Addressed all Claude review issues.

client/src/__tests__/proxyFetchEndpoint.test.ts

@@ -3,13 +3,21 @@
  * Spawns the server and hits it like any other HTTP client would.
  */
 import { spawn, type ChildProcess } from "child_process";
-import { createServer, type Server } from "http";
+import {
+  createServer,
+  type IncomingMessage,
+  type Server,
+  type ServerResponse,
+} from "http";
 import { resolve } from "path";
 
 const TEST_PORT = 16321;
 const TEST_TOKEN = "test-proxy-token-12345";
 const SERVER_PATH = resolve(__dirname, "../../../server/build/index.js");
 
+/** Placeholder URL for tests where auth fails before the proxy fetches (no network). */
+const UNUSED_UPSTREAM_URL = "http://127.0.0.1:1/unused";
+
 async function waitForServer(baseUrl: string, maxWaitMs = 5000): Promise<void> {
   const start = Date.now();
   while (Date.now() - start < maxWaitMs) {
@@ -23,6 +31,36 @@ async function waitForServer(baseUrl: string, maxWaitMs = 5000): Promise<void> {
   throw new Error("Server did not become ready");
 }
 
+/**
+ * Runs `fn` with a local HTTP server on 127.0.0.1:ephemeral-port.
+ * `origin` is `http://127.0.0.1:<port>` (no trailing path).
+ */
+async function withLocalUpstream(
+  onRequest: (req: IncomingMessage, res: ServerResponse) => void,
+  fn: (origin: string) => Promise<void>,
+): Promise<void> {
+  const upstream: Server = createServer(onRequest);
+
+  await new Promise<void>((resolve, reject) => {
+    upstream.once("error", reject);
+    upstream.listen(0, "127.0.0.1", () => resolve());
+  });
+
+  const addr = upstream.address();
+  if (!addr || typeof addr === "string") {
+    upstream.close();
+    throw new Error("Expected TCP listen address");
+  }
+
+  const origin = `http://127.0.0.1:${addr.port}`;
+
+  try {
+    await fn(origin);
+  } finally {
+    await new Promise<void>((r) => upstream.close(() => r()));
+  }
+}
+
 describe("POST /fetch endpoint", () => {
   let server: ChildProcess;
   const baseUrl = `http://localhost:${TEST_PORT}`;
@@ -48,7 +86,7 @@ describe("POST /fetch endpoint", () => {
       method: "POST",
       headers: { "Content-Type": "application/json" },
       body: JSON.stringify({
-        url: "http://example.com/",
+        url: UNUSED_UPSTREAM_URL,
         init: { method: "GET" },
       }),
     });
@@ -65,14 +103,14 @@ describe("POST /fetch endpoint", () => {
         "X-MCP-Proxy-Auth": "Bearer wrong-token",
       },
       body: JSON.stringify({
-        url: "http://example.com/",
+        url: UNUSED_UPSTREAM_URL,
         init: { method: "GET" },
       }),
     });
     expect(res.status).toBe(401);
   });
 
