diff --git a/auth/authorization_code.go b/auth/authorization_code.go
index 9b41e368..9cafb8df 100644
--- a/auth/authorization_code.go
+++ b/auth/authorization_code.go
@@ -14,6 +14,7 @@ import (
 	"net/url"
 	"slices"
 	"strings"
+	"sync"
 
 	"github.com/modelcontextprotocol/go-sdk/internal/util"
 	"github.com/modelcontextprotocol/go-sdk/oauthex"
@@ -128,6 +129,9 @@ type AuthorizationCodeHandler struct {
 
 	// tokenSource is the token source to use for authorization.
 	tokenSource oauth2.TokenSource
+
+	mu              sync.Mutex
+	requestedScopes []string
 }
 
 var _ OAuthHandler = (*AuthorizationCodeHandler)(nil)
@@ -288,6 +292,14 @@ func (h *AuthorizationCodeHandler) Authorize(ctx context.Context, req *http.Requ
 		scps = append(scps, "offline_access")
 	}
 
+	// Accumulate scopes: union previously requested scopes with the newly
+	// challenged scopes so that step-up authorization does not lose
+	// permissions granted in earlier rounds (SEP-2350).
+	h.mu.Lock()
+	scps = unionScopes(h.requestedScopes, scps)
+	h.requestedScopes = scps
+	h.mu.Unlock()
+
 	cfg := &oauth2.Config{
 		ClientID:     resolvedClientConfig.clientID,
 		ClientSecret: resolvedClientConfig.clientSecret,
@@ -343,6 +355,25 @@ func errorFromChallenges(cs []oauthex.Challenge) string {
 	return ""
 }
 
+// unionScopes returns the union of existing and challenged scopes,
+// preserving order (existing first, then new challenged scopes).
+func unionScopes(existing, challenged []string) []string {
+	if len(existing) == 0 {
+		return challenged
+	}
+	if len(challenged) == 0 {
+		return existing
+	}
+	result := make([]string, len(existing), len(existing)+len(challenged))
+	copy(result, existing)
+	for _, s := range challenged {
+		if !slices.Contains(result, s) {
+			result = append(result, s)
+		}
+	}
+	return result
+}
+
 // getProtectedResourceMetadata returns the protected resource metadata.
 // If no metadata was found or the fetched metadata fails security checks,
 // it returns an error.
diff --git a/auth/authorization_code_test.go b/auth/authorization_code_test.go
index d30cbf36..419a8389 100644
--- a/auth/authorization_code_test.go
+++ b/auth/authorization_code_test.go
@@ -113,6 +113,95 @@ 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"},
+				},
+			},
+		},
+	})
+	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
+
+	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)
+			return nil, fmt.Errorf("stop after capturing URL")
+		},
+	})
+	if err != nil {
+		t.Fatalf("NewAuthorizationCodeHandler failed: %v", err)
+	}
+
+	// First authorization: 401 with scope="read"
+	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", resource_metadata="%s/.well-known/oauth-protected-resource/resource"`, resourceServer.URL))
+	err = handler.Authorize(context.Background(), req, resp)
+	if err == nil || !strings.Contains(err.Error(), "stop after capturing URL") {
+		t.Fatalf("First Authorize expected error containing 'stop after capturing URL', got: %v", err)
+	}
+
+	// Verify first auth URL requested only "read".
+	firstURL, err := url.Parse(capturedAuthURLs[0])
+	if err != nil {
+		t.Fatalf("Failed to parse first auth URL: %v", err)
+	}
+	if got := firstURL.Query().Get("scope"); got != "read" {
+		t.Errorf("First auth scope = %q, want %q", got, "read")
+	}
+
+	// Second authorization: 403 insufficient_scope with scope="write"
+	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="write", resource_metadata="%s/.well-known/oauth-protected-resource/resource"`, resourceServer.URL))
+	err = handler.Authorize(context.Background(), req2, resp2)
+	if err == nil || !strings.Contains(err.Error(), "stop after capturing URL") {
+		t.Fatalf("Second Authorize expected error containing 'stop after capturing URL', got: %v", err)
+	}
+
+	// Verify second auth URL accumulated both scopes.
+	secondURL, err := url.Parse(capturedAuthURLs[1])
+	if err != nil {
+		t.Fatalf("Failed to parse second auth URL: %v", err)
+	}
+	if got := secondURL.Query().Get("scope"); got != "read write" {
+		t.Errorf("Second auth scope = %q, want %q", got, "read write")
+	}
+}
+
 func TestAuthorize_ForbiddenUnhandledError(t *testing.T) {
 	req := httptest.NewRequest(http.MethodGet, "http://example.com/resource", nil)
 	resp := &http.Response{
@@ -740,6 +829,67 @@ func TestApplicationTypeInference(t *testing.T) {
 	}
 }
 
