diff --git a/internal/config/config.go b/internal/config/config.go
index 20b611c..7a5831e 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -82,10 +82,15 @@ func Load() (*Config, error) {
 			return nil, err
 		}
 	}
-	applyDefaults(cfg)
+	changed := applyDefaults(cfg)
 	if err := validate(cfg); err != nil {
 		return nil, err
 	}
+	if changed {
+		if err := writeFile(path, cfg); err != nil {
+			return nil, err
+		}
+	}
 	return cfg, nil
 }
 
@@ -119,36 +124,46 @@ func migrateLegacy(path string, data []byte) error {
 	return writeFile(path, cfg)
 }
 
-func applyDefaults(c *Config) {
+func applyDefaults(c *Config) (changed bool) {
 	if c.BotName == "" {
 		c.BotName = "ai-bot"
+		changed = true
 	}
 	if c.Port == 0 {
 		c.Port = 8080
+		changed = true
 	}
 	if c.LogLevel == "" {
 		c.LogLevel = "info"
+		changed = true
 	}
 	if c.SystemPrompt == "" {
 		c.SystemPrompt = "你是一个乐于助人的 AI 助手。"
+		changed = true
 	}
 	if c.DefaultProvider == "" && len(c.Providers) > 0 {
 		c.DefaultProvider = c.Providers[0].Name
+		changed = true
 	}
 	if c.Database.Driver == "" {
 		c.Database.Driver = "sqlite3"
+		changed = true
 	}
 	if c.Database.File == "" {
 		c.Database.File = "data/memory.db"
+		changed = true
 	}
 	if c.Database.Driver == "mysql" {
 		if c.Database.Host == "" {
 			c.Database.Host = "127.0.0.1"
+			changed = true
 		}
 		if c.Database.Port == 0 {
 			c.Database.Port = 3306
+			changed = true
 		}
 	}
+	return changed
 }
 
 func validate(c *Config) error {
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 307a4ac..a7b88bf 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -1,10 +1,18 @@
 package config
 
-import "testing"
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
 
 func TestApplyDatabaseDefaults(t *testing.T) {
 	c := &Config{Providers: []Provider{{Name: "p", BaseURL: "x", Models: []string{"m"}}}}
-	applyDefaults(c)
+	changed := applyDefaults(c)
+	if !changed {
+		t.Error("缺失字段应返回 changed=true")
+	}
 	if c.Database.Driver != "sqlite3" {
 		t.Errorf("默认 driver = %q, want sqlite3", c.Database.Driver)
 	}
@@ -13,12 +21,30 @@ func TestApplyDatabaseDefaults(t *testing.T) {
 	}
 }
 
+func TestApplyDefaultsNoChange(t *testing.T) {
+	c := &Config{
+		BotName:         "x",
+		Port:            8081,
+		LogLevel:        "debug",
+		SystemPrompt:    "sp",
+		DefaultProvider: "p",
+		Providers:       []Provider{{Name: "p", BaseURL: "x", Models: []string{"m"}}},
+		Database:        DatabaseConfig{Driver: "mysql", File: "f", Host: "h", Port: 3307, Name: "n"},
+	}
+	if applyDefaults(c) {
+		t.Error("完整配置不应返回 changed")
+	}
+}
+
 func TestApplyMySQLDefaults(t *testing.T) {
 	c := &Config{
 		Providers: []Provider{{Name: "p", BaseURL: "x", Models: []string{"m"}}},
 		Database:  DatabaseConfig{Driver: "mysql", Name: "memory"},
 	}
-	applyDefaults(c)
+	changed := applyDefaults(c)
+	if !changed {
+		t.Error("mysql 缺 host/port 应返回 changed")
+	}
 	if c.Database.Host != "127.0.0.1" {
 		t.Errorf("默认 host = %q, want 127.0.0.1", c.Database.Host)
 	}
@@ -27,6 +53,66 @@ func TestApplyMySQLDefaults(t *testing.T) {
 	}
 }
 
+func TestLoadWritesBackDatabase(t *testing.T) {
+	t.Chdir(t.TempDir())
+	cfg = nil
+	path := filepath.Join("data", "config.yaml")
+	old := "bot_name: test-bot\ndefault_provider: deepseek\ndefault_model: deepseek-v4-flash\nproviders:\n    - name: deepseek\n      base_url: https://api.deepseek.com\n      models:\n        - deepseek-v4-flash\n"
+	if err := os.MkdirAll("data", 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte(old), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load 出错: %v", err)
+	}
+	if c.Database.Driver != "sqlite3" {
+		t.Errorf("默认 driver = %q, want sqlite3", c.Database.Driver)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("读取写回后的配置失败: %v", err)
+	}
+	if !strings.Contains(string(data), "database:") || !strings.Contains(string(data), "sqlite3") {
+		t.Errorf("写回的文件缺少 database 段:\n%s", data)
+	}
+	if !strings.Contains(string(data), "test-bot") {
+		t.Errorf("写回不应覆盖已有字段:\n%s", data)
+	}
+}
+
+func TestLoadNoRewriteWhenComplete(t *testing.T) {
+	t.Chdir(t.TempDir())
+	cfg = nil
+	path := filepath.Join("data", "config.yaml")
+	full := "bot_name: test-bot\ndefault_provider: deepseek\ndefault_model: deepseek-v4-flash\nproviders:\n    - name: deepseek\n      base_url: https://api.deepseek.com\n      models:\n        - deepseek-v4-flash\n"
+	if err := os.MkdirAll("data", 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte(full), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	// 完整配置（含 database 段）
+	complete := full + "database:\n    driver: sqlite3\n    file: data/memory.db\n"
+	if err := os.WriteFile(path, []byte(complete), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Load(); err != nil {
+		t.Fatalf("Load 出错: %v", err)
+	}
+	info, _ := os.Stat(path)
+	before := info.ModTime()
+	if _, err := Load(); err != nil {
+		t.Fatalf("第二次 Load 出错: %v", err)
+	}
+	info, _ = os.Stat(path)
+	if !info.ModTime().Equal(before) {
+		t.Error("完整配置不应重复写回文件")
+	}
+}
+
 func TestValidateDatabase(t *testing.T) {
 	c := &Config{
 		DefaultProvider: "p",