feature(auth): OAuthClientProvider.delegateAuthorization

An optional method that clients can use whenever the authorization should be delegated to an existing implementation.

Author: m-paternostro

src/client/auth.test.ts

@@ -1045,5 +1045,156 @@ describe("OAuth Authorization", () => {
         "https://different-resource.example.com/mcp-server"
       );
     });
+
+    describe("delegateAuthorization", () => {
+      const validMetadata = {
+        issuer: "https://auth.example.com",
+        authorization_endpoint: "https://auth.example.com/authorize",
+        token_endpoint: "https://auth.example.com/token",
+        registration_endpoint: "https://auth.example.com/register",
+        response_types_supported: ["code"],
+        code_challenge_methods_supported: ["S256"],
+      };
+
+      const validClientInfo = {
+        client_id: "client123",
+        client_secret: "secret123",
+        redirect_uris: ["http://localhost:3000/callback"],
+        client_name: "Test Client",
+      };
+
+      const validTokens = {
+        access_token: "access123",
+        token_type: "Bearer",
+        expires_in: 3600,
+        refresh_token: "refresh123",
+      };
+
+      // Setup shared mock function for all tests
+      beforeEach(() => {
+        // Reset mockFetch implementation
+        mockFetch.mockReset();
+
+        // Set up the mockFetch to respond to all necessary API calls
+        mockFetch.mockImplementation((url) => {
+          const urlString = url.toString();
+
+          if (urlString.includes("/.well-known/oauth-protected-resource")) {
+            return Promise.resolve({
+              ok: false,
+              status: 404
+            });
+          } else if (urlString.includes("/.well-known/oauth-authorization-server")) {
+            return Promise.resolve({
+              ok: true,
+              status: 200,
+              json: async () => validMetadata
+            });
+          } else if (urlString.includes("/token")) {
+            return Promise.resolve({
+              ok: true,
+              status: 200,
+              json: async () => validTokens
+            });
+          }
+
+          return Promise.reject(new Error(`Unexpected fetch call: ${urlString}`));
+        });
+      });
+
+      it("should use delegateAuthorization when implemented and return AUTHORIZED", async () => {
+        const mockProvider: OAuthClientProvider = {
+          redirectUrl: "http://localhost:3000/callback",
+          clientMetadata: {
+            redirect_uris: ["http://localhost:3000/callback"],
+            client_name: "Test Client"
+          },
+          clientInformation: () => validClientInfo,
+          tokens: () => validTokens,
+          saveTokens: jest.fn(),
+          redirectToAuthorization: jest.fn(),
+          saveCodeVerifier: jest.fn(),
+          codeVerifier: () => "test_verifier",
+          delegateAuthorization: jest.fn().mockResolvedValue("AUTHORIZED")
+        };
+
+        const result = await auth(mockProvider, { serverUrl: "https://auth.example.com" });
+
+        expect(result).toBe("AUTHORIZED");
+        expect(mockProvider.delegateAuthorization).toHaveBeenCalledWith(
+          "https://auth.example.com",
+          expect.objectContaining(validMetadata)
+        );
+        expect(mockProvider.redirectToAuthorization).not.toHaveBeenCalled();
+      });
+
+      it("should fall back to standard flow when delegateAuthorization returns undefined", async () => {
+        const mockProvider: OAuthClientProvider = {
+          redirectUrl: "http://localhost:3000/callback",
+          clientMetadata: {
+            redirect_uris: ["http://localhost:3000/callback"],
+            client_name: "Test Client"
+          },
+          clientInformation: () => validClientInfo,
+          tokens: () => validTokens,
+          saveTokens: jest.fn(),
+          redirectToAuthorization: jest.fn(),
+          saveCodeVerifier: jest.fn(),
+          codeVerifier: () => "test_verifier",
+          delegateAuthorization: jest.fn().mockResolvedValue(undefined)
+        };
+
+        const result = await auth(mockProvider, { serverUrl: "https://auth.example.com" });
+
+        expect(result).toBe("AUTHORIZED");
+        expect(mockProvider.delegateAuthorization).toHaveBeenCalled();
+        expect(mockProvider.saveTokens).toHaveBeenCalled();
+      });
+
+      it("should not call delegateAuthorization when processing authorizationCode", async () => {
+        const mockProvider: OAuthClientProvider = {
+          redirectUrl: "http://localhost:3000/callback",
+          clientMetadata: {
+            redirect_uris: ["http://localhost:3000/callback"],
+            client_name: "Test Client"
+          },
+          clientInformation: () => validClientInfo,
+          tokens: jest.fn(),
+          saveTokens: jest.fn(),
+          redirectToAuthorization: jest.fn(),
+          saveCodeVerifier: jest.fn(),
+          codeVerifier: () => "test_verifier",
+          delegateAuthorization: jest.fn()
+        };
+
+        await auth(mockProvider, {
+          serverUrl: "https://auth.example.com",
+          authorizationCode: "code123"
+        });
+
+        expect(mockProvider.delegateAuthorization).not.toHaveBeenCalled();
+        expect(mockProvider.saveTokens).toHaveBeenCalled();
+      });
+
+      it("should propagate errors from delegateAuthorization", async () => {
+        const mockProvider: OAuthClientProvider = {
+          redirectUrl: "http://localhost:3000/callback",
+          clientMetadata: {
+            redirect_uris: ["http://localhost:3000/callback"],
+            client_name: "Test Client"
+          },
+          clientInformation: () => validClientInfo,
+          tokens: jest.fn(),
+          saveTokens: jest.fn(),
+          redirectToAuthorization: jest.fn(),
+          saveCodeVerifier: jest.fn(),
+          codeVerifier: () => "test_verifier",
+          delegateAuthorization: jest.fn().mockRejectedValue(new Error("Delegation failed"))
+        };
+
+        await expect(auth(mockProvider, { serverUrl: "https://auth.example.com" }))
+          .rejects.toThrow("Delegation failed");
+      });
+    });
   });
 });

