Support upscoping on insufficient_scope 403

src/client/auth.test.ts

@@ -95,6 +95,16 @@ describe('OAuth Authorization', () => {
 
             expect(extractWWWAuthenticateParams(mockResponse)).toEqual({ scope: scope });
         });
+
+        it('returns error when present', async () => {
+            const mockResponse = {
+                headers: {
+                    get: vi.fn(name => (name === 'WWW-Authenticate' ? `Bearer error="insufficient_scope", scope="admin"` : null))
+                }
+            } as unknown as Response;
+
+            expect(extractWWWAuthenticateParams(mockResponse)).toEqual({ error: 'insufficient_scope', scope: 'admin' });
+        });
     });
 
     describe('discoverOAuthProtectedResourceMetadata', () => {

src/client/auth.ts

@@ -522,9 +522,9 @@ export async function selectResourceURL(
 }
 
 /**
- * Extract resource_metadata and scope from WWW-Authenticate header.
+ * Extract resource_metadata, scope, and error from WWW-Authenticate header.
  */
-export function extractWWWAuthenticateParams(res: Response): { resourceMetadataUrl?: URL; scope?: string } {
+export function extractWWWAuthenticateParams(res: Response): { resourceMetadataUrl?: URL; scope?: string; error?: string } {
     const authenticateHeader = res.headers.get('WWW-Authenticate');
     if (!authenticateHeader) {
         return {};
@@ -535,29 +535,51 @@ export function extractWWWAuthenticateParams(res: Response): { resourceMetadataU
         return {};
     }
 
-    const resourceMetadataRegex = /resource_metadata="([^"]*)"/;
-    const resourceMetadataMatch = resourceMetadataRegex.exec(authenticateHeader);
-
-    const scopeRegex = /scope="([^"]*)"/;
-    const scopeMatch = scopeRegex.exec(authenticateHeader);
+    const resourceMetadataMatch = extractFieldFromWwwAuth(res, 'resource_metadata') || undefined;
 
     let resourceMetadataUrl: URL | undefined;
     if (resourceMetadataMatch) {
         try {
-            resourceMetadataUrl = new URL(resourceMetadataMatch[1]);
+            resourceMetadataUrl = new URL(resourceMetadataMatch);
         } catch {
             // Ignore invalid URL
         }
     }
 
-    const scope = scopeMatch?.[1] || undefined;
+    const scope = extractFieldFromWwwAuth(res, 'scope') || undefined;
+    const error = extractFieldFromWwwAuth(res, 'error') || undefined;
 
     return {
         resourceMetadataUrl,
-        scope
+        scope,
+        error
     };
 }
 
+/**
+ * Extracts a specific field's value from the WWW-Authenticate header string.
+ *
+ * @param response The HTTP response object containing the headers.
+ * @param fieldName The name of the field to extract (e.g., "realm", "nonce").
+ * @returns The field value
+ */
+function extractFieldFromWwwAuth(response: Response, fieldName: string): string | null {
+    const wwwAuthHeader = response.headers.get('WWW-Authenticate');
+    if (!wwwAuthHeader) {
+        return null;
+    }
+
+    const pattern = new RegExp(`${fieldName}=(?:"([^"]+)"|([^\\s,]+))`);
+    const match = wwwAuthHeader.match(pattern);
+
+    if (match) {
+        // Pattern matches: field_name="value" or field_name=value (unquoted)
+        return match[1] || match[2];
+    }
+
+    return null;
+}
+
 /**
  * Extract resource_metadata from response header.
  * @deprecated Use `extractWWWAuthenticateParams` instead.

src/client/streamableHttp.test.ts

@@ -593,6 +593,98 @@ describe('StreamableHTTPClientTransport', () => {
         expect(mockAuthProvider.redirectToAuthorization.mock.calls).toHaveLength(1);
     });
 
