diff --git a/pets-be/internal/httpapi/auth.go b/pets-be/internal/httpapi/auth.go
index d63165e..299ad29 100644
--- a/pets-be/internal/httpapi/auth.go
+++ b/pets-be/internal/httpapi/auth.go
@@ -2,9 +2,11 @@ package httpapi
 
 import (
 	"context"
+	"crypto/rand"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"math/big"
 	"net/http"
 	"net/url"
 	"strings"
@@ -79,8 +81,9 @@ func (a *API) devLogin(c *gin.Context) {
 		return
 	}
 	name := strings.TrimSpace(in.Nickname)
-	if name == "" {
-		name = "柯基家长"
+	if len([]rune(name)) > 64 {
+		fail(c, http.StatusBadRequest, "昵称不能超过 64 字")
+		return
 	}
 	user, err := a.findOrCreateUser("dev:"+in.Key, name)
 	if err != nil {
@@ -123,7 +126,7 @@ func (a *API) login(c *gin.Context) {
 		fail(c, http.StatusUnauthorized, "微信登录凭证无效")
 		return
 	}
-	user, err := a.findOrCreateUser(wx.OpenID, "柯基家长")
+	user, err := a.findOrCreateUser(wx.OpenID, "")
 	if err != nil {
 		fail(c, http.StatusInternalServerError, "登录失败")
 		return
@@ -140,17 +143,61 @@ func (a *API) findOrCreateUser(openID, nickname string) (model.User, error) {
 	if !errors.Is(err, gorm.ErrRecordNotFound) {
 		return user, err
 	}
-	user = model.User{OpenID: openID, Nickname: nickname}
-	if err := a.db.Create(&user).Error; err != nil {
-		// A concurrent login may have inserted the same open_id.
-		if lookup := a.db.Where("open_id = ?", openID).First(&user).Error; lookup == nil {
-			return user, nil
+	// Serialize new-user naming across server instances without requiring custom nicknames to be unique.
+	err = a.db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Exec("SELECT pg_advisory_xact_lock(?)", int64(7270139527)).Error; err != nil {
+			return err
 		}
+		// Another login for this open ID may have completed while waiting for the lock.
+		if err := tx.Where("open_id = ?", openID).First(&user).Error; err == nil {
+			return nil
+		} else if !errors.Is(err, gorm.ErrRecordNotFound) {
+			return err
+		}
+		name := strings.TrimSpace(nickname)
+		if name == "" {
+			generated, err := defaultNickname(tx)
+			if err != nil {
+				return err
+			}
+			name = generated
+		}
+		user = model.User{OpenID: openID, Nickname: name}
+		return tx.Create(&user).Error
+	})
+	if err != nil {
 		return user, fmt.Errorf("create user: %w", err)
 	}
 	return user, nil
 }
 
+func defaultNickname(tx *gorm.DB) (string, error) {
+	// Prefer four digits. Expand when occupied names make retries frequent.
+	for digits := 4; digits <= 6; digits++ {
+		limit := int64(10000)
+		if digits == 5 {
+			limit = 100000
+		} else if digits == 6 {
+			limit = 1000000
+		}
+		for attempt := 0; attempt < 20; attempt++ {
+			number, err := rand.Int(rand.Reader, big.NewInt(limit))
+			if err != nil {
+				return "", fmt.Errorf("generate nickname: %w", err)
+			}
+			name := fmt.Sprintf("毛孩家长%0*d", digits, number.Int64())
+			var count int64
+			if err := tx.Model(&model.User{}).Where("nickname = ?", name).Count(&count).Error; err != nil {
+				return "", err
+			}
+			if count == 0 {
+				return name, nil
+			}
+		}
+	}
+	return "", errors.New("default nickname allocation failed")
+}
+
 func (a *API) loginUser(c *gin.Context, user model.User) {
 	if user.Disabled {
 		fail(c, http.StatusForbidden, "账号已停用")
diff --git a/pets-be/internal/httpapi/nickname_test.go b/pets-be/internal/httpapi/nickname_test.go
new file mode 100644
index 0000000..90c88b9
--- /dev/null
+++ b/pets-be/internal/httpapi/nickname_test.go
@@ -0,0 +1,69 @@
+package httpapi
+
+import (
+	"os"
+	"regexp"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/sundynix/pets-be/internal/config"
+	"github.com/sundynix/pets-be/internal/database"
+)
+
+func TestDefaultNicknamePostgres(t *testing.T) {
+	if os.Getenv("PETS_ADMIN_INTEGRATION") != "1" {
+		t.Skip("PostgreSQL integration disabled")
+	}
+	cfg, err := config.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	db, err := database.Open(cfg.Postgres, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sqlDB, _ := db.DB()
+	defer sqlDB.Close()
+	tx := db.Begin()
+	if tx.Error != nil {
+		t.Fatal(tx.Error)
+	}
+	defer tx.Rollback()
+	api := New(tx, nil, cfg)
+	seen := map[string]bool{}
+	for range 12 {
+		openID := "nickname-test:" + uuid.NewString()
+		user, err := api.findOrCreateUser(openID, "")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !regexp.MustCompile(`^毛孩家长\d{4,6}$`).MatchString(user.Nickname) {
+			t.Fatalf("unexpected name %q", user.Nickname)
+		}
+		if seen[user.Nickname] {
+			t.Fatalf("duplicate name %q", user.Nickname)
+		}
+		seen[user.Nickname] = true
+		again, err := api.findOrCreateUser(openID, "不应覆盖的名字")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if again.ID != user.ID || again.Nickname != user.Nickname {
+			t.Fatal("relogin changed existing user")
+		}
+	}
+	custom, err := api.findOrCreateUser("nickname-test:"+uuid.NewString(), "小豆的家长")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if custom.Nickname != "小豆的家长" {
+		t.Fatal("custom nickname lost")
+	}
+	again, err := api.findOrCreateUser(custom.OpenID, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if again.Nickname != custom.Nickname {
+		t.Fatal("relogin replaced custom nickname")
+	}
+}