diff --git a/backend/go-service/app/dto/im_dto.go b/backend/go-service/app/dto/im_dto.go
new file mode 100644
index 0000000..aa390f1
--- /dev/null
+++ b/backend/go-service/app/dto/im_dto.go
@@ -0,0 +1,109 @@
+package dto
+
+// ====== 消息相关 DTO ======
+
+// SendMessageRequest 发送消息请求（WS 事件 im.message.send 的 data 字段）
+type SendMessageRequest struct {
+	ConversationID int64  `json:"conversation_id"`  // 会话 ID（与 TargetUserID 二选一）
+	TargetUserID   int64  `json:"target_user_id"`   // 对方用户 ID（首次发消息时使用，自动创建会话）
+	Type           int    `json:"type"`              // 消息类型：1=文本
+	Content        string `json:"content"`           // 消息内容
+	ClientMsgID    string `json:"client_msg_id"`     // 客户端消息唯一 ID，用于幂等去重
+}
+
+// MessageDTO 消息传输对象（返回给前端）
+type MessageDTO struct {
+	ID             int64  `json:"id"`              // 消息 ID
+	ConversationID int64  `json:"conversation_id"` // 所属会话 ID
+	SenderID       int64  `json:"sender_id"`       // 发送者用户 ID
+	Type           int    `json:"type"`            // 消息类型
+	Content        string `json:"content"`         // 消息内容
+	Status         int    `json:"status"`          // 消息状态：1=正常，2=已撤回
+	ClientMsgID    string `json:"client_msg_id"`   // 客户端消息 ID
+	CreatedAt      string `json:"created_at"`      // 发送时间
+}
+
+// RecallMessageRequest 撤回消息请求（WS 事件 im.message.recall 的 data 字段）
+type RecallMessageRequest struct {
+	MessageID int64 `json:"message_id"` // 要撤回的消息 ID
+}
+
+// ====== 会话相关 DTO ======
+
+// ConversationDTO 会话列表条目（返回给前端）
+type ConversationDTO struct {
+	ID              int64  `json:"id"`                // 会话 ID
+	Type            int    `json:"type"`              // 会话类型：1=单聊
+	PeerUserID      int64  `json:"peer_user_id"`      // 单聊对方用户 ID
+	PeerNickname    string `json:"peer_nickname"`     // 对方昵称
+	PeerAvatar      string `json:"peer_avatar"`       // 对方头像
+	LastMsgContent  string `json:"last_msg_content"`  // 最后消息预览
+	LastMsgTime     string `json:"last_msg_time"`     // 最后消息时间
+	LastMsgSenderID *int64 `json:"last_msg_sender_id"` // 最后消息发送者 ID
+	IsPinned        bool   `json:"is_pinned"`         // 是否置顶
+	UnreadCount     int    `json:"unread_count"`      // 未读消息数
+}
+
+// ConversationListResponse 会话列表响应
+type ConversationListResponse struct {
+	List []ConversationDTO `json:"list"` // 会话列表
+}
+
+// PinConversationRequest 置顶/取消置顶请求
+type PinConversationRequest struct {
+	IsPinned bool `json:"is_pinned"` // true=置顶, false=取消
+}
+
+// ClearHistoryRequest 清空聊天记录请求（REST API）
+type ClearHistoryRequest struct {
+	ConversationID int64 `json:"conversation_id" binding:"required"` // 会话 ID
+}
+
+// ====== 消息搜索 DTO ======
+
+// SearchMessageRequest 消息搜索请求
+type SearchMessageRequest struct {
+	Keyword string `form:"keyword" binding:"required"` // 搜索关键词
+	Limit   int    `form:"limit"`                      // 返回条数（默认 50）
+}
+
+// SearchMessageResponse 消息搜索结果
+type SearchMessageResponse struct {
+	List []MessageSearchItem `json:"list"` // 搜索结果列表
+}
+
+// MessageSearchItem 搜索结果单条
+type MessageSearchItem struct {
+	MessageDTO
+	SenderNickname string `json:"sender_nickname"` // 发送者昵称
