96,10 @@ export function useInspectorClient(
       "instructionsChange",
       onInstructionsChange,
     );
+    inspectorClient.addEventListener(
+      "protocolVersionChange",
+      onProtocolVersionChange,
+    );
 
     return () => {
       inspectorClient.removeEventListener("statusChange", onStatusChange);
@@ -99,6 +115,10 @@ export function useInspectorClient(
         "instructionsChange",
         onInstructionsChange,
       );
+      inspectorClient.removeEventListener(
+        "protocolVersionChange",
+        onProtocolVersionChange,
+      );
     };
   }, [inspectorClient]);
 
@@ -117,6 +137,7 @@ export function useInspectorClient(
     capabilities,
     serverInfo,
     instructions,
+    protocolVersion,
     appRendererClient: inspectorClient?.getAppRendererClient() ?? null,
     connect,
     disconnect,