diff --git a/src/client/auth.test.ts b/src/client/auth.test.ts
index fc71b03d9..8124fe768 100644
--- a/src/client/auth.test.ts
+++ b/src/client/auth.test.ts
@@ -1585,10 +1585,81 @@ describe('OAuth Authorization', () => {
             // First call should be to protected resource metadata
             expect(mockFetch.mock.calls[0][0].toString()).toBe('https://resource.example.com/.well-known/oauth-protected-resource');
 
-            // Second call should be to oauth metadata
+            // Second call should be to oauth metadata at the root path
             expect(mockFetch.mock.calls[1][0].toString()).toBe('https://resource.example.com/.well-known/oauth-authorization-server');
         });
 
+        it('uses base URL (with root path) as authorization server when protected-resource-metadata discovery fails', async () => {
+            // Setup: First call to protected resource metadata fails (404)
+            // When no authorization_servers are found in protected resource metadata,
+            // the auth server URL should be set to the base URL with "/" path
+            let callCount = 0;
+            mockFetch.mockImplementation(url => {
+                callCount++;
+
+                const urlString = url.toString();
+
+                if (urlString.includes('/.well-known/oauth-protected-resource')) {
+                    // Protected resource metadata discovery attempts (both path-aware and root) fail with 404
+                    return Promise.resolve({
+                        ok: false,
+                        status: 404
+                    });
+                } else if (urlString === 'https://resource.example.com/.well-known/oauth-authorization-server') {
+                    // Should fetch from base URL with root path, not the full serverUrl path
+                    return Promise.resolve({
+                        ok: true,
+                        status: 200,
+                        json: async () => ({
+                            issuer: 'https://resource.example.com/',
+                            authorization_endpoint: 'https://resource.example.com/authorize',
+                            token_endpoint: 'https://resource.example.com/token',
+                            registration_endpoint: 'https://resource.example.com/register',
+                            response_types_supported: ['code'],
+                            code_challenge_methods_supported: ['S256']
+                        })
+                    });
+                } else if (urlString.includes('/register')) {
+                    // Client registration succeeds
+                    return Promise.resolve({
+                        ok: true,
+                        status: 200,
+                        json: async () => ({
+                            client_id: 'test-client-id',
+                            client_secret: 'test-client-secret',
+                            client_id_issued_at: 1612137600,
+                            client_secret_expires_at: 1612224000,
+                            redirect_uris: ['http://localhost:3000/callback'],
+                            client_name: 'Test Client'
+                        })
+                    });
+                }
+
+                return Promise.reject(new Error(`Unexpected fetch call #${callCount}: ${urlString}`));
+            });
+
+            // Mock provider methods
+            (mockProvider.clientInformation as jest.Mock).mockResolvedValue(undefined);
+            (mockProvider.tokens as jest.Mock).mockResolvedValue(undefined);
+            mockProvider.saveClientInformation = jest.fn();
+
+            // Call the auth function with a server URL that has a path
+            const result = await auth(mockProvider, {
+                serverUrl: 'https://resource.example.com/path/to/server'
+            });
+
+            // Verify the result
+            expect(result).toBe('REDIRECT');
+
+            // Verify that the oauth-authorization-server call uses the base URL
+            // This proves the fix: using new URL("/", serverUrl) instead of serverUrl
+            const authServerCall = mockFetch.mock.calls.find(call =>
+                call[0].toString().includes('/.well-known/oauth-authorization-server')
+            );
+            expect(authServerCall).toBeDefined();
+            expect(authServerCall[0].toString()).toBe('https://resource.example.com/.well-known/oauth-authorization-server');
+        });
+
         it('passes resource parameter through authorization flow', async () => {
             // Mock successful metadata discovery - need to include protected resource metadata
             mockFetch.mockImplementation(url => {
diff --git a/src/client/auth.ts b/src/client/auth.ts
index 6d4ede84b..fba0e7bf7 100644
--- a/src/client/auth.ts
+++ b/src/client/auth.ts
@@ -348,6 +348,7 @@ async function authInternal(
 ): Promise<AuthResult> {
     let resourceMetadata: OAuthProtectedResourceMetadata | undefined;
     let authorizationServerUrl: string | URL | undefined;
+
     try {
         resourceMetadata = await discoverOAuthProtectedResourceMetadata(serverUrl, { resourceMetadataUrl }, fetchFn);
         if (resourceMetadata.authorization_servers && resourceMetadata.authorization_servers.length > 0) {
@@ -359,10 +360,10 @@ async function authInternal(
 
     /**
      * If we don't get a valid authorization server metadata from protected resource metadata,
-     * fallback to the legacy MCP spec's implementation (version 2025-03-26): MCP server acts as the Authorization server.
+     * fallback to the legacy MCP spec's implementation (version 2025-03-26): MCP server base URL acts as the Authorization server.
      */
     if (!authorizationServerUrl) {
-        authorizationServerUrl = serverUrl;
+        authorizationServerUrl = new URL('/', serverUrl);
     }
 
     const resource: URL | undefined = await selectResourceURL(serverUrl, provider, resourceMetadata);