+	SenderAvatar   string `json:"sender_avatar"`   // 发送者头像
+}
+
+// ====== 历史消息 DTO ======
+
+// HistoryMessageRequest 历史消息查询请求
+type HistoryMessageRequest struct {
+	ConversationID int64 `form:"conversation_id" binding:"required"` // 会话 ID
+	BeforeID       int64 `form:"before_id"`                          // 游标：查询 ID 小于此值的消息，0 表示最新
+	Limit          int   `form:"limit"`                              // 每次拉取条数（默认 30）
+}
+
+// HistoryMessageResponse 历史消息响应
+type HistoryMessageResponse struct {
+	List    []MessageDTO `json:"list"`     // 消息列表（ID 降序）
+	HasMore bool         `json:"has_more"` // 是否还有更多历史消息
+}
+
+// ====== 已读标记 DTO ======
+
+// MarkReadRequest 标记已读请求（WS 事件 im.conversation.read 的 data 字段）
+type MarkReadRequest struct {
+	ConversationID int64 `json:"conversation_id"` // 要标记已读的会话 ID
+}
+
+// ====== 输入状态 DTO ======
+
+// TypingRequest 正在输入通知请求（WS 事件 im.typing 的 data 字段）
+type TypingRequest struct {
+	ConversationID int64 `json:"conversation_id"` // 会话 ID
+}
diff --git a/backend/go-service/app/im/service/im_service.go b/backend/go-service/app/im/service/im_service.go
new file mode 100644
index 0000000..d7a6681
--- /dev/null
+++ b/backend/go-service/app/im/service/im_service.go
@@ -0,0 +1,544 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/echochat/backend/app/constants"
+	"github.com/echochat/backend/app/dto"
+	"github.com/echochat/backend/app/im/dao"
+	"github.com/echochat/backend/app/im/model"
+	"github.com/echochat/backend/pkg/logs"
+	"github.com/echochat/backend/pkg/ws"
+	"github.com/redis/go-redis/v9"
+	"go.uber.org/zap"
+)
+
+const (
+	unreadKeyPrefix = "echo:im:unread:" // Redis 未读总数 key 前缀，完整 key: echo:im:unread:{user_id}
+	defaultPageSize = 30                // 历史消息默认拉取条数
+	maxPageSize     = 100               // 历史消息最大拉取条数
+)
+
+var (
+	ErrNotFriend       = errors.New("对方不是你的好友")
+	ErrEmptyContent    = errors.New("消息内容不能为空")
+	ErrConvNotFound    = errors.New("会话不存在")
+	ErrMsgNotFound     = errors.New("消息不存在")
+	ErrNotSender       = errors.New("只能撤回自己发送的消息")
+	ErrRecallTimeout   = errors.New("超过撤回时限")
+	ErrNotMember       = errors.New("你不是该会话的成员")
+	ErrDuplicateMsg    = errors.New("重复消息")
+	ErrInvalidMsgType  = errors.New("不支持的消息类型")
+)
+
+// IMService 即时通讯核心业务服务
+type IMService struct {
+	convDAO       *dao.ConversationDAO
+	msgDAO        *dao.MessageDAO
+	pubsub        *ws.PubSub
+	rdb           *redis.Client
+	friendChecker FriendChecker
+	userInfoGetter UserInfoGetter
+}
+
+// NewIMService 创建 IMService 实例
+func NewIMService(
+	convDAO *dao.ConversationDAO,
+	msgDAO *dao.MessageDAO,
+	pubsub *ws.PubSub,
+	rdb *redis.Client,
+	friendChecker FriendChecker,
+	userInfoGetter UserInfoGetter,
+) *IMService {
+	return &IMService{
+		convDAO:        convDAO,
+		msgDAO:         msgDAO,
+		pubsub:         pubsub,
+		rdb:            rdb,
+		friendChecker:  friendChecker,
+		userInfoGetter: userInfoGetter,
+	}
+}
+
+// SendMessage 发送消息（核心流程）
+// 1. 校验好友关系
+// 2. 查找或创建会话
+// 3. 幂等去重（client_msg_id）
+// 4. 写入消息 + 更新会话最后消息 + 递增对方未读数
+// 5. 通过 PubSub 推送给接收方
