diff --git a/internal/acp/relay.go b/internal/acp/relay.go
new file mode 100644
index 0000000..e288c2c
--- /dev/null
+++ b/internal/acp/relay.go
@@ -0,0 +1,189 @@
+// relay.go 实现 ACP 双向 JSON-RPC 中继：
+//   - reader: 子进程 stdout → 解析 JSON-RPC → 落 events → fanout / handler dispatch
+//   - writer: 服务端发起 / WS 客户端透传 → 子进程 stdin
+//   - subscribers: 多 WS tab 共享 reader 的 fanout
+//
+// id 命名空间（spec §5.2 / §7.2）：
+//   - 服务端发起：int64（atomic 递增）
+//   - 客户端 WS 发起：string（前端自生成）
+//   - inflight 仅存 int64 → response chan；string id 的 response 当 event fanout
+package acp
+
+import (
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"sync"
+	"sync/atomic"
+
+	"github.com/google/uuid"
+
+	"github.com/yan1h/agent-coding-workflow/internal/acp/handlers"
+)
+
+// EventEnvelope 是推到 WS 的事件结构（与落库的 Event 字段同步，但带 ID 数值化方便 JSON）。
+type EventEnvelope struct {
+	ID        int64           `json:"id"`
+	Direction string          `json:"direction"`        // 'in' | 'out'
+	Kind      string          `json:"kind"`             // 'request'|'response'|'notification'|'error'|'session_terminated'|'slow_consumer_disconnect'
+	Method    string          `json:"method,omitempty"`
+	Payload   json.RawMessage `json:"payload"`
+	Truncated bool            `json:"truncated"`
+}
+
+// Subscriber 是单个 WS tab 的订阅句柄。
+type Subscriber struct {
+	ID     uuid.UUID
+	UserID uuid.UUID
+	Send   chan *EventEnvelope
+	closed atomic.Bool
+}
+
+// Relay 是单 session 的中继器。每个活跃 session 一个 Relay 实例。
+type Relay struct {
+	sessionID uuid.UUID
+
+	// 协议层
+	enc *Encoder
+	dec *Decoder
+
+	// 服务端发起请求等响应（id=int64）
+	inflight sync.Map // int64 → chan *Message
+	nextID   atomic.Int64
+
+	// WS subscribers（多 tab fanout）
+	subsMu sync.Mutex
+	subs   map[uuid.UUID]*Subscriber
+
+	// 写入路径
+	serverInitiated chan *Message // 服务端发起的 request/notification
+	clientInitiated chan *Message // WS 上行（已校验 method 白名单）
+
+	// 依赖
+	repo        Repository
+	fsHandler   *handlers.FsHandler
+	permHandler *handlers.PermissionHandler
+	log         *slog.Logger
+	cfg         RelayConfig
+
+	// 终止控制
+	closed atomic.Bool
+	done   chan struct{}
+}
+
+// RelayConfig 影响事件入库截断行为。
+type RelayConfig struct {
+	EventMaxPayload    int
+	EventTruncateField int
+	WSSendBuffer       int
+}
+
+// NewRelay 构造一个空 Relay；启动 goroutine 走 Run（D7 实现）。
+func NewRelay(sessionID uuid.UUID, dec *Decoder, enc *Encoder,
+	repo Repository, fs *handlers.FsHandler, perm *handlers.PermissionHandler,
+	cfg RelayConfig, log *slog.Logger) *Relay {
+	if log == nil {
+		log = slog.Default()
+	}
+	return &Relay{
+		sessionID:       sessionID,
+		enc:             enc,
+		dec:             dec,
+		subs:            map[uuid.UUID]*Subscriber{},
+		serverInitiated: make(chan *Message, 32),
+		clientInitiated: make(chan *Message, 32),
+		repo:            repo,
+		fsHandler:       fs,
+		permHandler:     perm,
+		log:             log,
+		cfg:             cfg,
+		done:            make(chan struct{}),
+	}
+}
+
+// Subscribe 注册一个 WS tab 订阅。
