mcp: Add client-side scope accumulation

auth/authorization_code.go

@@ -128,6 +128,8 @@ type AuthorizationCodeHandler struct {
 
 	// tokenSource is the token source to use for authorization.
 	tokenSource oauth2.TokenSource
+
+	grantedScopes map[string][]string
 }
 
 var _ OAuthHandler = (*AuthorizationCodeHandler)(nil)
@@ -187,7 +189,10 @@ func NewAuthorizationCodeHandler(config *AuthorizationCodeHandlerConfig) (*Autho
 	if config.Client == nil {
 		config.Client = http.DefaultClient
 	}
-	return &AuthorizationCodeHandler{config: config}, nil
+	return &AuthorizationCodeHandler{
+		config:        config,
+		grantedScopes: make(map[string][]string),
+	}, nil
 }
 
 func isNonRootHTTPSURL(u string) bool {
@@ -275,19 +280,24 @@ func (h *AuthorizationCodeHandler) Authorize(ctx context.Context, req *http.Requ
 		return err
 	}
 
-	scps := scopesFromChallenges(wwwChallenges)
-	if len(scps) == 0 && len(prm.ScopesSupported) > 0 {
-		scps = prm.ScopesSupported
+	requestedScopes := scopesFromChallenges(wwwChallenges)
+	if len(requestedScopes) == 0 && len(prm.ScopesSupported) > 0 {
+		requestedScopes = prm.ScopesSupported
 	}
 
 	// SEP-2207: when the client desires refresh tokens and the Authorization
 	// Server advertises offline_access support, add it to the requested scopes.
 	if h.config.RequestRefreshToken &&
 		slices.Contains(asm.ScopesSupported, "offline_access") &&
-		!slices.Contains(scps, "offline_access") {
-		scps = append(scps, "offline_access")
+		!slices.Contains(requestedScopes, "offline_access") {
+		requestedScopes = append(requestedScopes, "offline_access")
 	}
 
+	// Accumulate scopes: union previously granted scopes with the newly
+	// challenged scopes so that step-up authorization does not lose
+	// permissions granted in earlier rounds (SEP-2350).
+	requestedScopes = UnionScopes(h.grantedScopes[asm.Issuer], requestedScopes)
+
 	cfg := &oauth2.Config{
 		ClientID:     resolvedClientConfig.clientID,
 		ClientSecret: resolvedClientConfig.clientSecret,
@@ -298,7 +308,7 @@ func (h *AuthorizationCodeHandler) Authorize(ctx context.Context, req *http.Requ
 			AuthStyle: resolvedClientConfig.authStyle,
 		},
 		RedirectURL: h.config.RedirectURL,
-		Scopes:      scps,
+		Scopes:      requestedScopes,
 	}
 
 	authRes, err := h.getAuthorizationCode(ctx, cfg, prm.Resource)
@@ -307,7 +317,12 @@ func (h *AuthorizationCodeHandler) Authorize(ctx context.Context, req *http.Requ
 		return err
 	}
 
-	return h.exchangeAuthorizationCode(ctx, cfg, authRes, prm.Resource)
+	err = h.exchangeAuthorizationCode(ctx, cfg, authRes, prm.Resource)
+	if err != nil {
+		return err
+	}
+
+	return h.updateGrantedScopes(asm.Issuer, requestedScopes)
 }
 
 // resourceMetadataURLFromChallenges returns a resource metadata URL from the given "WWW-Authenticate" header challenges,
@@ -558,3 +573,20 @@ func (h *AuthorizationCodeHandler) exchangeAuthorizationCode(ctx context.Context
 	h.tokenSource = cfg.TokenSource(clientCtx, token)
 	return nil
 }
+
+// updateGrantedScopes updates the granted scopes based on the token source and requested scopes
+func (h *AuthorizationCodeHandler) updateGrantedScopes(issuer string, requestedScopes []string) error {
+	if h.tokenSource == nil {
+		return nil
+	}
+	tok, err := h.tokenSource.Token()
+	if err != nil {
+		return err
+	}
+	if tokenScopes := ScopesFromToken(tok); tokenScopes == nil {
+		h.grantedScopes[issuer] = requestedScopes
+	} else {
+		h.grantedScopes[issuer] = tokenScopes
+	}
+	return nil
+}

auth/authorization_code_test.go

@@ -16,6 +16,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/modelcontextprotocol/go-sdk/internal/oauthtest"
 	"github.com/modelcontextprotocol/go-sdk/oauthex"
 	"golang.org/x/oauth2"
@@ -113,6 +114,131 @@ func TestAuthorize(t *testing.T) {
 	}
 }
 