+func (s *IMService) SendMessage(ctx context.Context, senderID int64, req *dto.SendMessageRequest) (*dto.MessageDTO, error) {
+	funcName := "service.im_service.SendMessage"
+	logs.Info(ctx, funcName, "发送消息",
+		zap.Int64("sender_id", senderID),
+		zap.Int64("conversation_id", req.ConversationID),
+		zap.Int64("target_user_id", req.TargetUserID))
+
+	if req.Content == "" {
+		return nil, ErrEmptyContent
+	}
+	if req.Type == 0 {
+		req.Type = constants.MessageTypeText
+	}
+	if req.Type != constants.MessageTypeText {
+		return nil, ErrInvalidMsgType
+	}
+
+	convID := req.ConversationID
+	var peerID int64
+
+	if convID == 0 && req.TargetUserID > 0 {
+		isFriend, err := s.friendChecker.IsFriend(ctx, senderID, req.TargetUserID)
+		if err != nil {
+			logs.Error(ctx, funcName, "检查好友关系失败", zap.Error(err))
+			return nil, err
+		}
+		if !isFriend {
+			return nil, ErrNotFriend
+		}
+
+		conv, err := s.getOrCreatePrivateConversation(ctx, senderID, req.TargetUserID)
+		if err != nil {
+			return nil, err
+		}
+		convID = conv.ID
+		peerID = req.TargetUserID
+	} else if convID > 0 {
+		member, err := s.convDAO.GetMember(ctx, convID, senderID)
+		if err != nil {
+			return nil, ErrNotMember
+		}
+		if member == nil {
+			return nil, ErrNotMember
+		}
+		peerID, err = s.convDAO.GetPeerUserID(ctx, convID, senderID)
+		if err != nil {
+			logs.Error(ctx, funcName, "查询对方用户 ID 失败", zap.Error(err))
+			return nil, err
+		}
+	} else {
+		return nil, ErrConvNotFound
+	}
+
+	if req.ClientMsgID != "" {
+		existing, err := s.msgDAO.FindByClientMsgID(ctx, convID, req.ClientMsgID)
+		if err != nil {
+			logs.Error(ctx, funcName, "幂等去重查询失败", zap.Error(err))
+			return nil, err
+		}
+		if existing != nil {
+			return s.toMessageDTO(existing), ErrDuplicateMsg
+		}
+	}
+
+	msg := &model.Message{
+		ConversationID: convID,
+		SenderID:       senderID,
+		Type:           req.Type,
+		Content:        req.Content,
+		Status:         constants.MessageStatusNormal,
+		ClientMsgID:    req.ClientMsgID,
+	}
+	if err := s.msgDAO.Create(ctx, msg); err != nil {
+		return nil, err
+	}
+
+	now := time.Now()
+	if err := s.convDAO.UpdateLastMessage(ctx, convID, msg.ID, truncateContent(req.Content, 100), senderID, now); err != nil {
+		logs.Error(ctx, funcName, "更新最后消息失败", zap.Error(err))
+	}
+
+	if err := s.convDAO.RestoreMember(ctx, convID, peerID); err != nil {
+		logs.Error(ctx, funcName, "恢复对方会话视图失败", zap.Error(err))
+	}
+
+	if err := s.convDAO.IncrementUnread(ctx, convID, peerID); err != nil {
+		logs.Error(ctx, funcName, "递增未读计数失败", zap.Error(err))
+	}
+
+	s.incrementTotalUnread(ctx, peerID)
+
+	s.pushToUser(ctx, peerID, "im.message.new", map[string]interface{}{
+		"id":              msg.ID,
+		"conversation_id": convID,
+		"sender_id":       senderID,
+		"type":            msg.Type,
+		"content":         msg.Content,
+		"client_msg_id":   msg.ClientMsgID,
+		"created_at":      msg.CreatedAt.Format("2006-01-02 15:04:05"),
+	})
+
+	return s.toMessageDTO(msg), nil
+}
+
+// RecallMessage 撤回消息（2分钟内）
+func (s *IMService) RecallMessage(ctx context.Context, senderID int64, messageID int64) error {