+func (r *Relay) Subscribe(userID uuid.UUID) *Subscriber {
+	sub := &Subscriber{
+		ID:     uuid.New(),
+		UserID: userID,
+		Send:   make(chan *EventEnvelope, r.cfg.WSSendBuffer),
+	}
+	r.subsMu.Lock()
+	r.subs[sub.ID] = sub
+	r.subsMu.Unlock()
+	return sub
+}
+
+// Unsubscribe 移除订阅；幂等。
+func (r *Relay) Unsubscribe(subID uuid.UUID) {
+	r.subsMu.Lock()
+	defer r.subsMu.Unlock()
+	if sub, ok := r.subs[subID]; ok {
+		if sub.closed.CompareAndSwap(false, true) {
+			close(sub.Send)
+		}
+		delete(r.subs, subID)
+	}
+}
+
+// fanout 把一条 envelope 推给所有 subscribers。慢消费者按 §7.6 自动断开。
+func (r *Relay) fanout(env *EventEnvelope) {
+	r.subsMu.Lock()
+	defer r.subsMu.Unlock()
+	for id, sub := range r.subs {
+		select {
+		case sub.Send <- env:
+		default:
+			// buffer 满 → 慢消费者，标 closed + 通知 + close channel
+			if sub.closed.CompareAndSwap(false, true) {
+				disc := &EventEnvelope{Kind: "slow_consumer_disconnect"}
+				select {
+				case sub.Send <- disc:
+				default:
+				}
+				close(sub.Send)
+				delete(r.subs, id)
+				r.log.Warn("acp.relay.slow_subscriber_dropped",
+					"session_id", r.sessionID, "sub_id", id)
+			}
+		}
+	}
+}
+
+// Close 优雅关闭：先广播 final event，再关 channel，再标 closed。
+// supervisor.onExit 调用，确保所有 WS 看到 session_terminated。
+func (r *Relay) Close(status string, exitCode *int32) {
+	if !r.closed.CompareAndSwap(false, true) {
+		return
+	}
+	final := &EventEnvelope{
+		Kind: "session_terminated",
+		Payload: mustJSON(map[string]any{
+			"status":    status,
+			"exit_code": exitCode,
+		}),
+	}
+	r.fanout(final)
+
+	r.subsMu.Lock()
+	for id, sub := range r.subs {
+		if sub.closed.CompareAndSwap(false, true) {
+			close(sub.Send)
+		}
+		delete(r.subs, id)
+	}
+	r.subsMu.Unlock()
+
+	close(r.done)
+}
+
+// Done returns a channel closed when Relay is terminated.
+func (r *Relay) Done() <-chan struct{} { return r.done }
+
+func mustJSON(v any) json.RawMessage {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return json.RawMessage(fmt.Sprintf(`{"_marshal_err":%q}`, err.Error()))
+	}
+	return b
+}
diff --git a/internal/acp/supervisor.go b/internal/acp/supervisor.go
index 54053c4..5071538 100644
--- a/internal/acp/supervisor.go
+++ b/internal/acp/supervisor.go
@@ -83,6 +83,7 @@ type Process struct {
 	Stdin  io.WriteCloser
 	Stdout io.ReadCloser
 	Stderr io.ReadCloser
+	Relay  *Relay
 
 	StderrBuf  *stderrRing  // 排障 ring buffer
 	StartedAt  time.Time
@@ -121,6 +122,16 @@ func (r *stderrRing) Tail(maxBytes int) string {
 	return out
 }
 
-// Relay field + GetRelay method added in Task D7 when Relay type is wired.
+// GetRelay returns the relay for an active session, or nil if not running.
+// WS handler 用此判断是否进 live 模式。
+func (s *Supervisor) GetRelay(sid uuid.UUID) *Relay {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	p, ok := s.procs[sid]
+	if !ok {
+		return nil
+	}
+	return p.Relay
+}
 
 // Spawn / Kill / ShutdownAll / monitorProcess / handshake are implemented in subsequent tasks (E2).