-  it("forwards request when auth token is valid", async () => {
+  it("returns 400 for non-http(s) URL when auth token is valid", async () => {
     const res = await fetch(`${baseUrl}/fetch`, {
       method: "POST",
       headers: {
@@ -80,59 +118,117 @@ describe("POST /fetch endpoint", () => {
         "X-MCP-Proxy-Auth": `Bearer ${TEST_TOKEN}`,
       },
       body: JSON.stringify({
-        url: "http://example.com/",
+        url: "file:///etc/passwd",
         init: { method: "GET" },
       }),
     });
-    expect(res.status).toBe(200);
+    expect(res.status).toBe(400);
-    const body = await res.json();
+    const body = (await res.json()) as { error: string };
-    expect(body.ok).toBe(true);
+    expect(body.error).toBe("Only http/https URLs are allowed");
-    expect(body.status).toBe(200);
+  });
-    expect(body.body).toBeDefined();
+
+  it("returns 400 for invalid URL string when auth token is valid", async () => {
+    const res = await fetch(`${baseUrl}/fetch`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        "X-MCP-Proxy-Auth": `Bearer ${TEST_TOKEN}`,
+      },
+      body: JSON.stringify({
+        url: "not a valid url",
+        init: { method: "GET" },
+      }),
+    });
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toBe("Invalid URL");
+  });
+
+  it("returns 400 when url is missing when auth token is valid", async () => {
+    const res = await fetch(`${baseUrl}/fetch`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        "X-MCP-Proxy-Auth": `Bearer ${TEST_TOKEN}`,
+      },
+      body: JSON.stringify({ init: { method: "GET" } }),
+    });
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toBe("Missing or invalid url");
+  });
+
+  it("forwards request when auth token is valid", async () => {
+    const upstreamPayload = JSON.stringify({ hello: "proxy-fetch-test" });
+
+    await withLocalUpstream(
+      (req, res) => {
+        res.writeHead(200, { "Content-Type": "application/json" });
+        res.end(upstreamPayload);
+      },
+      async (origin) => {
+        const upstreamUrl = `${origin}/ok`;
+
+        const res = await fetch(`${baseUrl}/fetch`, {
+          method: "POST",
+          headers: {
+            "Content-Type": "application/json",
+            "X-MCP-Proxy-Auth": `Bearer ${TEST_TOKEN}`,
+          },
+          body: JSON.stringify({
+            url: upstreamUrl,
+            init: { method: "GET" },
+          }),
+        });
+
+        expect(res.status).toBe(200);
+        const body = (await res.json()) as {
+          ok: boolean;
+          status: number;
+          statusText: string;
+          body: string;
+          headers: Record<string, string>;
+        };
+        expect(body.ok).toBe(true);
+        expect(body.status).toBe(200);
+        expect(body.statusText).toBe("OK");
+        expect(body.body).toBe(upstreamPayload);
+        expect(body.headers["content-type"]).toMatch(/application\/json/i);
+      },
+    );
   });
 
   it("mirrors upstream 404 (non-2xx) when auth token is valid", async () => {
-    const upstream: Server = createServer((req, res) => {
+    await withLocalUpstream(
-      res.writeHead(404, { "Content-Type": "application/json" });
+      (req, res) => {
-      res.end('{"error":"not_found"}');
+        res.writeHead(404, { "Content-Type": "application/json" });
-    });
+        res.end('{"error":"not_found"}');
+      },
+      async (origin) => {
+        const upstreamUrl = `${origin}/missing`;
 
-    await new Promise<void>((resolve, reject) => {
+        const res = await fetch(`${baseUrl}/fetch`, {
-      upstream.once("error", reject);
+          method: "POST",
-      upstream.listen(0, "127.0.0.1", () => resolve());
+          headers: {
-    });
+            "Content-Type": "application/json",
+            "X-MCP-Proxy-Auth": `Bearer ${TEST_TOKEN}`,
+          },
+          body: JSON.stringify({
+            url: upstreamUrl,
+            init: { method: "GET" },
+          }),
+        });
 
-    const addr = upstream.address();
+        expect(res.status).toBe(404);
-    if (!addr || typeof addr === "string") {
+        const body = (await res.json()) as {
-      upstream.close();
+          ok: boolean;
-      throw new Error("Expected TCP listen address");
+          status: number;
-    }
+          body: string;
-    const upstreamUrl = `http://127.0.0.1:${addr.port}/missing`;
+        };
-
+        expect(body.ok).toBe(false);
-    try {
+        expect(body.status).toBe(404);
-      const res = await fetch(`${baseUrl}/fetch`, {
+        expect(JSON.parse(body.body)).toEqual({ error: "not_found" });
-        method: "POST",
+      },
-        headers: {
+    );
-          "Content-Type": "application/json",
-          "X-MCP-Proxy-Auth": `Bearer ${TEST_TOKEN}`,
-        },
-        body: JSON.stringify({
-          url: upstreamUrl,
-          init: { method: "GET" },
-        }),
-      });
-
-      expect(res.status).toBe(404);
-      const body = (await res.json()) as {
-        ok: boolean;
-        status: number;
-        body: string;
-      };
-      expect(body.ok).toBe(false);
-      expect(body.status).toBe(404);
-      expect(JSON.parse(body.body)).toEqual({ error: "not_found" });
-    } finally {
-      await new Promise<void>((r) => upstream.close(() => r()));
-    }
   });
 });