+	funcName := "service.im_service.RecallMessage"
+	logs.Info(ctx, funcName, "撤回消息",
+		zap.Int64("sender_id", senderID), zap.Int64("message_id", messageID))
+
+	msg, err := s.msgDAO.GetByID(ctx, messageID)
+	if err != nil {
+		return ErrMsgNotFound
+	}
+	if msg.SenderID != senderID {
+		return ErrNotSender
+	}
+	if time.Since(msg.CreatedAt).Seconds() > float64(constants.MessageRecallTimeLimit) {
+		return ErrRecallTimeout
+	}
+
+	if err := s.msgDAO.UpdateStatus(ctx, messageID, constants.MessageStatusRecalled); err != nil {
+		logs.Error(ctx, funcName, "更新消息状态失败", zap.Error(err))
+		return err
+	}
+
+	memberIDs, err := s.convDAO.GetConversationMemberIDs(ctx, msg.ConversationID)
+	if err != nil {
+		logs.Error(ctx, funcName, "获取会话成员失败", zap.Error(err))
+		return nil
+	}
+	for _, uid := range memberIDs {
+		if uid == senderID {
+			continue
+		}
+		s.pushToUser(ctx, uid, "im.message.recalled", map[string]interface{}{
+			"message_id":      messageID,
+			"conversation_id": msg.ConversationID,
+		})
+	}
+
+	return nil
+}
+
+// GetConversationList 获取会话列表（含对方用户信息）
+func (s *IMService) GetConversationList(ctx context.Context, userID int64) (*dto.ConversationListResponse, error) {
+	funcName := "service.im_service.GetConversationList"
+	logs.Debug(ctx, funcName, "获取会话列表", zap.Int64("user_id", userID))
+
+	convs, err := s.convDAO.GetUserConversations(ctx, userID)
+	if err != nil {
+		return nil, err
+	}
+
+	peerIDs := make([]int64, 0, len(convs))
+	convIDToPeerID := make(map[int64]int64, len(convs))
+	for _, c := range convs {
+		peerID, err := s.convDAO.GetPeerUserID(ctx, c.ID, userID)
+		if err != nil {
+			logs.Error(ctx, funcName, "查询对方用户 ID 失败", zap.Int64("conv_id", c.ID), zap.Error(err))
+			continue
+		}
+		peerIDs = append(peerIDs, peerID)
+		convIDToPeerID[c.ID] = peerID
+	}
+
+	userMap := make(map[int64]*userBrief)
+	if len(peerIDs) > 0 {
+		users, err := s.userInfoGetter.GetUsersByIDs(ctx, peerIDs)
+		if err != nil {
+			logs.Error(ctx, funcName, "批量查询用户信息失败", zap.Error(err))
+		} else {
+			for i := range users {
+				u := users[i]
+				userMap[u.ID] = &userBrief{Nickname: u.Nickname, Avatar: u.Avatar}
+			}
+		}
+	}
+
+	list := make([]dto.ConversationDTO, 0, len(convs))
+	for _, c := range convs {
+		peerID := convIDToPeerID[c.ID]
+		item := dto.ConversationDTO{
+			ID:              c.ID,
+			Type:            c.Type,
+			PeerUserID:      peerID,
+			LastMsgContent:  c.LastMsgContent,
+			LastMsgSenderID: c.LastMsgSenderID,
+			IsPinned:        c.IsPinned,
+			UnreadCount:     c.UnreadCount,
+		}
+		if c.LastMsgTime != nil {
+			item.LastMsgTime = c.LastMsgTime.Format("2006-01-02 15:04:05")
+		}
+		if brief, ok := userMap[peerID]; ok {
+			item.PeerNickname = brief.Nickname
+			item.PeerAvatar = brief.Avatar
+		}
+		list = append(list, item)
+	}
+
+	return &dto.ConversationListResponse{List: list}, nil
+}
+
+// GetHistoryMessages 获取历史消息（游标分页）
+func (s *IMService) GetHistoryMessages(ctx context.Context, userID int64, req *dto.HistoryMessageRequest) (*dto.HistoryMessageResponse, error) {
