diff --git a/packages/core/src/mcp/oauth-utils.test.ts b/packages/core/src/mcp/oauth-utils.test.ts
index 6dab62a3386..8d7b16aaf48 100644
--- a/packages/core/src/mcp/oauth-utils.test.ts
+++ b/packages/core/src/mcp/oauth-utils.test.ts
@@ -300,6 +300,47 @@ describe('OAuthUtils', () => {
         scopes: ['read', 'write'],
       });
     });
+
+    it('should validate against root resource when path metadata falls back to root discovery', async () => {
+      mockFetch
+        // path-based protected resource metadata is unavailable
+        .mockResolvedValueOnce({
+          ok: false,
+        })
+        // root-based protected resource metadata succeeds
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              resource: 'https://example.com',
+              authorization_servers: ['https://auth.example.com'],
+              bearer_methods_supported: ['header'],
+            }),
+        })
+        // discoverAuthorizationServerMetadata
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockAuthServerMetadata),
+        });
+
+      await expect(
+        OAuthUtils.discoverOAuthConfig('https://example.com/mcp'),
+      ).resolves.toEqual({
+        authorizationUrl: 'https://auth.example.com/authorize',
+        issuer: 'https://auth.example.com',
+        tokenUrl: 'https://auth.example.com/token',
+        scopes: ['read', 'write'],
+      });
+
+      expect(mockFetch).nthCalledWith(
+        1,
+        'https://example.com/.well-known/oauth-protected-resource/mcp',
+      );
+      expect(mockFetch).nthCalledWith(
+        2,
+        'https://example.com/.well-known/oauth-protected-resource',
+      );
+    });
   });
 
   describe('metadataToOAuthConfig', () => {
@@ -401,6 +442,36 @@ describe('OAuthUtils', () => {
         scopes: ['read', 'write'],
       });
     });
+
+    it('should validate against resource_metadata URL instead of MCP server path', async () => {
+      mockFetch
+        // fetchProtectedResourceMetadata(resource_metadata URL)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              resource: 'https://example.com',
+              authorization_servers: ['https://auth.example.com'],
+            }),
+        })
+        // discoverAuthorizationServerMetadata(auth server well-known URL)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockAuthServerMetadata),
+        });
+
+      await expect(
+        OAuthUtils.discoverOAuthFromWWWAuthenticate(
+          'Bearer realm="example", resource_metadata="https://example.com/.well-known/oauth-protected-resource"',
+          'https://example.com/mcp',
+        ),
+      ).resolves.toEqual({
+        authorizationUrl: 'https://auth.example.com/authorize',
+        issuer: 'https://auth.example.com',
+        tokenUrl: 'https://auth.example.com/token',
+        scopes: ['read', 'write'],
+      });
+    });
   });
 
   describe('extractBaseUrl', () => {
@@ -471,6 +542,40 @@ describe('OAuthUtils', () => {
     });
   });
 
+  describe('buildResourceParameterFromMetadataUrl', () => {
+    it('should infer a root resource from a root protected resource metadata URL', () => {
+      const result = OAuthUtils.buildResourceParameterFromMetadataUrl(
+        'https://example.com/.well-known/oauth-protected-resource',
+      );
+
+      expect(result).toBe('https://example.com/');
+    });
+
+    it('should infer a path resource from a path-based protected resource metadata URL', () => {
+      const result = OAuthUtils.buildResourceParameterFromMetadataUrl(
+        'https://example.com/.well-known/oauth-protected-resource/mcp/v1',
+      );
+
+      expect(result).toBe('https://example.com/mcp/v1');
+    });
+
+    it('should reject non-protected-resource metadata URLs', () => {
+      expect(() =>
+        OAuthUtils.buildResourceParameterFromMetadataUrl(
+          'https://example.com/.well-known/oauth-authorization-server',
+        ),
+      ).toThrow(/Invalid protected resource metadata URL/);
+    });
+
+    it('should reject protocol-relative resource paths', () => {
+      expect(() =>
+        OAuthUtils.buildResourceParameterFromMetadataUrl(
+          'https://example.com/.well-known/oauth-protected-resource//attacker.com/api',
+        ),
+      ).toThrow(/Invalid protected resource metadata URL/);
+    });
+  });
+
   describe('parseTokenExpiry', () => {
     it('should return the expiry time in milliseconds for a valid token', () => {
       // Corresponds to a date of 2100-01-01T00:00:00Z
diff --git a/packages/core/src/mcp/oauth-utils.ts b/packages/core/src/mcp/oauth-utils.ts
index 12ab2bd9ff6..26dc7c9b0bf 100644
--- a/packages/core/src/mcp/oauth-utils.ts
+++ b/packages/core/src/mcp/oauth-utils.ts
@@ -87,6 +87,42 @@ export class OAuthUtils {
     };
   }
 