+    it('attempts upscoping on 403 with WWW-Authenticate header', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'test',
+            params: {},
+            id: 'test-id'
+        };
+
+        const fetchMock = global.fetch as Mock;
+        fetchMock
+            // First call: returns 403 with insufficient_scope
+            .mockResolvedValueOnce({
+                ok: false,
+                status: 403,
+                statusText: 'Forbidden',
+                headers: new Headers({
+                    'WWW-Authenticate':
+                        'Bearer error="insufficient_scope", scope="new_scope", resource_metadata="http://example.com/resource"'
+                }),
+                text: () => Promise.resolve('Insufficient scope')
+            })
+            // Second call: successful after upscoping
+            .mockResolvedValueOnce({
+                ok: true,
+                status: 202,
+                headers: new Headers()
+            });
+
+        // Spy on the imported auth function and mock successful authorization
+        const authModule = await import('./auth.js');
+        const authSpy = vi.spyOn(authModule, 'auth');
+        authSpy.mockResolvedValue('AUTHORIZED');
+
+        await transport.send(message);
+
+        // Verify fetch was called twice
+        expect(fetchMock).toHaveBeenCalledTimes(2);
+
+        // Verify auth was called with the new scope
+        expect(authSpy).toHaveBeenCalledWith(
+            mockAuthProvider,
+            expect.objectContaining({
+                scope: 'new_scope',
+                resourceMetadataUrl: new URL('http://example.com/resource')
+            })
+        );
+
+        authSpy.mockRestore();
+    });
+
+    it('prevents infinite upscoping on repeated 403', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'test',
+            params: {},
+            id: 'test-id'
+        };
+
+        // Mock fetch calls to always return 403 with insufficient_scope
+        const fetchMock = global.fetch as Mock;
+        fetchMock.mockResolvedValue({
+            ok: false,
+            status: 403,
+            statusText: 'Forbidden',
+            headers: new Headers({
+                'WWW-Authenticate': 'Bearer error="insufficient_scope", scope="new_scope"'
+            }),
+            text: () => Promise.resolve('Insufficient scope')
+        });
+
+        // Spy on the imported auth function and mock successful authorization
+        const authModule = await import('./auth.js');
+        const authSpy = vi.spyOn(authModule, 'auth');
+        authSpy.mockResolvedValue('AUTHORIZED');
+
+        // First send: should trigger upscoping
+        await expect(transport.send(message)).rejects.toThrow('Server returned 403 after trying upscoping');
+
+        expect(fetchMock).toHaveBeenCalledTimes(2); // Initial call + one retry after auth
+        expect(authSpy).toHaveBeenCalledTimes(1); // Auth called once
+
+        // Second send: should fail immediately without re-calling auth
+        fetchMock.mockClear();
+        authSpy.mockClear();
+        await expect(transport.send(message)).rejects.toThrow('Server returned 403 after trying upscoping');
+
+        expect(fetchMock).toHaveBeenCalledTimes(1); // Only one fetch call
+        expect(authSpy).not.toHaveBeenCalled(); // Auth not called again
+
+        authSpy.mockRestore();
+    });
+
     describe('Reconnection Logic', () => {
         let transport: StreamableHTTPClientTransport;
 

src/client/streamableHttp.ts

@@ -134,6 +134,7 @@ export class StreamableHTTPClientTransport implements Transport {
     private _reconnectionOptions: StreamableHTTPReconnectionOptions;
     private _protocolVersion?: string;
     private _hasCompletedAuthFlow = false; // Circuit breaker: detect auth success followed by immediate 401
+    private _lastUpscopingHeader?: string; // Track last upscoping header to prevent infinite upscoping.
 
     onclose?: () => void;
     onerror?: (error: Error) => void;
@@ -452,12 +453,49 @@ export class StreamableHTTPClientTransport implements Transport {
                     return this.send(message);
                 }
 
+                if (response.status === 403 && this._authProvider) {
+                    const { resourceMetadataUrl, scope, error } = extractWWWAuthenticateParams(response);
+
+                    if (error === 'insufficient_scope') {
+                        const wwwAuthHeader = response.headers.get('WWW-Authenticate');
+
+                        // Check if we've already tried upscoping with this header to prevent infinite loops.
+                        if (this._lastUpscopingHeader === wwwAuthHeader) {
+                            throw new StreamableHTTPError(403, 'Server returned 403 after trying upscoping');
+                        }
+
+                        if (scope) {
+                            this._scope = scope;
+                        }
+
+                        if (resourceMetadataUrl) {
+                            this._resourceMetadataUrl = resourceMetadataUrl;
+                        }
+
+                        // Mark that upscoping was tried.
+                        this._lastUpscopingHeader = wwwAuthHeader ?? undefined;
+                        const result = await auth(this._authProvider, {
+                            serverUrl: this._url,
+                            resourceMetadataUrl: this._resourceMetadataUrl,
+                            scope: this._scope,
+                            fetchFn: this._fetch
+                        });
+
+                        if (result !== 'AUTHORIZED') {
+                            throw new UnauthorizedError();
+                        }
+
+                        return this.send(message);
+                    }
+                }
+
                 const text = await response.text().catch(() => null);
                 throw new Error(`Error POSTing to endpoint (HTTP ${response.status}): ${text}`);
             }
 
             // Reset auth loop flag on successful response
             this._hasCompletedAuthFlow = false;
+            this._lastUpscopingHeader = undefined;
 
             // If the response is 202 Accepted, there's no body to process
             if (response.status === 202) {