diff --git a/backend/internal/pkg/claude/constants.go b/backend/internal/pkg/claude/constants.go
index 0c6e9b4c..fb95ffe2 100644
--- a/backend/internal/pkg/claude/constants.go
+++ b/backend/internal/pkg/claude/constants.go
@@ -16,7 +16,12 @@ const (
 const DefaultBetaHeader = BetaClaudeCode + "," + BetaOAuth + "," + BetaInterleavedThinking + "," + BetaFineGrainedToolStreaming
 
 // MessageBetaHeaderNoTools /v1/messages 在无工具时的 beta header
-const MessageBetaHeaderNoTools = BetaOAuth + "," + BetaInterleavedThinking
+//
+// NOTE: Claude Code OAuth credentials are scoped to Claude Code. When we "mimic"
+// Claude Code for non-Claude-Code clients, we must include the claude-code beta
+// even if the request doesn't use tools, otherwise upstream may reject the
+// request as a non-Claude-Code API request.
+const MessageBetaHeaderNoTools = BetaClaudeCode + "," + BetaOAuth + "," + BetaInterleavedThinking
 
 // MessageBetaHeaderWithTools /v1/messages 在有工具时的 beta header
 const MessageBetaHeaderWithTools = BetaClaudeCode + "," + BetaOAuth + "," + BetaInterleavedThinking
diff --git a/backend/internal/service/gateway_oauth_metadata_test.go b/backend/internal/service/gateway_oauth_metadata_test.go
new file mode 100644
index 00000000..ed6f1887
--- /dev/null
+++ b/backend/internal/service/gateway_oauth_metadata_test.go
@@ -0,0 +1,62 @@
+package service
+
+import (
+	"regexp"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildOAuthMetadataUserID_FallbackWithoutAccountUUID(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		Model:          "claude-sonnet-4-5",
+		Stream:         true,
+		MetadataUserID: "",
+		System:         nil,
+		Messages:       nil,
+	}
+
+	account := &Account{
+		ID:    123,
+		Type:  AccountTypeOAuth,
+		Extra: map[string]any{}, // intentionally missing account_uuid / claude_user_id
+	}
+
+	fp := &Fingerprint{ClientID: "deadbeef"} // should be used as user id in legacy format
+
+	got := svc.buildOAuthMetadataUserID(parsed, account, fp)
+	require.NotEmpty(t, got)
+
+	// Legacy format: user_{client}_account__session_{uuid}
+	re := regexp.MustCompile(`^user_[a-zA-Z0-9]+_account__session_[a-f0-9-]{36}$`)
+	require.True(t, re.MatchString(got), "unexpected user_id format: %s", got)
+}
+
+func TestBuildOAuthMetadataUserID_UsesAccountUUIDWhenPresent(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		Model:          "claude-sonnet-4-5",
+		Stream:         true,
+		MetadataUserID: "",
+	}
+
+	account := &Account{
+		ID:   123,
+		Type: AccountTypeOAuth,
+		Extra: map[string]any{
+			"account_uuid":      "acc-uuid",
+			"claude_user_id":    "clientid123",
+			"anthropic_user_id": "",
+		},
+	}
+
+	got := svc.buildOAuthMetadataUserID(parsed, account, nil)
+	require.NotEmpty(t, got)
+
+	// New format: user_{client}_account_{account_uuid}_session_{uuid}
+	re := regexp.MustCompile(`^user_clientid123_account_acc-uuid_session_[a-f0-9-]{36}$`)
+	require.True(t, re.MatchString(got), "unexpected user_id format: %s", got)
+}
diff --git a/backend/internal/service/gateway_service.go b/backend/internal/service/gateway_service.go
index 01663ae7..1ebd1246 100644
--- a/backend/internal/service/gateway_service.go
+++ b/backend/internal/service/gateway_service.go
@@ -795,17 +795,15 @@ func (s *GatewayService) buildOAuthMetadataUserID(parsed *ParsedRequest, account
 	if parsed.MetadataUserID != "" {
 		return ""
 	}
-	accountUUID := account.GetExtraString("account_uuid")
-	if accountUUID == "" {
-		return ""
-	}
 
 	userID := strings.TrimSpace(account.GetClaudeUserID())
 	if userID == "" && fp != nil {
 		userID = fp.ClientID
 	}
 	if userID == "" {
-		return ""
+		// Fall back to a random, well-formed client id so we can still satisfy
+		// Claude Code OAuth requirements when account metadata is incomplete.
+		userID = generateClientID()
 	}
 
 	sessionHash := s.GenerateSessionHash(parsed)
@@ -814,7 +812,14 @@ func (s *GatewayService) buildOAuthMetadataUserID(parsed *ParsedRequest, account
 		seed := fmt.Sprintf("%d::%s", account.ID, sessionHash)
 		sessionID = generateSessionUUID(seed)
 	}
-	return fmt.Sprintf("user_%s_account_%s_session_%s", userID, accountUUID, sessionID)
+
+	// Prefer the newer format that includes account_uuid (if present),
+	// otherwise fall back to the legacy Claude Code format.
+	accountUUID := strings.TrimSpace(account.GetExtraString("account_uuid"))
+	if accountUUID != "" {
+		return fmt.Sprintf("user_%s_account_%s_session_%s", userID, accountUUID, sessionID)
+	}
+	return fmt.Sprintf("user_%s_account__session_%s", userID, sessionID)
 }
 
 func generateSessionUUID(seed string) string {