diff --git a/.gitignore b/.gitignore
index 1290bde7..be0ff49b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,6 +40,7 @@ coverage.out
 coverage.html
 
 # Logs and temporary files
+/log/
 *.log
 *.bak
 *~
diff --git a/cmd/mcp-stdio/main.go b/cmd/mcp-stdio/main.go
index ad7b8f63..94f91ec0 100644
--- a/cmd/mcp-stdio/main.go
+++ b/cmd/mcp-stdio/main.go
@@ -25,7 +25,12 @@ func main() {
 	}
 
 	// 初始化日志（stdio 模式下使用 stderr 输出日志，避免干扰 JSON-RPC 通信）
-	log := logger.New(cfg.Log.Level, "stderr")
+	log := logger.New(cfg.Log.Level, "stderr", logger.DiagnosticOptions{
+		Dir:           cfg.Log.DiagnosticDir,
+		Disabled:      cfg.Log.DiagnosticDisabled,
+		RetentionDays: cfg.Log.DiagnosticRetentionDays,
+	})
+	defer log.Sync()
 
 	// 创建MCP服务器
 	mcpServer := mcp.NewServer(log.Logger)
diff --git a/cmd/server/main.go b/cmd/server/main.go
index d45619a7..194b449b 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -101,7 +101,12 @@ func main() {
 	}
 
 	// 初始化日志
-	log := logger.New(cfg.Log.Level, cfg.Log.Output)
+	log := logger.New(cfg.Log.Level, cfg.Log.Output, logger.DiagnosticOptions{
+		Dir:           cfg.Log.DiagnosticDir,
+		Disabled:      cfg.Log.DiagnosticDisabled,
+		RetentionDays: cfg.Log.DiagnosticRetentionDays,
+	})
+	defer log.Sync()
 
 	// 创建可取消的根 context，用于优雅关闭
 	ctx, cancel := context.WithCancel(context.Background())
diff --git a/config.example.yaml b/config.example.yaml
index 40e091e8..f9254a31 100644
--- a/config.example.yaml
+++ b/config.example.yaml
@@ -36,6 +36,9 @@ auth:
 log:
   level: info # 日志级别: debug(调试), info(信息), warn(警告), error(错误)
   output: stdout # 日志输出位置: stdout(标准输出), stderr(标准错误), 或文件路径
+  diagnostic_dir: log # 额外保存 warn 及以上诊断日志，按本地日期拆分；相对于进程工作目录
+  diagnostic_retention_days: 14 # 保留天数（含当天）；省略或 <= 0 使用 14 天
+  diagnostic_disabled: false # true 关闭额外诊断日志；修改后需重启
 # 平台操作审计（系统设置 -> 日志审计；不记录对话正文与每次工具调用）
 audit:
   enabled: true
diff --git a/docs/en-US/configuration.md b/docs/en-US/configuration.md
index f04c1da1..f49d0129 100644
--- a/docs/en-US/configuration.md
+++ b/docs/en-US/configuration.md
@@ -143,3 +143,9 @@ After changing, validate the specific subsystem rather than trusting the save me
