Log error.Error with Any values in JSON

handler.go

@@ -403,3 +403,23 @@ type jsonLog struct {
 	Attrs   map[string]any `json:"attrs,omitempty"`
 	Pid     string         `json:"pid,omitempty"`
 }
+
+func (j jsonLog) MarshalJSON() ([]byte, error) {
+	a := map[string]any{}
+	for key, value := range j.Attrs {
+		if err, ok := value.(error); ok {
+			a[key] = err.Error()
+		} else {
+			a[key] = value
+		}
+	}
+
+	return json.Marshal(jsonLog{
+		Level:   j.Level,
+		Time:    j.Time,
+		Message: j.Message,
+		Group:   j.Group,
+		Attrs:   a,
+		Pid:     j.Pid,
+	})
+}

handler_test.go

@@ -3,6 +3,7 @@ package shandler_test
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -249,6 +250,37 @@ func TestErrorTags(t *testing.T) {
 	assert.Contains(t, stderr.String(), fmt.Sprintf("[ERROR] %s - test error_id=", now))
 }
 
+func TestSlogAnyError(t *testing.T) {
+	now := time.Now().Format(time.TimeOnly)
+	tests := []struct {
+		name     string
+		opts     []handler.HandlerOption
+		expected string
+	}{
+		{
+			name:     "text",
+			opts:     []handler.HandlerOption{},
+			expected: fmt.Sprintf(`[ERROR] %s - test err=big error`, now),
+		},
+		{
+			name:     "json",
+			opts:     []handler.HandlerOption{handler.WithJSON()},
+			expected: fmt.Sprintf(`{"level":"ERROR","time":"%s","message":"test","attrs":{"err":"big error"}}`, now),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var stderr bytes.Buffer
+			tt.opts = append(tt.opts, handler.WithStdErr(&stderr))
+			logger := slog.New(handler.NewHandler(tt.opts...))
+			err := errors.New("big error")
+			logger.Error("test", slog.Any("err", err))
+			actual := strings.TrimRight(stderr.String(), "\n")
+			assert.Equal(t, tt.expected, actual)
+		})
+	}
+}
+
 func BenchmarkHandlers(b *testing.B) {
 	var stdout bytes.Buffer
 	bt := []struct {