+func TestUnionScopes(t *testing.T) {
+	tests := []struct {
+		name       string
+		existing   []string
+		challenged []string
+		want       []string
+	}{
+		{
+			name:       "both empty",
+			existing:   nil,
+			challenged: nil,
+			want:       nil,
+		},
+		{
+			name:       "existing only",
+			existing:   []string{"read"},
+			challenged: nil,
+			want:       []string{"read"},
+		},
+		{
+			name:       "challenged only",
+			existing:   nil,
+			challenged: []string{"write"},
+			want:       []string{"write"},
+		},
+		{
+			name:       "disjoint scopes",
+			existing:   []string{"read"},
+			challenged: []string{"write"},
+			want:       []string{"read", "write"},
+		},
+		{
+			name:       "overlapping scopes",
+			existing:   []string{"read", "write"},
+			challenged: []string{"write", "admin"},
+			want:       []string{"read", "write", "admin"},
+		},
+		{
+			name:       "identical scopes",
+			existing:   []string{"read", "write"},
+			challenged: []string{"read", "write"},
+			want:       []string{"read", "write"},
+		},
+		{
+			name:       "preserves order",
+			existing:   []string{"b", "a"},
+			challenged: []string{"c", "a"},
+			want:       []string{"b", "a", "c"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := unionScopes(tt.existing, tt.challenged)
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Errorf("unionScopes() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestAuthorize_OfflineAccessScope(t *testing.T) {
 	tests := []struct {
 		name                string
diff --git a/auth/extauth/client_credentials.go b/auth/extauth/client_credentials.go
index fefa29b9..34bcd627 100644
--- a/auth/extauth/client_credentials.go
+++ b/auth/extauth/client_credentials.go
@@ -12,6 +12,7 @@ import (
 	"net/url"
 	"slices"
 	"strings"
+	"sync"
 
 	"github.com/modelcontextprotocol/go-sdk/auth"
 	"github.com/modelcontextprotocol/go-sdk/oauthex"
@@ -47,6 +48,9 @@ type ClientCredentialsHandlerConfig struct {
 type ClientCredentialsHandler struct {
 	config      *ClientCredentialsHandlerConfig
 	tokenSource oauth2.TokenSource
+
+	mu              sync.Mutex
+	requestedScopes []string
 }
 
 // Compile-time check that ClientCredentialsHandler implements auth.OAuthHandler.
@@ -127,6 +131,14 @@ func (h *ClientCredentialsHandler) Authorize(ctx context.Context, req *http.Requ
 		scopes = prm.ScopesSupported
 	}
 
+	// Accumulate scopes: union previously requested scopes with the newly
+	// challenged scopes so that step-up authorization does not lose
+	// permissions granted in earlier rounds (SEP-2350).
+	h.mu.Lock()
+	scopes = unionScopes(h.requestedScopes, scopes)
+	h.requestedScopes = scopes
+	h.mu.Unlock()
+
 	// Step 3: Exchange client credentials for an access token.
 	creds := h.config.Credentials
 	cfg := &clientcredentials.Config{
@@ -229,6 +241,25 @@ func scopesFromChallenges(cs []oauthex.Challenge) []string {
 	return nil
 }
 
+// unionScopes returns the union of existing and challenged scopes,
+// preserving order (existing first, then new challenged scopes).
+func unionScopes(existing, challenged []string) []string {
+	if len(existing) == 0 {
+		return challenged
+	}
+	if len(challenged) == 0 {
+		return existing
+	}
+	result := make([]string, len(existing), len(existing)+len(challenged))
+	copy(result, existing)
+	for _, s := range challenged {
+		if !slices.Contains(result, s) {
+			result = append(result, s)
+		}
+	}
+	return result
+}
+
 // selectTokenAuthMethod selects the preferred token endpoint auth method based on
 // the authorization server's supported methods. Prefers client_secret_post over
 // client_secret_basic per the OAuth 2.1 draft.
diff --git a/auth/extauth/client_credentials_test.go b/auth/extauth/client_credentials_test.go
index 91ae7c04..fe46c48e 100644
--- a/auth/extauth/client_credentials_test.go
+++ b/auth/extauth/client_credentials_test.go
@@ -10,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/modelcontextprotocol/go-sdk/auth"
 	"github.com/modelcontextprotocol/go-sdk/internal/oauthtest"
 	"github.com/modelcontextprotocol/go-sdk/oauthex"
@@ -255,6 +256,135 @@ func TestClientCredentialsHandler_Authorize(t *testing.T) {
 	})
 }
 