 - Config API and apply: `internal/handler/config.go`
 - Route registration: `internal/app/app.go`
 - C2 reconciliation: `internal/app/c2_lifecycle.go`
+
+## Diagnostic logs
+
+Alongside `log.output` (controlled by `log.level`), warnings and errors are saved as JSON Lines in `log/diagnostic-YYYY-MM-DD.log`, using the server’s local date. This independent warn-and-above output preserves existing context, caller information, and error stack traces; ordinary info/debug records are excluded and no extra request bodies or tool output are collected.
+
+Configure `log.diagnostic_dir` (default `log`, relative to the working directory), `log.diagnostic_retention_days` (default 14, including today; nonpositive values use the default), or `log.diagnostic_disabled: true` to disable it. Restart after changing these settings. Files are created only when a diagnostic record is written; the first write each day removes expired files matching `diagnostic-YYYY-MM-DD.log`. Cleanup does not run while no diagnostic records are written. Write failures are reported to stderr without interrupting the primary log output.
diff --git a/docs/zh-CN/configuration.md b/docs/zh-CN/configuration.md
index 5c9dc1ad..0db87d4a 100644
--- a/docs/zh-CN/configuration.md
+++ b/docs/zh-CN/configuration.md
@@ -27,7 +27,11 @@ log:
 - Chromium 浏览器插件的合法 `chrome-extension://<32位插件ID>` Origin 会被自动识别，无需配置。插件仍需按域授权，并使用密码登录与 Bearer Token 调用 API。
 - `server.cors_allowed_origins`：仅供其他可信 Web 集成使用的额外 Origin 精确白名单；不支持 `*`，修改后需重启服务。
 - `auth.session_duration_hours`：登录会话有效期（小时）。登录密码由 RBAC 用户管理，首次启动时在控制台输出 `admin` 初始密码。
-- `log.output`：可以是 `stdout`、`stderr` 或文件路径。
+- `log.output`：可以是 `stdout`、`stderr` 或文件路径，由 `log.level` 控制级别。
+- 额外诊断日志默认开启，仅记录 `warn` 及以上（包括重试、连接异常和错误），独立于 `log.level`，不保存普通 `info` / `debug` 日志。保留原有结构化字段、时间、代码位置和 Error 及以上堆栈，不额外采集请求正文或工具输出。
+- `log.diagnostic_dir`：默认 `log`，相对于进程工作目录，文件名为 `diagnostic-YYYY-MM-DD.log`（JSON Lines，按服务器本地日期拆分）。只有出现诊断日志时才创建目录和文件；跨天后首次写入切换文件。
+- `log.diagnostic_retention_days`：默认 14 天（含当天）；省略或小于等于 0 时使用默认值。每天首次写入时清理此目录内过期的 `diagnostic-日期.log`，不删除其他文件；没有新诊断日志时不执行清理。
+- `log.diagnostic_disabled: true`：关闭额外诊断落盘。以上日志配置修改后需重启；目录无法写入时保留原输出，并由 Zap 向 stderr 报告写入失败。
 
