fix: surface MCP initialize errors in UI

client/e2e/fixtures/unsupported-protocol-server.js

@@ -0,0 +1,89 @@
+import http from "node:http";
+import { once } from "node:events";
+
+const ERROR_MESSAGE =
+  "Unsupported protocol version: 2025-11-25 - supported versions: 2025-06-18,2025-03-26,2024-11-05,2024-10-07";
+
+const applyCors = (res) => {
+  res.setHeader("Access-Control-Allow-Origin", "*");
+  res.setHeader(
+    "Access-Control-Allow-Headers",
+    "content-type, accept, mcp-session-id, mcp-protocol-version, authorization",
+  );
+  res.setHeader("Access-Control-Allow-Methods", "GET,POST,OPTIONS");
+};
+
+export async function startUnsupportedProtocolServer() {
+  const server = http.createServer(async (req, res) => {
+    applyCors(res);
+
+    if (req.method === "OPTIONS") {
+      res.statusCode = 204;
+      res.end();
+      return;
+    }
+
+    // Streamable HTTP transport does an optional GET to establish an SSE stream.
+    // Returning 405 is an expected case handled by the SDK.
+    if (req.method === "GET") {
+      res.statusCode = 405;
+      res.end();
+      return;
+    }
+
+    // Accepts any path; the Inspector URL field can include arbitrary endpoints.
+    if (req.method !== "POST") {
+      res.statusCode = 404;
+      res.end();
+      return;
+    }
+
+    let body = "";
+    req.setEncoding("utf8");
+    req.on("data", (chunk) => {
+      body += chunk;
+    });
+    await once(req, "end");
+
+    let parsed;
+    try {
+      parsed = JSON.parse(body);
+    } catch {
+      res.statusCode = 400;
+      res.setHeader("content-type", "application/json");
+      res.end(JSON.stringify({ error: "Invalid JSON" }));
+      return;
+    }
+
+    res.statusCode = 200;
+    res.setHeader("content-type", "application/json");
+    res.end(
+      JSON.stringify({
+        jsonrpc: "2.0",
+        id: parsed?.id ?? null,
+        error: {
+          code: -32602,
+          message: ERROR_MESSAGE,
+        },
+      }),
+    );
+  });
+
+  server.listen(0, "127.0.0.1");
+  await once(server, "listening");
+
+  const address = server.address();
+  if (!address || typeof address === "string") {
+    throw new Error("Failed to start unsupported protocol fixture server");
+  }
+
+  const baseUrl = `http://127.0.0.1:${address.port}`;
+
+  return {
+    baseUrl,
+    close: () =>
+      new Promise((resolve, reject) =>
+        server.close((e) => (e ? reject(e) : resolve())),
+      ),
+  };
+}

client/e2e/protocol-version-error.spec.ts

@@ -0,0 +1,43 @@
+import { test, expect } from "@playwright/test";
+import { startUnsupportedProtocolServer } from "./fixtures/unsupported-protocol-server.js";
+
+const APP_URL = "http://localhost:6274/";
+
+test.describe("Protocol version negotiation errors", () => {
+  test("surfaces -32602 initialize error and hides proxy token hint", async ({
+    page,
+  }) => {
+    const fixture = await startUnsupportedProtocolServer();
+    try {
+      await page.goto(APP_URL);
+
+      const transportSelect = page.getByLabel("Transport Type");
+      await expect(transportSelect).toBeVisible();
+      await transportSelect.click();
+      await page.getByRole("option", { name: "Streamable HTTP" }).click();
+
+      const connectionTypeSelect = page.getByLabel("Connection Type");
+      await expect(connectionTypeSelect).toBeVisible();
+      await connectionTypeSelect.click();
+      await page.getByRole("option", { name: "Direct" }).click();
+
+      await page.locator("#sse-url-input").fill(fixture.baseUrl);
+
+      await page.getByRole("button", { name: "Connect" }).click();
+
+      await expect(page.getByText(/MCP error\s*-32602/i).first()).toBeVisible({
+        timeout: 10000,
+      });
+
+      await expect(
+        page.getByText(/Did you add the proxy session token in Configuration/i),
+      ).toHaveCount(0);
+
+      await expect(
+        page.locator('[data-testid="connection-error-details"]'),
+      ).toBeVisible();
+    } finally {
+      await fixture.close();
+    }
+  });
+});