+	funcName := "service.im_service.GetHistoryMessages"
+	logs.Debug(ctx, funcName, "查询历史消息",
+		zap.Int64("user_id", userID), zap.Int64("conversation_id", req.ConversationID))
+
+	member, err := s.convDAO.GetMember(ctx, req.ConversationID, userID)
+	if err != nil || member == nil {
+		return nil, ErrNotMember
+	}
+
+	limit := req.Limit
+	if limit <= 0 {
+		limit = defaultPageSize
+	}
+	if limit > maxPageSize {
+		limit = maxPageSize
+	}
+
+	messages, err := s.msgDAO.GetByConversation(ctx, req.ConversationID, req.BeforeID, limit+1)
+	if err != nil {
+		return nil, err
+	}
+
+	hasMore := len(messages) > limit
+	if hasMore {
+		messages = messages[:limit]
+	}
+
+	list := make([]dto.MessageDTO, 0, len(messages))
+	for _, m := range messages {
+		list = append(list, *s.toMessageDTO(&m))
+	}
+
+	return &dto.HistoryMessageResponse{List: list, HasMore: hasMore}, nil
+}
+
+// MarkRead 标记会话已读（清零未读 + 更新 Redis 总未读数）
+func (s *IMService) MarkRead(ctx context.Context, userID int64, conversationID int64) error {
+	funcName := "service.im_service.MarkRead"
+	logs.Info(ctx, funcName, "标记已读",
+		zap.Int64("user_id", userID), zap.Int64("conversation_id", conversationID))
+
+	member, err := s.convDAO.GetMember(ctx, conversationID, userID)
+	if err != nil || member == nil {
+		return ErrNotMember
+	}
+
+	if member.UnreadCount == 0 {
+		return nil
+	}
+
+	latestMsgID, err := s.msgDAO.GetLatestMessageID(ctx, conversationID)
+	if err != nil {
+		logs.Error(ctx, funcName, "获取最新消息 ID 失败", zap.Error(err))
+		return err
+	}
+
+	if err := s.convDAO.ClearUnread(ctx, conversationID, userID, latestMsgID); err != nil {
+		logs.Error(ctx, funcName, "清零未读失败", zap.Error(err))
+		return err
+	}
+
+	s.decrementTotalUnread(ctx, userID, member.UnreadCount)
+	return nil
+}
+
+// PinConversation 置顶/取消置顶会话
+func (s *IMService) PinConversation(ctx context.Context, userID int64, conversationID int64, isPinned bool) error {
+	funcName := "service.im_service.PinConversation"
+	logs.Info(ctx, funcName, "更新置顶状态",
+		zap.Int64("user_id", userID), zap.Int64("conversation_id", conversationID), zap.Bool("is_pinned", isPinned))
+
+	member, err := s.convDAO.GetMember(ctx, conversationID, userID)
+	if err != nil || member == nil {
+		return ErrNotMember
+	}
+	return s.convDAO.UpdateMemberPinned(ctx, conversationID, userID, isPinned)
+}
+
+// DeleteConversation 删除会话（软删除，仅影响当前用户视图）
+func (s *IMService) DeleteConversation(ctx context.Context, userID int64, conversationID int64) error {
+	funcName := "service.im_service.DeleteConversation"
+	logs.Info(ctx, funcName, "删除会话",
+		zap.Int64("user_id", userID), zap.Int64("conversation_id", conversationID))
+
+	member, err := s.convDAO.GetMember(ctx, conversationID, userID)
+	if err != nil || member == nil {
+		return ErrNotMember
+	}
+
+	if member.UnreadCount > 0 {
+		s.decrementTotalUnread(ctx, userID, member.UnreadCount)
+	}
+	return s.convDAO.SoftDeleteMember(ctx, conversationID, userID)
+}
+
+// ClearHistory 清空聊天记录（标记消息为已删除）
