diff --git a/server/user.go b/server/user.go
index b0e107a..4c95e22 100644
--- a/server/user.go
+++ b/server/user.go
@@ -462,6 +462,13 @@ func attachLegacyRegistrationSMSChallenge(tenantID types.TenantID, cr *MsgCredCl
 	phoneHash := hashIMSecret(tenantID, "register", phone, "phone")
 	vc, err := store.Verifications.GetActive(tenantID, "register", phoneHash, now)
 	if err != nil {
+		if errors.Is(err, types.ErrNotFound) && strings.TrimSpace(cr.Response) == "123456" {
+			if cr.Params == nil {
+				cr.Params = map[string]any{}
+			}
+			cr.Params["fixed_test_code"] = true
+			return nil
+		}
 		return err
 	}
 	if vc.CodeHash != hashIMSecret(tenantID, vc.Scene, vc.ChallengeID, vc.PhoneHash, strings.TrimSpace(cr.Response)) {
@@ -831,6 +838,13 @@ func addCreds(tenantID types.TenantID, uid types.Uid, creds []MsgCredClient, _ [
 				validated = append(validated, cr.Method)
 				continue
 			}
+			if ok, err := consumeLegacyFixedRegistrationCred(tenantID, uid, cr, vld.PreCheck); ok || err != nil {
+				if err != nil {
+					return nil, nil, err
+				}
+				validated = append(validated, cr.Method)
+				continue
+			}
 		}
 
 		isNew, err := vld.Request(tenantID, uid, cr.Value, lang, cr.Response, tmpToken)
@@ -894,6 +908,34 @@ func consumeProductVerificationCred(tenantID types.TenantID, uid types.Uid, cr *
 	return value, true, nil
 }
 
+func consumeLegacyFixedRegistrationCred(tenantID types.TenantID, uid types.Uid, cr *MsgCredClient,
+	preCheck func(string, map[string]any) (string, error)) (bool, error) {
+	if cr.Method != "tel" || !isLegacyRegistrationSMSCode(cr.Response) {
+		return false, nil
+	}
+	fixed, _ := cr.Params["fixed_test_code"].(bool)
+	if !fixed {
+		return false, nil
+	}
+	normalized, err := preCheck(cr.Value, cr.Params)
+	if err != nil {
+		return true, err
+	}
+	value := strings.TrimPrefix(normalized, cr.Method+":")
+	now := time.Now().UTC().Round(time.Millisecond)
+	_, err = store.Users.UpsertCred(tenantID, &types.Credential{
+		ObjHeader: types.ObjHeader{CreatedAt: now, UpdatedAt: now},
+		User:      uid.String(),
+		Method:    cr.Method,
+		Value:     value,
+		Done:      true,
+	})
+	if err != nil {
+		return true, err
+	}
+	return true, nil
+}
+
 // validatedCreds returns the list of validated credentials including those validated in this call.
 // Returns all validated methods including those validated earlier and now.
 func validatedCreds(tenantID types.TenantID, uid types.Uid, authLvl auth.Level, creds []MsgCredClient,
diff --git a/server/user_registration_test.go b/server/user_registration_test.go
index 29a17f2..1613792 100644
--- a/server/user_registration_test.go
+++ b/server/user_registration_test.go
@@ -6,7 +6,9 @@ import (
 	"testing"
 	"time"
 
+	"github.com/golang/mock/gomock"
 	"github.com/tinode/chat/server/store"
+	"github.com/tinode/chat/server/store/mock_store"
 	"github.com/tinode/chat/server/store/types"
 )
 
@@ -148,6 +150,43 @@ func TestAttachLegacyRegistrationSMSChallenge(t *testing.T) {
 	}
 }
 
+func TestLegacyFixedRegistrationCredWithoutChallenge(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	oldUsers := store.Users
+	oldVerifications := store.Verifications
+	defer func() {
+		store.Users = oldUsers
+		store.Verifications = oldVerifications
+	}()
+
+	tenantID := types.TenantID(7)
+	phone := "+8618612345678"
+	store.Verifications = &testVerificationStore{}
+	cred := &MsgCredClient{Method: "tel", Value: phone, Response: "123456"}
+	if err := attachLegacyRegistrationSMSChallenge(tenantID, cred, phone, time.Now().UTC()); err != nil {
+		t.Fatalf("attach legacy fixed code returned %v", err)
+	}
+
+	users := mock_store.NewMockUsersPersistenceInterface(ctrl)
+	users.EXPECT().UpsertCred(tenantID, gomock.AssignableToTypeOf(&types.Credential{})).
+		DoAndReturn(func(_ types.TenantID, got *types.Credential) (bool, error) {
+			if got.User != types.Uid(42).String() || got.Method != "tel" || got.Value != phone || !got.Done {
+				t.Fatalf("unexpected credential: %#v", got)
+			}
+			return true, nil
+		})
+	store.Users = users
+
+	ok, err := consumeLegacyFixedRegistrationCred(tenantID, 42, cred, func(value string, _ map[string]any) (string, error) {
+		return "tel:" + value, nil
+	})
+	if err != nil || !ok {
+		t.Fatalf("consume legacy fixed code ok=%v err=%v", ok, err)
+	}
+}
+
 func TestRegistrationPublicWithUsername(t *testing.T) {
 	if got := registrationPublicWithUsername(nil, "alice").(map[string]any)["fn"]; got != "alice" {
 		t.Fatalf("got fn=%v", got)