diff --git a/clients/web/src/test/core/auth/cimd.test.ts b/clients/web/src/test/core/auth/cimd.test.ts
index caf2e30e..63bc64a5 100644
--- a/clients/web/src/test/core/auth/cimd.test.ts
+++ b/clients/web/src/test/core/auth/cimd.test.ts
@@ -105,24 +105,21 @@ describe("ensureCimdClientRegistration", () => {
     const metadataUrl = new URL(
       "http://127.0.0.1:9999/custom/protected-resource",
     );
-    // `ensureCimdClientRegistration` runs protected-resource discovery through
-    // the SDK's default fetch, so the advertised URL is observable only on the
-    // global — the AS-metadata leg is the one that takes `fetchFn`.
-    const globalFetch = vi.fn(async (input: RequestInfo | URL) =>
-      String(input) === metadataUrl.href
-        ? new Response(
-            JSON.stringify({
-              resource: SERVER_URL,
-              authorization_servers: ["http://127.0.0.1:9999"],
-            }),
-          )
-        : new Response(null, { status: 404 }),
-    );
-    vi.stubGlobal("fetch", globalFetch);
-
-    const fetchFn = vi.fn(
-      async () =>
-        new Response(
+    // Both discovery legs must run through the configured fetch — on web that
+    // is the backend proxy, so a leg that fell back to the global would fail
+    // on CORS and be swallowed.
+    const fetchFn = vi.fn(async (input: RequestInfo | URL) => {
+      const url = String(input);
+      if (url === metadataUrl.href) {
+        return new Response(
+          JSON.stringify({
+            resource: SERVER_URL,
+            authorization_servers: ["http://127.0.0.1:9999"],
+          }),
+        );
+      }
+      if (url.includes("/.well-known/oauth-authorization-server")) {
+        return new Response(
           JSON.stringify({
             issuer: "http://127.0.0.1:9999",
             authorization_endpoint: "http://127.0.0.1:9999/oauth/authorize",
@@ -130,8 +127,14 @@ describe("ensureCimdClientRegistration", () => {
             response_types_supported: ["code"],
             client_id_metadata_document_supported: true,
           }),
-        ),
-    );
+        );
+      }
+      return new Response(null, { status: 404 });
+    });
+    // Nothing may reach the network directly; a call here means a leg fell
+    // back to the global fetch.
+    const globalFetch = vi.fn();
+    vi.stubGlobal("fetch", globalFetch);
 
     await ensureCimdClientRegistration({
       serverUrl: SERVER_URL,
@@ -140,10 +143,14 @@ describe("ensureCimdClientRegistration", () => {
       resourceMetadataUrl: metadataUrl,
     });
 
-    expect(globalFetch).toHaveBeenCalled();
-    for (const [input] of globalFetch.mock.calls) {
-      expect(String(input)).toBe(metadataUrl.href);
-    }
+    expect(globalFetch).not.toHaveBeenCalled();
+    const requested = fetchFn.mock.calls.map(([input]) => String(input));
+    expect(requested).toContain(metadataUrl.href);
+    expect(
+      requested.filter((url) =>
+        url.includes("/.well-known/oauth-protected-resource"),
+      ),
+    ).toEqual([]);
     expect(storage.saveClientInformation).toHaveBeenCalledWith(
       SERVER_URL,
       { client_id: METADATA_URL },
diff --git a/clients/web/src/test/integration/mcp/oauth-resource-metadata-challenge.test.ts b/clients/web/src/test/integration/mcp/oauth-resource-metadata-challenge.test.ts
index 909ac187..bda34f61 100644
--- a/clients/web/src/test/integration/mcp/oauth-resource-metadata-challenge.test.ts
+++ b/clients/web/src/test/integration/mcp/oauth-resource-metadata-challenge.test.ts
@@ -17,6 +17,7 @@ import {
   TestServerHttp,
   getDefaultServerConfig,
   createOAuthTestServerConfig,
+  isOriginRelativePath,
   loadConfig,
   resolveConfig,
   waitForOAuthWellKnown,
@@ -103,4 +104,20 @@ describe("OAuth challenge resource_metadata (RFC 9728)", () => {
       resourceMetadataPath: METADATA_PATH,
     });
   });
+
+  it.each([
+    ["/custom/protected-resource", true],
+    ["/a", true],
+    // Both re-point the origin when resolved against the request base while
+    // Express still registers the route locally, so the server would advertise
+    // a document it does not serve (Copilot).
+    ["//other-host/doc", false],
+    ["/\\other-host/doc", false],
+    ["https://other-host/doc", false],
+    ["custom/protected-resource", false],
+    ["/has a space", false],
+    ["", false],
+  ])("origin-relative path check: %j -> %s", (value, expected) => {
+    expect(isOriginRelativePath(value)).toBe(expected);
+  });
 });
diff --git a/core/auth/cimd.ts b/core/auth/cimd.ts
index 1b140d2b..d8ccc8bf 100644
--- a/core/auth/cimd.ts
+++ b/core/auth/cimd.ts
@@ -38,6 +38,12 @@ export async function ensureCimdClientRegistration(params: {
     resourceMetadata = await discoverOAuthProtectedResourceMetadata(
       params.serverUrl,
       { resourceMetadataUrl: params.resourceMetadataUrl },
+      // The same fetch the AS-metadata leg below uses. On web that is
+      // `createRemoteFetch`, which proxies through the backend to sidestep
+      // CORS — on the global `fetch` this leg would fail in the browser, be
+      // swallowed by the catch, and leave CIMD probing the wrong
+      // authorization server (Copilot).
+      params.fetchFn,
     );
   } catch {
     resourceMetadata = undefined;
diff --git a/test-servers/src/load-config.ts b/test-servers/src/load-config.ts
index c791eef2..ca1f75ef 100644
--- a/test-servers/src/load-config.ts
+++ b/test-servers/src/load-config.ts
@@ -6,6 +6,7 @@
 import { readFileSync } from "fs";
 import path from "path";
 import YAML from "yaml";
+import { isOriginRelativePath } from "./test-server-oauth.js";
 