client/src/components/AuthDebugger.tsx

@@ -108,8 +108,13 @@ const AuthDebugger = ({
     });
   }, [serverUrl, updateAuthState]);
 
-  const fetchFn =
+  const fetchFn = useMemo(
-    connectionType === "proxy" && config ? createProxyFetch(config) : undefined;
+    () =>
+      connectionType === "proxy" && config
+        ? createProxyFetch(config)
+        : undefined,
+    [connectionType, config],
+  );
 
   const stateMachine = useMemo(
     () => new OAuthStateMachine(serverUrl, updateAuthState, fetchFn),

client/src/lib/__tests__/mcpProxyTransportErrorCode.test.ts

@@ -0,0 +1,24 @@
+/**
+ * Compare client vs server numeric literal without importing server/mcpProxy.ts:
+ * that module pulls in the SDK graph and breaks Jest (e.g. optional peer deps).
+ */
+import { readFileSync } from "fs";
+import { resolve } from "path";
+import { MCP_PROXY_TRANSPORT_ERROR_CODE as clientCode } from "../constants";
+
+const EXPORT_RE =
+  /export const MCP_PROXY_TRANSPORT_ERROR_CODE\s*=\s*(-?\d+)\s*;/;
+
+describe("MCP_PROXY_TRANSPORT_ERROR_CODE", () => {
+  it("matches server/src/mcpProxy.ts (avoid silent drift between client and server)", () => {
+    const serverSrcPath = resolve(
+      __dirname,
+      "../../../../server/src/mcpProxy.ts",
+    );
+    const serverSrc = readFileSync(serverSrcPath, "utf-8");
+    const match = serverSrc.match(EXPORT_RE);
+    expect(match).not.toBeNull();
+    const serverCode = Number(match![1]);
+    expect(clientCode).toBe(serverCode);
+  });
+});

client/src/lib/__tests__/proxyFetch.test.ts

@@ -221,4 +221,24 @@ describe("createProxyFetch", () => {
     const callBody = JSON.parse(mockFetch.mock.calls[0][1].body);
     expect(callBody.url).toBe("https://example.com/discovery");
   });
+
+  it("uses Request.url when input is a Request (not [object Request])", async () => {
+    mockFetch.mockResolvedValue({
+      ok: true,
+      json: () =>
+        Promise.resolve({
+          ok: true,
+          status: 200,
+          statusText: "OK",
+          headers: {},
+          body: "",
+        }),
+    });
+
+    const fetchFn = createProxyFetch(configWithProxy);
+    await fetchFn(new Request("https://example.com/from-request"));
+
+    const callBody = JSON.parse(mockFetch.mock.calls[0][1].body);
+    expect(callBody.url).toBe("https://example.com/from-request");
+  });
 });

client/src/lib/constants.ts

@@ -41,7 +41,8 @@ export const DEFAULT_MCP_PROXY_LISTEN_PORT = "6277";
  * JSON-RPC error code when the inspector proxy cannot complete a forward to the upstream MCP server.
  * **-32099** — high end of JSON-RPC’s -32000..-32099 server-error band to avoid MCP-assigned codes
  * (see comment on `server/src/mcpProxy.ts` `MCP_PROXY_TRANSPORT_ERROR_CODE`). Duplicated like
- * `DEFAULT_MCP_PROXY_LISTEN_PORT`.
+ * `DEFAULT_MCP_PROXY_LISTEN_PORT`. Keep in sync with the server value;
+ * `src/lib/__tests__/mcpProxyTransportErrorCode.test.ts` fails if they drift.
  */
 export const MCP_PROXY_TRANSPORT_ERROR_CODE = -32099;
 

client/src/lib/hooks/useConnection.ts

@@ -379,8 +379,6 @@ export function useConnection({
     }
   };
 