client/src/App.tsx

@@ -299,6 +299,7 @@ const App = () => {
 
   const {
     connectionStatus,
+    connectionError,
     serverCapabilities,
     serverImplementation,
     mcpClient,
@@ -956,6 +957,7 @@ const App = () => {
       >
         <Sidebar
           connectionStatus={connectionStatus}
+          connectionError={connectionError}
           transportType={transportType}
           setTransportType={setTransportType}
           command={command}

client/src/components/Sidebar.tsx

@@ -33,6 +33,10 @@ import { InspectorConfig } from "@/lib/configurationTypes";
 import { ConnectionStatus } from "@/lib/constants";
 import useTheme from "../lib/hooks/useTheme";
 import { version } from "../../../package.json";
+import {
+  getMcpErrorInfo,
+  parseUnsupportedProtocolVersionError,
+} from "@/utils/mcpErrorUtils";
 import {
   Tooltip,
   TooltipTrigger,
@@ -45,6 +49,7 @@ import IconDisplay, { WithIcons } from "./IconDisplay";
 
 interface SidebarProps {
   connectionStatus: ConnectionStatus;
+  connectionError?: unknown | null;
   transportType: "stdio" | "sse" | "streamable-http";
   setTransportType: (type: "stdio" | "sse" | "streamable-http") => void;
   command: string;
@@ -80,6 +85,7 @@ interface SidebarProps {
 
 const Sidebar = ({
   connectionStatus,
+  connectionError,
   transportType,
   setTransportType,
   command,
@@ -119,6 +125,22 @@ const Sidebar = ({
   const [copiedServerFile, setCopiedServerFile] = useState(false);
   const { toast } = useToast();
 
+  const mcpError = connectionError ? getMcpErrorInfo(connectionError) : null;
+  const mcpErrorDisplayMessage = mcpError
+    ? mcpError.message
+        .replace(/^McpError:\s*/i, "")
+        .replace(/^MCP error\s+-?\d+:\s*/i, "")
+    : "";
+  const protocolVersionDetails = mcpError
+    ? parseUnsupportedProtocolVersionError(mcpError.message)
+    : null;
+
+  const shouldShowProxyTokenHint =
+    connectionStatus === "error" &&
+    connectionType === "proxy" &&
+    !mcpError &&
+    !config.MCP_PROXY_AUTH_TOKEN?.value;
+
   const connectionTypeTip =
     "Connect to server directly (requires CORS config on server) or via MCP Inspector Proxy";
   // Reusable error reporter for copy actions
@@ -753,7 +775,6 @@ const Sidebar = ({
                     case "connected":
                       return "bg-green-500";
                     case "error":
-                      return "bg-red-500";
                     case "error-connecting-to-proxy":
                       return "bg-red-500";
                     default:
@@ -767,10 +788,18 @@ const Sidebar = ({
                     case "connected":
                       return "Connected";
                     case "error": {
-                      const hasProxyToken = config.MCP_PROXY_AUTH_TOKEN?.value;
-                      if (!hasProxyToken) {
+                      if (mcpError && typeof mcpError.code === "number") {
+                        return `MCP error ${mcpError.code}: ${mcpErrorDisplayMessage}`;
+                      }
+
+                      if (mcpError) {
+                        return `MCP error: ${mcpErrorDisplayMessage}`;
+                      }
+
+                      if (shouldShowProxyTokenHint) {
                         return "Connection Error - Did you add the proxy session token in Configuration?";
                       }
+
                       return "Connection Error - Check if your MCP server is running and proxy token is correct";
                     }
                     case "error-connecting-to-proxy":
@@ -782,6 +811,53 @@ const Sidebar = ({
               </span>
             </div>
 