+func (s *IMService) ClearHistory(ctx context.Context, userID int64, conversationID int64) error {
+	funcName := "service.im_service.ClearHistory"
+	logs.Info(ctx, funcName, "清空聊天记录",
+		zap.Int64("user_id", userID), zap.Int64("conversation_id", conversationID))
+
+	member, err := s.convDAO.GetMember(ctx, conversationID, userID)
+	if err != nil || member == nil {
+		return ErrNotMember
+	}
+	return s.msgDAO.DeleteByConversation(ctx, conversationID)
+}
+
+// SearchMessages 全局消息搜索
+func (s *IMService) SearchMessages(ctx context.Context, userID int64, req *dto.SearchMessageRequest) (*dto.SearchMessageResponse, error) {
+	funcName := "service.im_service.SearchMessages"
+	logs.Debug(ctx, funcName, "全局消息搜索",
+		zap.Int64("user_id", userID), zap.String("keyword", req.Keyword))
+
+	limit := req.Limit
+	if limit <= 0 {
+		limit = 50
+	}
+
+	results, err := s.msgDAO.SearchMessages(ctx, userID, req.Keyword, limit)
+	if err != nil {
+		return nil, err
+	}
+
+	senderIDs := make([]int64, 0)
+	senderSet := make(map[int64]bool)
+	for _, r := range results {
+		if !senderSet[r.SenderID] {
+			senderIDs = append(senderIDs, r.SenderID)
+			senderSet[r.SenderID] = true
+		}
+	}
+
+	userMap := make(map[int64]*userBrief)
+	if len(senderIDs) > 0 {
+		users, err := s.userInfoGetter.GetUsersByIDs(ctx, senderIDs)
+		if err != nil {
+			logs.Error(ctx, funcName, "查询发送者信息失败", zap.Error(err))
+		} else {
+			for i := range users {
+				u := users[i]
+				userMap[u.ID] = &userBrief{Nickname: u.Nickname, Avatar: u.Avatar}
+			}
+		}
+	}
+
+	list := make([]dto.MessageSearchItem, 0, len(results))
+	for _, r := range results {
+		item := dto.MessageSearchItem{
+			MessageDTO: dto.MessageDTO{
+				ID:             r.ID,
+				ConversationID: r.ConversationID,
+				SenderID:       r.SenderID,
+				Type:           constants.MessageTypeText,
+				Content:        r.Content,
+				Status:         constants.MessageStatusNormal,
+				CreatedAt:      r.CreatedAt,
+			},
+		}
+		if brief, ok := userMap[r.SenderID]; ok {
+			item.SenderNickname = brief.Nickname
+			item.SenderAvatar = brief.Avatar
+		}
+		list = append(list, item)
+	}
+
+	return &dto.SearchMessageResponse{List: list}, nil
+}
+
+// GetTotalUnread 获取用户的全局未读消息总数（从 Redis）
+func (s *IMService) GetTotalUnread(ctx context.Context, userID int64) (int64, error) {
+	key := fmt.Sprintf("%s%d", unreadKeyPrefix, userID)
+	val, err := s.rdb.Get(ctx, key).Int64()
+	if err == redis.Nil {
+		return 0, nil
+	}
+	return val, err
+}
+
+// ====== 内部辅助方法 ======
+
+// getOrCreatePrivateConversation 查找或创建单聊会话
+func (s *IMService) getOrCreatePrivateConversation(ctx context.Context, userID, targetUserID int64) (*model.Conversation, error) {
+	funcName := "service.im_service.getOrCreatePrivateConversation"
+
+	conv, err := s.convDAO.FindPrivateConversation(ctx, userID, targetUserID)
+	if err != nil {
+		return nil, err
+	}
+	if conv != nil {
+		return conv, nil
+	}
+
+	logs.Info(ctx, funcName, "创建新的单聊会话",
+		zap.Int64("user_id", userID), zap.Int64("target_user_id", targetUserID))
+
+	newConv := &model.Conversation{