+func TestAuthorize_ScopeAccumulation(t *testing.T) {
+	authServer := oauthtest.NewFakeAuthorizationServer(oauthtest.Config{
+		RegistrationConfig: &oauthtest.RegistrationConfig{
+			PreregisteredClients: map[string]oauthtest.ClientInfo{
+				"test_client_id": {
+					Secret:       "test_client_secret",
+					RedirectURIs: []string{"http://localhost:12345/callback"},
+				},
+			},
+		},
+		TokenScopeFunc: func(requestedScope string) string {
+			// Simulate a server that never grants "write".
+			var granted []string
+			for _, s := range strings.Fields(requestedScope) {
+				if s != "write" {
+					granted = append(granted, s)
+				}
+			}
+			return strings.Join(granted, " ")
+		},
+	})
+	authServer.Start(t)
+
+	resourceMux := http.NewServeMux()
+	resourceServer := httptest.NewServer(resourceMux)
+	t.Cleanup(resourceServer.Close)
+	resourceURL := resourceServer.URL + "/resource"
+
+	resourceMux.Handle("/.well-known/oauth-protected-resource/resource", ProtectedResourceMetadataHandler(&oauthex.ProtectedResourceMetadata{
+		Resource:             resourceURL,
+		AuthorizationServers: []string{authServer.URL()},
+	}))
+
+	var capturedAuthURLs []string
+	noRedirectClient := &http.Client{
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
+
+	handler, err := NewAuthorizationCodeHandler(&AuthorizationCodeHandlerConfig{
+		RedirectURL: "http://localhost:12345/callback",
+		PreregisteredClient: &oauthex.ClientCredentials{
+			ClientID: "test_client_id",
+			ClientSecretAuth: &oauthex.ClientSecretAuth{
+				ClientSecret: "test_client_secret",
+			},
+		},
+		AuthorizationCodeFetcher: func(ctx context.Context, args *AuthorizationArgs) (*AuthorizationResult, error) {
+			capturedAuthURLs = append(capturedAuthURLs, args.URL)
+			resp, err := noRedirectClient.Get(args.URL)
+			if err != nil {
+				return nil, err
+			}
+			defer resp.Body.Close()
+			loc, err := resp.Location()
+			if err != nil {
+				return nil, err
+			}
+			return &AuthorizationResult{
+				Code:  loc.Query().Get("code"),
+				State: loc.Query().Get("state"),
+			}, nil
+		},
+	})
+	if err != nil {
+		t.Fatalf("NewAuthorizationCodeHandler failed: %v", err)
+	}
+
+	// First authorization: 401 with scope="read write".
+	// The token response will only grant "read" (TokenScopeFunc strips "write").
+	req := httptest.NewRequest(http.MethodGet, resourceURL, nil)
+	resp := &http.Response{
+		StatusCode: http.StatusUnauthorized,
+		Header:     make(http.Header),
+		Body:       http.NoBody,
+	}
+	resp.Header.Set("WWW-Authenticate",
+		fmt.Sprintf(`Bearer scope="read write", resource_metadata="%s/.well-known/oauth-protected-resource/resource"`, resourceServer.URL))
+	if err := handler.Authorize(context.Background(), req, resp); err != nil {
+		t.Fatalf("First Authorize failed: %v", err)
+	}
+
+	// Verify first auth URL requested "read" and "write".
+	firstURL, err := url.Parse(capturedAuthURLs[0])
+	if err != nil {
+		t.Fatalf("Failed to parse first auth URL: %v", err)
+	}
+	firstScopes := strings.Fields(firstURL.Query().Get("scope"))
+	if diff := cmp.Diff([]string{"read", "write"}, firstScopes, cmpopts.SortSlices(func(a, b string) bool { return a < b })); diff != "" {
+		t.Errorf("First auth scopes mismatch (-want +got):\n%s", diff)
+	}
+
+	// Verify only "read" was granted (the token omitted "write").
+	issuer := authServer.URL()
+	if diff := cmp.Diff([]string{"read"}, handler.grantedScopes[issuer], cmpopts.SortSlices(func(a, b string) bool { return a < b })); diff != "" {
+		t.Errorf("After first Authorize, grantedScopes mismatch (-want +got):\n%s", diff)
+	}
+
+	// Second authorization: 403 insufficient_scope with scope="admin".
+	// Accumulated scopes should be "read" (previously granted) + "admin" (new).
+	req2 := httptest.NewRequest(http.MethodGet, resourceURL, nil)
+	resp2 := &http.Response{
+		StatusCode: http.StatusForbidden,
+		Header:     make(http.Header),
+		Body:       http.NoBody,
+	}
+	resp2.Header.Set("WWW-Authenticate",
+		fmt.Sprintf(`Bearer error="insufficient_scope", scope="admin", resource_metadata="%s/.well-known/oauth-protected-resource/resource"`, resourceServer.URL))
+	if err := handler.Authorize(context.Background(), req2, resp2); err != nil {
+		t.Fatalf("Second Authorize failed: %v", err)
+	}
+
+	// Verify second auth URL accumulated "read" (granted) + "admin" (challenged),
+	// but NOT "write" (requested but never granted).
+	secondURL, err := url.Parse(capturedAuthURLs[1])
+	if err != nil {
+		t.Fatalf("Failed to parse second auth URL: %v", err)
+	}
+	secondScopes := strings.Fields(secondURL.Query().Get("scope"))
+	if diff := cmp.Diff([]string{"admin", "read"}, secondScopes, cmpopts.SortSlices(func(a, b string) bool { return a < b })); diff != "" {
+		t.Errorf("Second auth scopes mismatch (-want +got):\n%s", diff)
+	}
+}
+
 func TestAuthorize_ForbiddenUnhandledError(t *testing.T) {
 	req := httptest.NewRequest(http.MethodGet, "http://example.com/resource", nil)
 	resp := &http.Response{