 ## AI 通道与模型配置
 
diff --git a/internal/config/config.go b/internal/config/config.go
index 1b32666d..4fb988a4 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -807,8 +807,11 @@ type ServerConfig struct {
 }
 
 type LogConfig struct {
-	Level  string `yaml:"level"`
-	Output string `yaml:"output"`
+	Level                   string `yaml:"level"`
+	Output                  string `yaml:"output"`
+	DiagnosticDir           string `yaml:"diagnostic_dir"`
+	DiagnosticDisabled      bool   `yaml:"diagnostic_disabled"`
+	DiagnosticRetentionDays int    `yaml:"diagnostic_retention_days"`
 }
 
 type MCPConfig struct {
diff --git a/internal/logger/daily_writer.go b/internal/logger/daily_writer.go
new file mode 100644
index 00000000..a5f4ccce
--- /dev/null
+++ b/internal/logger/daily_writer.go
@@ -0,0 +1,88 @@
+package logger
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+)
+
+// DiagnosticOptions controls the additional warn-and-above diagnostic output.
+type DiagnosticOptions struct {
+	Dir           string
+	Disabled      bool
+	RetentionDays int // Values <= 0 use the default of 14 calendar days.
+}
+
+// dailyWriter opens lazily: healthy runs create no diagnostic files. Opening
+// per write also avoids keeping descriptors open across rotation or shutdown.
+type dailyWriter struct {
+	mu            sync.Mutex
+	dir           string
+	retentionDays int
+	cleanedDay    string
+	now           func() time.Time
+}
+
+func newDailyWriter(options DiagnosticOptions) *dailyWriter {
+	if options.Dir == "" {
+		options.Dir = "log"
+	}
+	if options.RetentionDays <= 0 {
+		options.RetentionDays = 14
+	}
+	return &dailyWriter{dir: options.Dir, retentionDays: options.RetentionDays, now: time.Now}
+}
+
+func (w *dailyWriter) Write(p []byte) (int, error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	now := w.now()
+	day := now.Format(time.DateOnly)
+	if err := os.MkdirAll(w.dir, 0700); err != nil {
+		return 0, fmt.Errorf("create diagnostic log directory: %w", err)
+	}
+	f, err := os.OpenFile(filepath.Join(w.dir, "diagnostic-"+day+".log"), os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0600)
+	if err != nil {
+		return 0, fmt.Errorf("open diagnostic log: %w", err)
+	}
+	n, writeErr := f.Write(p)
+	closeErr := f.Close()
+	var cleanupErr error
+	if w.cleanedDay != day {
+		cleanupErr = w.cleanup(now)
+		if cleanupErr == nil {
+			w.cleanedDay = day
+		}
+	}
+	return n, errors.Join(writeErr, closeErr, cleanupErr)
+}
+
+// Writes are unbuffered and files are closed before Write returns.
+func (w *dailyWriter) Sync() error { return nil }
+
+func (w *dailyWriter) cleanup(now time.Time) error {
+	entries, err := os.ReadDir(w.dir)
+	if err != nil {
+		return err
+	}
+	cutoff := now.AddDate(0, 0, -(w.retentionDays - 1)).Format(time.DateOnly)
+	var errs []error
+	for _, entry := range entries {
+		name := entry.Name()
+		if !entry.Type().IsRegular() || !strings.HasPrefix(name, "diagnostic-") || !strings.HasSuffix(name, ".log") {
+			continue
+		}
+		day := strings.TrimSuffix(strings.TrimPrefix(name, "diagnostic-"), ".log")
+		if _, err := time.Parse(time.DateOnly, day); err != nil || day >= cutoff {
+			continue
+		}
+		if err := os.Remove(filepath.Join(w.dir, name)); err != nil && !os.IsNotExist(err) {
+			errs = append(errs, fmt.Errorf("remove expired diagnostic log %s: %w", name, err))
+		}
+	}
+	return errors.Join(errs...)
+}
diff --git a/internal/logger/logger.go b/internal/logger/logger.go
index 7e306fab..1a47e275 100644
--- a/internal/logger/logger.go
+++ b/internal/logger/logger.go
@@ -11,7 +11,7 @@ type Logger struct {
 	*zap.Logger
 }
 
-func New(level, output string) *Logger {
+func New(level, output string, diagnostics ...DiagnosticOptions) *Logger {
 	var zapLevel zapcore.Level
 	switch level {
 	case "debug":
@@ -34,6 +34,8 @@ func New(level, output string) *Logger {
 	var writeSyncer zapcore.WriteSyncer
 	if output == "stdout" {
 		writeSyncer = zapcore.AddSync(os.Stdout)
+	} else if output == "stderr" {
+		writeSyncer = zapcore.AddSync(os.Stderr)
 	} else {
 		file, err := os.OpenFile(output, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
 		if err != nil {
@@ -49,6 +51,18 @@ func New(level, output string) *Logger {
 		zapLevel,
 	)
 
+	options := DiagnosticOptions{}
+	if len(diagnostics) > 0 {
+		options = diagnostics[0]
+	}
+	if !options.Disabled {
+		// The diagnostic threshold is independent of the primary output level.
+		core = zapcore.NewTee(core, zapcore.NewCore(
+			zapcore.NewJSONEncoder(config.EncoderConfig),
+			newDailyWriter(options), zapcore.WarnLevel,
+		))
+	}
+
 	logger := zap.New(core, zap.AddCaller(), zap.AddStacktrace(zapcore.ErrorLevel))
 