+            {connectionStatus === "error" && mcpError && (
+              <details
+                className="bg-gray-50 dark:bg-gray-900 p-3 rounded-lg mb-4 text-sm"
+                data-testid="connection-error-details"
+              >
+                <summary className="cursor-pointer font-medium text-gray-800 dark:text-gray-200">
+                  Error details
+                </summary>
+                <div className="mt-2 space-y-2 text-xs text-gray-700 dark:text-gray-300">
+                  <div className="font-mono break-words">
+                    MCP error
+                    {typeof mcpError.code === "number"
+                      ? ` ${mcpError.code}`
+                      : ""}
+                    : {mcpErrorDisplayMessage}
+                  </div>
+
+                  {protocolVersionDetails?.supportedProtocolVersions?.length ? (
+                    <div>
+                      Supported versions:{" "}
+                      <span className="font-mono">
+                        {protocolVersionDetails.supportedProtocolVersions.join(
+                          ", ",
+                        )}
+                      </span>
+                    </div>
+                  ) : null}
+
+                  {mcpError.code === -32602 ? (
+                    <div className="text-gray-600 dark:text-gray-400">
+                      The server returned an error for{" "}
+                      <span className="font-mono">initialize</span> instead of
+                      negotiating a compatible protocol version.{" "}
+                      <a
+                        className="text-blue-600 dark:text-blue-400 hover:underline"
+                        href="https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#version-negotiation"
+                        target="_blank"
+                        rel="noopener noreferrer"
+                      >
+                        Spec: version negotiation
+                      </a>
+                    </div>
+                  ) : null}
+                </div>
+              </details>
+            )}
+
             {connectionStatus === "connected" && serverImplementation && (
               <div className="bg-gray-50 dark:bg-gray-900 p-3 rounded-lg mb-4">
                 <div className="flex items-center gap-2 mb-1">

client/src/components/__tests__/Sidebar.test.tsx

@@ -1,10 +1,11 @@
-import { render, screen, fireEvent, act } from "@testing-library/react";
+import { render, screen, fireEvent, act, within } from "@testing-library/react";
 import "@testing-library/jest-dom";
 import { describe, it, beforeEach, jest } from "@jest/globals";
 import Sidebar from "../Sidebar";
 import { DEFAULT_INSPECTOR_CONFIG } from "@/lib/constants";
 import { InspectorConfig } from "@/lib/configurationTypes";
 import { TooltipProvider } from "@/components/ui/tooltip";
+import { McpError } from "@modelcontextprotocol/sdk/types.js";
 
 // Mock theme hook
 jest.mock("../../lib/hooks/useTheme", () => ({
@@ -1046,4 +1047,69 @@ describe("Sidebar", () => {
       );
     });
   });
+
+  describe("Connection status errors", () => {
+    it("shows MCP error details and hides proxy token hint", () => {
+      const mcpError = new McpError(
+        -32602,
+        "Unsupported protocol version: 2025-11-25 - supported versions: 2025-06-18,2025-03-26,2024-11-05,2024-10-07",
+      );
+
+      renderSidebar({
+        connectionStatus: "error",
+        connectionError: mcpError,
+        config: {
+          ...DEFAULT_INSPECTOR_CONFIG,
+          MCP_PROXY_AUTH_TOKEN: {
+            ...DEFAULT_INSPECTOR_CONFIG.MCP_PROXY_AUTH_TOKEN,
+            value: "",
+          },
+        },
+      });
+
+      expect(
+        screen.getAllByText(
+          /MCP error -32602: Unsupported protocol version: 2025-11-25/i,
+        ).length,
+      ).toBeGreaterThan(0);
+
+      expect(
+        within(screen.getByTestId("connection-error-details")).getAllByText(
+          /Supported versions:/i,
+        ).length,
+      ).toBeGreaterThan(0);
+
+      const details = within(screen.getByTestId("connection-error-details"));
+      expect(details.getAllByText(/2025-06-18/).length).toBeGreaterThan(0);
+      expect(details.getAllByText(/2025-03-26/).length).toBeGreaterThan(0);
+      expect(details.getAllByText(/2024-11-05/).length).toBeGreaterThan(0);
+      expect(details.getAllByText(/2024-10-07/).length).toBeGreaterThan(0);
+
+      expect(
+        screen.queryByText(
+          /Did you add the proxy session token in Configuration\?/i,
+        ),
+      ).not.toBeInTheDocument();
+    });
+
+    it("does not show proxy token hint for direct connections", () => {
+      renderSidebar({
+        connectionStatus: "error",
+        connectionType: "direct",
+        config: {
+          ...DEFAULT_INSPECTOR_CONFIG,
+          MCP_PROXY_AUTH_TOKEN: {
+            ...DEFAULT_INSPECTOR_CONFIG.MCP_PROXY_AUTH_TOKEN,
+            value: "",
+          },
+        },
+      });
+
+      expect(
+        screen.queryByText(
+          /Did you add the proxy session token in Configuration\?/i,
+        ),
+      ).not.toBeInTheDocument();
+    });
+  });
 });