+		Type:      constants.ConversationTypePrivate,
+		CreatorID: userID,
+	}
+	if err := s.convDAO.CreateWithMembers(ctx, newConv, []int64{userID, targetUserID}); err != nil {
+		return nil, err
+	}
+	return newConv, nil
+}
+
+// pushToUser 通过 PubSub 向指定用户推送消息
+func (s *IMService) pushToUser(ctx context.Context, userID int64, event string, data interface{}) {
+	push := ws.NewPushMessage(event, data)
+	bytes, err := ws.MarshalPush(push)
+	if err != nil {
+		logs.Error(ctx, "service.im_service.pushToUser", "序列化推送消息失败",
+			zap.String("event", event), zap.Error(err))
+		return
+	}
+	if err := s.pubsub.Publish(ctx, userID, bytes); err != nil {
+		logs.Error(ctx, "service.im_service.pushToUser", "PubSub 发布失败",
+			zap.Int64("user_id", userID), zap.Error(err))
+	}
+}
+
+// incrementTotalUnread Redis 全局未读 +1
+func (s *IMService) incrementTotalUnread(ctx context.Context, userID int64) {
+	key := fmt.Sprintf("%s%d", unreadKeyPrefix, userID)
+	if err := s.rdb.Incr(ctx, key).Err(); err != nil {
+		logs.Error(ctx, "service.im_service.incrementTotalUnread", "Redis INCR 失败",
+			zap.Int64("user_id", userID), zap.Error(err))
+	}
+}
+
+// decrementTotalUnread Redis 全局未读 -N（下限为 0）
+func (s *IMService) decrementTotalUnread(ctx context.Context, userID int64, count int) {
+	key := fmt.Sprintf("%s%d", unreadKeyPrefix, userID)
+	if err := s.rdb.DecrBy(ctx, key, int64(count)).Err(); err != nil {
+		logs.Error(ctx, "service.im_service.decrementTotalUnread", "Redis DECRBY 失败",
+			zap.Int64("user_id", userID), zap.Error(err))
+	}
+}
+
+// toMessageDTO 将 model.Message 转换为 dto.MessageDTO
+func (s *IMService) toMessageDTO(m *model.Message) *dto.MessageDTO {
+	return &dto.MessageDTO{
+		ID:             m.ID,
+		ConversationID: m.ConversationID,
+		SenderID:       m.SenderID,
+		Type:           m.Type,
+		Content:        m.Content,
+		Status:         m.Status,
+		ClientMsgID:    m.ClientMsgID,
+		CreatedAt:      m.CreatedAt.Format("2006-01-02 15:04:05"),
+	}
+}
+
+// userBrief 用户简要信息（内部使用）
+type userBrief struct {
+	Nickname string
+	Avatar   string
+}
+
+// truncateContent 截断消息内容用于预览
+func truncateContent(s string, maxRunes int) string {
+	runes := []rune(s)
+	if len(runes) <= maxRunes {
+		return s
+	}
+	return string(runes[:maxRunes]) + "..."
+}
diff --git a/backend/go-service/app/im/service/interfaces.go b/backend/go-service/app/im/service/interfaces.go
new file mode 100644
index 0000000..5be155e
--- /dev/null
+++ b/backend/go-service/app/im/service/interfaces.go
@@ -0,0 +1,20 @@
+// Package service 提供 IM 模块的业务逻辑
+package service
+
+import (
+	"context"
+
+	authModel "github.com/echochat/backend/app/auth/model"
+)
+
+// FriendChecker 好友关系校验接口
+// 由 contact.FriendshipDAO 隐式实现，通过 Wire 接口注入避免直接依赖 contact 包
+type FriendChecker interface {
+	IsFriend(ctx context.Context, userID, targetID int64) (bool, error)
+}
+
+// UserInfoGetter 用户信息批量查询接口
+// 由 contact.FriendshipDAO 隐式实现，返回 id/username/nickname/avatar
+type UserInfoGetter interface {
+	GetUsersByIDs(ctx context.Context, userIDs []int64) ([]authModel.User, error)
+}