-  const is401Error = isConnectionAuthError;
-
   const isProxyAuthError = (error: unknown): boolean => {
     return (
       error instanceof Error &&
@@ -389,7 +387,7 @@ export function useConnection({
   };
 
   const handleAuthError = async (error: unknown) => {
-    if (is401Error(error)) {
+    if (isConnectionAuthError(error)) {
       let scope = oauthScope?.trim();
       const fetchFn =
         connectionType === "proxy" ? createProxyFetch(config) : undefined;
@@ -820,7 +818,7 @@ export function useConnection({
         if (shouldRetry) {
           return connect(undefined, retryCount + 1);
         }
-        if (is401Error(error)) {
+        if (isConnectionAuthError(error)) {
           // Don't set error state if we're about to redirect for auth
 
           return;

client/src/lib/proxyFetch.ts

@@ -92,7 +92,12 @@ export function createProxyFetch(config: InspectorConfig): typeof fetch {
     input: RequestInfo | URL,
     init?: RequestInit,
   ): Promise<Response> => {
-    const url = typeof input === "string" ? input : input.toString();
+    const url =
+      typeof input === "string"
+        ? input
+        : input instanceof Request
+          ? input.url
+          : input.toString();
 
     // Serialize body for JSON transport. URLSearchParams and similar don't
     // JSON-serialize (they become {}), so we must convert to string first.

server/src/index.ts

@@ -844,6 +844,24 @@ app.post(
     try {
       const { url, init } = req.body as { url: string; init?: RequestInit };
 
+      if (typeof url !== "string" || url.length === 0) {
+        res.status(400).json({ error: "Missing or invalid url" });
+        return;
+      }
+
+      let parsedUrl: URL;
+      try {
+        parsedUrl = new URL(url);
+      } catch {
+        res.status(400).json({ error: "Invalid URL" });
+        return;
+      }
+
+      if (!["http:", "https:"].includes(parsedUrl.protocol)) {
+        res.status(400).json({ error: "Only http/https URLs are allowed" });
+        return;
+      }
+
       const response = await fetch(url, {
         method: init?.method ?? "GET",
         headers: (init?.headers as Record<string, string>) ?? {},

server/src/mcpProxy.ts

@@ -12,6 +12,9 @@ import { isJSONRPCRequest } from "@modelcontextprotocol/sdk/types.js";
  * the band are assigned over time — e.g. **-32002** is used for **ResourceNotFound** in parts of
  * the MCP ecosystem). We use **-32099** so this inspector-only bridge error stays clear of those
  * registered meanings; the client still keys off `error.data` shape, not the number alone.
+ *
+ * Keep in sync with `client/src/lib/constants.ts`; drift is caught by
+ * `client/src/lib/__tests__/mcpProxyTransportErrorCode.test.ts`.
  */
 export const MCP_PROXY_TRANSPORT_ERROR_CODE = -32099;
 
@@ -62,8 +65,8 @@ export function serializeProxyTransportError(
   } else if (error instanceof SseError) {
     if (error.code !== undefined) attachHttpStatusIfValid(error.code);
   }
-  const u = headerHolder?.lastUpstream401;
+  if (headerHolder?.lastUpstream401) {
-  if (u && headerHolder) {
+    const u = headerHolder.lastUpstream401;
     data.upstream401 = {
       wwwAuthenticate: u.wwwAuthenticate,
       body: u.body,