src/client/auth.ts

@@ -81,6 +81,30 @@ export interface OAuthClientProvider {
    * Implementations must verify the returned resource matches the MCP server.
    */
   validateResourceURL?(serverUrl: string | URL, resource?: string): Promise<URL | undefined>;
+
+  /**
+   * Optional method that allows the OAuth client to delegate authorization
+   * to an existing implementation, such as a platform or app-level identity provider.
+   *
+   * If this method returns "AUTHORIZED", the standard authorization flow will be bypassed.
+   * If it returns `undefined`, the SDK will proceed with its default OAuth implementation.
+   *
+   * When returning "AUTHORIZED", the implementation must ensure tokens have been saved
+   * through the provider's saveTokens method, or are accessible via the tokens() method.
+   *
+   * This method is useful when the host application already manages OAuth tokens or user sessions
+   * and does not need the SDK to handle the entire authorization flow directly.
+   *
+   * For example, in a mobile app, this could delegate to the native platform authentication,
+   * or in a browser application, it could use existing tokens from localStorage.
+   *
+   * Note: This method will NOT be called when processing an authorization code callback.
+   *
+   * @param serverUrl The URL of the authorization server.
+   * @param metadata The OAuth metadata if available.
+   * @returns "AUTHORIZED" if delegation succeeded and tokens are already available; otherwise `undefined`.
+   */
+  delegateAuthorization?(serverUrl: string | URL, metadata: OAuthMetadata | undefined): "AUTHORIZED" | undefined | Promise<"AUTHORIZED" | undefined>;
 }
 
 export type AuthResult = "AUTHORIZED" | "REDIRECT";
@@ -124,6 +148,14 @@ export async function auth(
 
   const metadata = await discoverOAuthMetadata(authorizationServerUrl);
 
+  // Delegate the authorization if supported and if not already in the middle of the standard flow
+  if (provider.delegateAuthorization && authorizationCode === undefined) {
+    const result = await provider.delegateAuthorization(authorizationServerUrl, metadata);
+    if (result === "AUTHORIZED") {
+      return "AUTHORIZED";
+    }
+  }
+
   // Handle client registration if needed
   let clientInformation = await Promise.resolve(provider.clientInformation());
   if (!clientInformation) {