diff --git a/internal/bootstrap/config.go b/internal/bootstrap/config.go
index f757c59..e3500e9 100644
--- a/internal/bootstrap/config.go
+++ b/internal/bootstrap/config.go
@@ -25,6 +25,7 @@ type Config struct {
 	RateLimit     RateLimitConfig     `yaml:"rate_limit"`
 	Message       MessageConfig       `yaml:"message"`
 	ObjectStorage ObjectStorageConfig `yaml:"object_storage"`
+	Media         MediaConfig         `yaml:"media"`
 	SMS           SMSConfig           `yaml:"sms"`
 }
 
@@ -135,6 +136,23 @@ type ObjectStorageConfig struct {
 	ChatExpireDays int           `yaml:"chat_expire_days"`
 }
 
+// MediaConfig is server-side media detect (users) + compress (bots).
+// Client App still compresses user uploads; see docs/media.md.
+type MediaConfig struct {
+	Enabled        bool   `yaml:"enabled"`
+	UserReject     bool   `yaml:"user_reject"` // false=log only; true=return 40005
+	BotCompress    bool   `yaml:"bot_compress"`
+	ImageMaxEdge   int    `yaml:"image_max_edge"`
+	VideoMaxEdge   int    `yaml:"video_max_edge"`
+	AvatarMaxEdge  int    `yaml:"avatar_max_edge"`
+	ImageMaxBytes  int64  `yaml:"image_max_bytes"`
+	VideoMaxBytes  int64  `yaml:"video_max_bytes"`
+	AvatarMaxBytes int64  `yaml:"avatar_max_bytes"`
+	JPEGQuality    int    `yaml:"jpeg_quality"`
+	VideoCRF       int    `yaml:"video_crf"`
+	FFmpegPath     string `yaml:"ffmpeg_path"`
+}
+
 type MessageConfig struct {
 	MaxContentSize int   `yaml:"max_content_size"`
 	SyncLimit      int   `yaml:"sync_limit"`
@@ -314,6 +332,42 @@ func applyDefaults(cfg *Config) {
 	if cfg.RateLimit.UploadPerSecond < 1 {
 		cfg.RateLimit.UploadPerSecond = 10
 	}
+	applyMediaDefaults(&cfg.Media)
+}
+
+func applyMediaDefaults(m *MediaConfig) {
+	if m == nil {
+		return
+	}
+	// Numeric/path defaults only. Toggle via YAML media.enabled / media.bot_compress
+	// (configs ship with enabled: true). Explicit enabled: false stays off.
+	if m.ImageMaxEdge <= 0 {
+		m.ImageMaxEdge = 1080
+	}
+	if m.VideoMaxEdge <= 0 {
+		m.VideoMaxEdge = 720
+	}
+	if m.AvatarMaxEdge <= 0 {
+		m.AvatarMaxEdge = 200
+	}
+	if m.ImageMaxBytes <= 0 {
+		m.ImageMaxBytes = 10 << 20
+	}
+	if m.VideoMaxBytes <= 0 {
+		m.VideoMaxBytes = 100 << 20
+	}
+	if m.AvatarMaxBytes <= 0 {
+		m.AvatarMaxBytes = 2 << 20
+	}
+	if m.JPEGQuality <= 0 {
+		m.JPEGQuality = 85
+	}
+	if m.VideoCRF <= 0 {
+		m.VideoCRF = 23
+	}
+	if strings.TrimSpace(m.FFmpegPath) == "" {
+		m.FFmpegPath = "ffmpeg"
+	}
 }
 
 func parseEnv(raw string) string {
diff --git a/internal/module/upload/logic/credential.go b/internal/module/upload/logic/credential.go
index 996a096..81197ab 100644
--- a/internal/module/upload/logic/credential.go
+++ b/internal/module/upload/logic/credential.go
@@ -7,8 +7,11 @@ import (
 	"spark-server/internal/module/upload/entity"
 	"spark-server/internal/pkg/errs"
 	"spark-server/internal/pkg/i18n"
+	"spark-server/internal/pkg/media"
 	"spark-server/internal/pkg/reqctx"
 	"spark-server/internal/pkg/validate"
+
+	"go.uber.org/zap"
 )
 
 // CreateInput requests a single-object Presigned PUT. The client does not choose the key.
@@ -43,6 +46,9 @@ func (s *Service) issuePut(ctx context.Context, userID int64, in *CreateInput) (
 	if err != nil {
 		return nil, err
 	}
+	if err := s.checkDeclaredSize(ctx, in.Purpose, in.Size); err != nil {
+		return nil, err
+	}
 	name := reqctx.NewObjectName()
 	ownerID := userID
 	if in.Purpose == entity.PurposeGroupAvatar {
@@ -69,6 +75,41 @@ func (s *Service) issuePut(ctx context.Context, userID int64, in *CreateInput) (
 	}, nil
 }
 
+// checkDeclaredSize applies soft media policy on credential size (no dimensions at this stage).
+func (s *Service) checkDeclaredSize(ctx context.Context, purpose string, size int64) error {
+	if s == nil || !s.media.Enabled {
+		return nil
+	}
+	var v *media.Violation
+	switch purpose {
+	case entity.PurposeAvatar, entity.PurposeGroupAvatar, entity.PurposeMomentCover:
+		v = s.media.CheckAvatarMeta(0, 0, size)
+	case entity.PurposeChatVideo, entity.PurposeMomentVideo, entity.PurposeSquareVideo:
+		v = s.media.CheckVideoMeta(0, 0, size, 0)
+	case entity.PurposeChatImage, entity.PurposeMomentImage, entity.PurposeSquareImage, entity.PurposeChatGIF:
+		v = s.media.CheckImageMeta(0, 0, size)
+	default:
+		return nil
+	}
+	if v == nil {
+		return nil
+	}
+	log := s.log
+	if log == nil {
+		log = zap.NewNop()
+	}
+	log.Warn("upload media size soft-check",
+		zap.String("purpose", purpose),
+		zap.String("reason", v.Reason),
+		zap.Int64("size", v.Size),
+		zap.Int64("max_size", v.MaxSize),
+	)
+	if !s.media.UserReject {
+		return nil
+	}
+	return errs.ErrMediaOversized.WithMsg(i18n.T(reqctx.Lang(ctx), i18n.KeyMediaOversized, nil))
+}
+
 func matchSpec(ctx context.Context, in *CreateInput) (entity.Spec, string, error) {
 	lang := reqctx.Lang(ctx)
 	spec, ok := entity.LookupSpec(in.Purpose)