+func TestClientCredentialsHandler_ScopeAccumulation(t *testing.T) {
+	authServer := oauthtest.NewFakeAuthorizationServer(oauthtest.Config{
+		MetadataEndpointConfig: &oauthtest.MetadataEndpointConfig{
+			ServeOAuthInsertedEndpoint: true,
+		},
+		RegistrationConfig: &oauthtest.RegistrationConfig{
+			PreregisteredClients: map[string]oauthtest.ClientInfo{
+				"test-client": {Secret: "test-secret"},
+			},
+		},
+		ClientCredentialsConfig: &oauthtest.ClientCredentialsConfig{
+			Enabled: true,
+		},
+	})
+	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", auth.ProtectedResourceMetadataHandler(&oauthex.ProtectedResourceMetadata{
+		Resource:             resourceURL,
+		AuthorizationServers: []string{authServer.URL()},
+	}))
+
+	handler, err := NewClientCredentialsHandler(validClientCredentialsConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// First authorization: 401 with scope="read"
+	resp := &http.Response{
+		StatusCode: http.StatusUnauthorized,
+		Header:     make(http.Header),
+		Body:       http.NoBody,
+	}
+	resp.Header.Set("WWW-Authenticate", `Bearer scope="read"`)
+	req := httptest.NewRequest("GET", resourceURL, nil)
+	if err := handler.Authorize(t.Context(), req, resp); err != nil {
+		t.Fatalf("First Authorize failed: %v", err)
+	}
+
+	// Verify handler tracked the requested scopes.
+	handler.mu.Lock()
+	firstScopes := append([]string{}, handler.requestedScopes...)
+	handler.mu.Unlock()
+	wantFirst := []string{"read"}
+	if diff := cmp.Diff(wantFirst, firstScopes); diff != "" {
+		t.Errorf("After first Authorize, requestedScopes mismatch (-want +got):\n%s", diff)
+	}
+
+	// Second authorization: 401 with scope="write" (simulating step-up)
+	resp2 := &http.Response{
+		StatusCode: http.StatusUnauthorized,
+		Header:     make(http.Header),
+		Body:       http.NoBody,
+	}
+	resp2.Header.Set("WWW-Authenticate", `Bearer scope="write"`)
+	req2 := httptest.NewRequest("GET", resourceURL, nil)
+	if err := handler.Authorize(t.Context(), req2, resp2); err != nil {
+		t.Fatalf("Second Authorize failed: %v", err)
+	}
+
+	// Verify handler accumulated both scopes.
+	handler.mu.Lock()
+	secondScopes := append([]string{}, handler.requestedScopes...)
+	handler.mu.Unlock()
+	wantSecond := []string{"read", "write"}
+	if diff := cmp.Diff(wantSecond, secondScopes); diff != "" {
+		t.Errorf("After second Authorize, requestedScopes mismatch (-want +got):\n%s", diff)
+	}
+}
+
+func TestUnionScopes(t *testing.T) {
+	tests := []struct {
+		name       string
+		existing   []string
+		challenged []string
+		want       []string
+	}{
+		{
+			name:       "both empty",
+			existing:   nil,
+			challenged: nil,
+			want:       nil,
+		},
+		{
+			name:       "existing only",
+			existing:   []string{"read"},
+			challenged: nil,
+			want:       []string{"read"},
+		},
+		{
+			name:       "challenged only",
+			existing:   nil,
+			challenged: []string{"write"},
+			want:       []string{"write"},
+		},
+		{
+			name:       "disjoint scopes",
+			existing:   []string{"read"},
+			challenged: []string{"write"},
+			want:       []string{"read", "write"},
+		},
+		{
+			name:       "overlapping scopes",
+			existing:   []string{"read", "write"},
+			challenged: []string{"write", "admin"},
+			want:       []string{"read", "write", "admin"},
+		},
+		{
+			name:       "identical scopes",
+			existing:   []string{"read", "write"},
+			challenged: []string{"read", "write"},
+			want:       []string{"read", "write"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := unionScopes(tt.existing, tt.challenged)
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Errorf("unionScopes() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestSelectTokenAuthMethod(t *testing.T) {
 	tests := []struct {
 		name      string