 	return &Logger{Logger: logger}
diff --git a/internal/logger/logger_test.go b/internal/logger/logger_test.go
new file mode 100644
index 00000000..f0d07d48
--- /dev/null
+++ b/internal/logger/logger_test.go
@@ -0,0 +1,120 @@
+package logger
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+func TestDiagnosticFiltering(t *testing.T) {
+	for _, level := range []string{"debug", "error"} {
+		t.Run(level, func(t *testing.T) {
+			root := t.TempDir()
+			dir := filepath.Join(root, "log")
+			log := New(level, filepath.Join(root, "primary.log"), DiagnosticOptions{Dir: dir})
+			log.Debug("debug")
+			log.Info("info")
+			if _, err := os.Stat(dir); !os.IsNotExist(err) {
+				t.Fatalf("ordinary logs created diagnostic directory: %v", err)
+			}
+			child := log.With(zap.String("conversation_id", "test-id"))
+			child.Warn("retry", zap.Int("attempt", 2))
+			child.Error("failed", zap.Error(fmt.Errorf("test failure")))
+			files, _ := filepath.Glob(filepath.Join(dir, "*.log"))
+			if len(files) != 1 {
+				t.Fatalf("files: %v", files)
+			}
+			data, err := os.ReadFile(files[0])
+			if err != nil {
+				t.Fatal(err)
+			}
+			lines := strings.Split(strings.TrimSpace(string(data)), "\n")
+			if len(lines) != 2 {
+				t.Fatalf("unexpected diagnostic records: %s", data)
+			}
+			for i, line := range lines {
+				var record map[string]interface{}
+				if err := json.Unmarshal([]byte(line), &record); err != nil {
+					t.Fatal(err)
+				}
+				if record["conversation_id"] != "test-id" || record["timestamp"] == nil || record["caller"] == nil {
+					t.Fatalf("missing diagnostic context: %v", record)
+				}
+				if i == 1 && (record["stacktrace"] == nil || record["error"] != "test failure") {
+					t.Fatalf("missing error details: %v", record)
+				}
+			}
+		})
+	}
+}
+
+func TestDiagnosticDisabled(t *testing.T) {
+	dir := filepath.Join(t.TempDir(), "log")
+	log := New("error", os.DevNull, DiagnosticOptions{Dir: dir, Disabled: true})
+	log.Error("failure")
+	if _, err := os.Stat(dir); !os.IsNotExist(err) {
+		t.Fatalf("disabled diagnostics wrote files: %v", err)
+	}
+}
+
+func TestDailyRotationRetentionAndConcurrency(t *testing.T) {
+	dir := t.TempDir()
+	w := newDailyWriter(DiagnosticOptions{Dir: dir, RetentionDays: 2})
+	now := time.Date(2026, 9, 8, 23, 59, 59, 0, time.Local)
+	w.now = func() time.Time { return now }
+	for _, name := range []string{"diagnostic-2026-09-06.log", "diagnostic-2026-09-07.log", "other.log", "diagnostic-invalid.log"} {
+		if err := os.WriteFile(filepath.Join(dir, name), nil, 0600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if _, err := w.Write([]byte("before midnight\n")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "diagnostic-2026-09-06.log")); !os.IsNotExist(err) {
+		t.Fatal("expired file remains")
+	}
+	now = now.Add(2 * time.Second)
+	var wg sync.WaitGroup
+	for i := 0; i < 50; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, err := w.Write([]byte("after midnight\n")); err != nil {
+				t.Error(err)
+			}
+		}()
+	}
+	wg.Wait()
+	for name, count := range map[string]int{"diagnostic-2026-09-08.log": 1, "diagnostic-2026-09-09.log": 50} {
+		data, err := os.ReadFile(filepath.Join(dir, name))
+		if err != nil || strings.Count(string(data), "\n") != count {
+			t.Fatalf("%s: %q, %v", name, data, err)
+		}
+	}
+	if _, err := os.Stat(filepath.Join(dir, "diagnostic-2026-09-07.log")); !os.IsNotExist(err) {
+		t.Fatal("rotation did not expire old file")
+	}
+	for _, name := range []string{"other.log", "diagnostic-invalid.log"} {
+		if _, err := os.Stat(filepath.Join(dir, name)); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestDiagnosticWriteFailureKeepsPrimaryOutput(t *testing.T) {
+	root := t.TempDir()
+	primary := filepath.Join(root, "primary.log")
+	log := New("info", primary, DiagnosticOptions{Dir: filepath.Join(primary, "invalid")})
+	log.Error("still visible")
+	data, err := os.ReadFile(primary)
+	if err != nil || !strings.Contains(string(data), "still visible") {
+		t.Fatalf("primary output lost: %s, %v", data, err)
+	}
+}