+  /**
+   * Extract the resource identifier prefix represented by a protected resource metadata URL.
+   *
+   * RFC 9728 §3.1 constructs metadata URLs by inserting
+   * /.well-known/oauth-protected-resource between the origin and resource path.
+   * RFC 9728 §7.3 then validates the resource metadata against that prefix.
+   *
+   * @param metadataUrl The protected resource metadata URL
+   * @returns The resource identifier prefix represented by the metadata URL
+   */
+  static buildResourceParameterFromMetadataUrl(metadataUrl: string): string {
+    const url = new URL(metadataUrl);
+    const wellKnownPrefix = '/.well-known/oauth-protected-resource';
+
+    if (
+      url.pathname !== wellKnownPrefix &&
+      !url.pathname.startsWith(`${wellKnownPrefix}/`)
+    ) {
+      throw new Error(
+        `Invalid protected resource metadata URL: ${metadataUrl}`,
+      );
+    }
+
+    const resourcePath = url.pathname.slice(wellKnownPrefix.length) || '/';
+    if (resourcePath.startsWith('//')) {
+      throw new Error(
+        `Invalid protected resource metadata URL: ${metadataUrl}`,
+      );
+    }
+
+    url.pathname = resourcePath;
+    url.search = '';
+    url.hash = '';
+    return this.buildResourceParameter(url.toString());
+  }
+
   /**
    * Fetch OAuth protected resource metadata.
    *
@@ -236,9 +272,9 @@ export class OAuthUtils {
       // RFC 9728 §3.1: Construct well-known URL by inserting /.well-known/oauth-protected-resource
       // between the host and path. This is the RFC-compliant approach.
       const wellKnownUrls = this.buildWellKnownUrls(serverUrl);
-      let resourceMetadata = await this.fetchProtectedResourceMetadata(
-        wellKnownUrls.protectedResource,
-      );
+      let resourceMetadataUrl = wellKnownUrls.protectedResource;
+      let resourceMetadata =
+        await this.fetchProtectedResourceMetadata(resourceMetadataUrl);
 
       // Fallback: If path-based discovery fails and we have a path, try root-based discovery
       // for backwards compatibility with servers that don't implement RFC 9728 path handling
@@ -249,6 +285,9 @@ export class OAuthUtils {
           resourceMetadata = await this.fetchProtectedResourceMetadata(
             rootBasedUrls.protectedResource,
           );
+          if (resourceMetadata) {
+            resourceMetadataUrl = rootBasedUrls.protectedResource;
+          }
         }
       }
 
@@ -256,7 +295,8 @@ export class OAuthUtils {
         // RFC 9728 Section 7.3: The client MUST ensure that the resource identifier URL
         // it is using as the prefix for the metadata request exactly matches the value
         // of the resource metadata parameter in the protected resource metadata document.
-        const expectedResource = this.buildResourceParameter(serverUrl);
+        const expectedResource =
+          this.buildResourceParameterFromMetadataUrl(resourceMetadataUrl);
         if (
           !this.isEquivalentResourceIdentifier(
             resourceMetadata.resource,
@@ -352,7 +392,8 @@ export class OAuthUtils {
 
     if (resourceMetadata && mcpServerUrl) {
       // Validate resource parameter per RFC 9728 Section 7.3
-      const expectedResource = this.buildResourceParameter(mcpServerUrl);
+      const expectedResource =
+        this.buildResourceParameterFromMetadataUrl(resourceMetadataUri);
       if (
         !this.isEquivalentResourceIdentifier(
           resourceMetadata.resource,