auth/extauth/client_credentials.go

@@ -47,6 +47,8 @@ type ClientCredentialsHandlerConfig struct {
 type ClientCredentialsHandler struct {
 	config      *ClientCredentialsHandlerConfig
 	tokenSource oauth2.TokenSource
+
+	grantedScopes map[string][]string
 }
 
 // Compile-time check that ClientCredentialsHandler implements auth.OAuthHandler.
@@ -67,7 +69,10 @@ func NewClientCredentialsHandler(config *ClientCredentialsHandlerConfig) (*Clien
 	if config.Credentials.ClientSecretAuth == nil {
 		return nil, fmt.Errorf("clientSecretAuth is required for client credentials grant")
 	}
-	return &ClientCredentialsHandler{config: config}, nil
+	return &ClientCredentialsHandler{
+		config:        config,
+		grantedScopes: make(map[string][]string),
+	}, nil
 }
 
 // TokenSource returns the token source for outgoing requests.
@@ -121,30 +126,47 @@ func (h *ClientCredentialsHandler) Authorize(ctx context.Context, req *http.Requ
 		}
 	}
 
-	// Determine scopes: use PRM's scopes_supported if available.
-	scopes := scopesFromChallenges(wwwChallenges)
-	if len(scopes) == 0 && len(prm.ScopesSupported) > 0 {
-		scopes = prm.ScopesSupported
+	// Determine requestedScopes: use PRM's scopes_supported if available.
+	requestedScopes := scopesFromChallenges(wwwChallenges)
+	if len(requestedScopes) == 0 && len(prm.ScopesSupported) > 0 {
+		requestedScopes = prm.ScopesSupported
 	}
 
+	// Accumulate scopes: union previously granted scopes with the newly
+	// challenged scopes so that step-up authorization does not lose
+	// permissions granted in earlier rounds (SEP-2350).
+	requestedScopes = auth.UnionScopes(h.grantedScopes[asm.Issuer], requestedScopes)
+
 	// Step 3: Exchange client credentials for an access token.
 	creds := h.config.Credentials
 	cfg := &clientcredentials.Config{
 		ClientID:     creds.ClientID,
 		ClientSecret: creds.ClientSecretAuth.ClientSecret,
 		TokenURL:     asm.TokenEndpoint,
-		Scopes:       scopes,
+		Scopes:       requestedScopes,
 		AuthStyle:    selectTokenAuthMethod(asm.TokenEndpointAuthMethodsSupported),
 	}
 
 	ctxWithClient := context.WithValue(ctx, oauth2.HTTPClient, httpClient)
 	h.tokenSource = cfg.TokenSource(ctxWithClient)
 
-	// Eagerly fetch a token to surface errors immediately.
-	if _, err := h.tokenSource.Token(); err != nil {
+	return h.updateGrantedScopes(asm.Issuer, requestedScopes)
+}
+
+func (h *ClientCredentialsHandler) updateGrantedScopes(issuer string, requestedScopes []string) error {
+	if h.tokenSource == nil {
+		return nil
+	}
+	tok, err := h.tokenSource.Token()
+	if err != nil {
 		h.tokenSource = nil
 		return fmt.Errorf("client credentials token request failed: %w", err)
 	}
+	if tokenScopes := auth.ScopesFromToken(tok); tokenScopes == nil {
+		h.grantedScopes[issuer] = requestedScopes
+	} else {
+		h.grantedScopes[issuer] = tokenScopes
+	}
 	return nil
 }