 export interface PresetRef {
   preset: string;
@@ -203,12 +204,9 @@ function validateConfig(
       }
     }
     const metadataPath = oauth.resourceMetadataPath;
-    if (
-      metadataPath !== undefined &&
-      (typeof metadataPath !== "string" || !metadataPath.startsWith("/"))
-    ) {
+    if (metadataPath !== undefined && !isOriginRelativePath(metadataPath)) {
       throw new Error(
-        `Invalid config in ${filePath}: oauth.resourceMetadataPath must be an absolute path starting with "/"`,
+        `Invalid config in ${filePath}: oauth.resourceMetadataPath must be an origin-relative path (e.g. "/custom/protected-resource") — a value such as "//host/doc" would advertise a document the server does not serve`,
       );
     }
     if (transportType === "stdio" && oauth.enabled === true) {
diff --git a/test-servers/src/test-server-oauth.ts b/test-servers/src/test-server-oauth.ts
index 946f7030..71280861 100644
--- a/test-servers/src/test-server-oauth.ts
+++ b/test-servers/src/test-server-oauth.ts
@@ -27,6 +27,54 @@ export function getOAuthMode(
   return config.mode ?? "combined";
 }
 
+const PATH_VALIDATION_BASE = "http://config.invalid";
+
+/**
+ * True for a path that resolves under its own origin — the only shape safe to
+ * use as both an Express route and a `resource_metadata` value.
+ *
+ * A leading-slash check is not enough: `//other-host/doc` and `/\other-host/doc`
+ * both re-point the origin when resolved against the request base (the URL
+ * parser folds a backslash into a slash for special schemes), while Express
+ * still registers the route locally — so the server would advertise a document
+ * it does not serve (Copilot). Comparing the resolved href against the literal
+ * also rejects anything the parser would rewrite (spaces, unescaped
+ * characters), which an Express route would not match either.
+ */
+export function isOriginRelativePath(value: unknown): value is string {
+  if (typeof value !== "string" || !value.startsWith("/")) {
+    return false;
+  }
+  try {
+    const resolved = new URL(value, PATH_VALIDATION_BASE);
+    return (
+      resolved.origin === PATH_VALIDATION_BASE &&
+      resolved.href === `${PATH_VALIDATION_BASE}${value}`
+    );
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * The configured metadata path, validated. Throws at server-setup time rather
+ * than serving a route that contradicts the challenge — the JSON-config path
+ * is validated earlier by `load-config`, so this covers a `ServerConfig`
+ * built programmatically.
+ */
+function resourceMetadataPath(config: OAuthConfig): string | undefined {
+  const path = config.resourceMetadataPath;
+  if (path === undefined) {
+    return undefined;
+  }
+  if (!isOriginRelativePath(path)) {
+    throw new Error(
+      `oauth.resourceMetadataPath must be an origin-relative path (got ${JSON.stringify(path)})`,
+    );
+  }
+  return path;
+}
+
 /**
  * The `WWW-Authenticate` challenge sent with every 401.
  *
@@ -36,7 +84,7 @@ export function getOAuthMode(
  * `Bearer` challenge keeps the existing fixtures byte-identical.
  */
 function bearerChallenge(config: OAuthConfig, req: Request): string {
-  const path = config.resourceMetadataPath;
+  const path = resourceMetadataPath(config);
   if (!path) {
     return "Bearer";
   }
@@ -206,7 +254,7 @@ function setupMetadataEndpoints(
   // a client that ignores the advertised `resource_metadata` URL gets a 404
   // — see the field's doc comment.
   app.get(
-    config.resourceMetadataPath ?? "/.well-known/oauth-protected-resource",
+    resourceMetadataPath(config) ?? "/.well-known/oauth-protected-resource",
     (req: Request, res: Response) => {
       const requestBaseUrl = `${req.protocol}://${req.get("host")}`;
       const resourceUrl = config.resource ?? new URL("/", requestBaseUrl).href;