diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 005423882..0a24cc8e3 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -220,23 +220,29 @@ func (al *AgentLoop) SetChannelManager(cm *channels.Manager) {
 
 // SetHooks installs a hook registry. Must be called before Run starts.
 func (al *AgentLoop) SetHooks(h *hooks.HookRegistry) {
+	if al.running.Load() {
+		panic("SetHooks must be called before Run starts")
+	}
 	al.hooks = h
 
 	// Rewire MessageTool callbacks to route through sendOutbound for hook interception.
 	for _, agentID := range al.registry.ListAgentIDs() {
 		if agent, ok := al.registry.GetAgent(agentID); ok {
 			if tool, ok := agent.Tools.Get("message"); ok {
-				if mt, ok := tool.(*tools.MessageTool); ok {
-					mt.SetSendCallback(func(channel, chatID, content string) error {
-						if !al.sendOutbound(context.Background(), bus.OutboundMessage{
-							Channel: channel,
-							ChatID:  chatID,
-							Content: content,
-						}) {
-							return fmt.Errorf("message canceled by hook")
-						}
-						return nil
-					})
+					if mt, ok := tool.(*tools.MessageTool); ok {
+						mt.SetSendCallback(func(channel, chatID, content string) error {
+							if sent, reason := al.sendOutbound(context.Background(), bus.OutboundMessage{
+								Channel: channel,
+								ChatID:  chatID,
+								Content: content,
+							}); !sent {
+								if strings.TrimSpace(reason) == "" {
+									reason = "unspecified"
+								}
+								return fmt.Errorf("message canceled by hook: %s", reason)
+							}
+							return nil
+						})
 				}
 			}
 		}
@@ -265,8 +271,8 @@ func (al *AgentLoop) EnablePlugins(plugins ...plugin.Plugin) error {
 }
 
 // sendOutbound wraps bus.PublishOutbound with the message_sending hook.
-// Returns true if the message was sent, false if canceled by a hook.
-func (al *AgentLoop) sendOutbound(ctx context.Context, msg bus.OutboundMessage) bool {
+// Returns whether the message was sent and, if canceled, the cancel reason.
+func (al *AgentLoop) sendOutbound(ctx context.Context, msg bus.OutboundMessage) (bool, string) {
 	if al.hooks != nil {
 		event := &hooks.MessageSendingEvent{Channel: msg.Channel, ChatID: msg.ChatID, Content: msg.Content}
 		al.hooks.TriggerMessageSending(ctx, event)
@@ -281,12 +287,12 @@ func (al *AgentLoop) sendOutbound(ctx context.Context, msg bus.OutboundMessage)
 					"chat_id": msg.ChatID,
 					"reason":  reason,
 				})
-			return false
+			return false, reason
 		}
 		msg.Content = event.Content
 	}
 	al.bus.PublishOutbound(msg)
-	return true
+	return true, ""
 }
 
 // RecordLastChannel records the last active channel for this workspace.
diff --git a/pkg/agent/plugin_test.go b/pkg/agent/plugin_test.go
index f872364c7..7bbcd3624 100644
--- a/pkg/agent/plugin_test.go
+++ b/pkg/agent/plugin_test.go
@@ -61,7 +61,7 @@ func TestSetPluginManagerInstallsHookRegistry(t *testing.T) {
 		t.Fatal("expected agent loop hooks to use plugin manager registry")
 	}
 
-	sent := al.sendOutbound(context.Background(), bus.OutboundMessage{
+	sent, reason := al.sendOutbound(context.Background(), bus.OutboundMessage{
 		Channel: "cli",
 		ChatID:  "direct",
 		Content: "hello",
@@ -69,5 +69,38 @@ func TestSetPluginManagerInstallsHookRegistry(t *testing.T) {
 	if sent {
 		t.Fatal("expected outbound message to be blocked by plugin")
 	}
+	if reason == "" {
+		t.Fatal("expected cancel reason to be propagated")
+	}
 }
 
+func TestSetHooksPanicsWhenRunning(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "agent-plugin-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	cfg := &config.Config{
+		Agents: config.AgentsConfig{
+			Defaults: config.AgentDefaults{
+				Workspace:         tmpDir,
+				Model:             "test-model",
+				MaxTokens:         4096,
+				MaxToolIterations: 10,
+			},
+		},
+	}
+
+	msgBus := bus.NewMessageBus()
+	al := NewAgentLoop(cfg, msgBus, &mockProvider{})
+	al.running.Store(true)
+
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal("expected panic when calling SetHooks while running")
+		}
+	}()
+
+	al.SetHooks(hooks.NewHookRegistry())
+}
diff --git a/pkg/hooks/hooks_test.go b/pkg/hooks/hooks_test.go
index eb80c2955..427ec0d07 100644
--- a/pkg/hooks/hooks_test.go
+++ b/pkg/hooks/hooks_test.go
@@ -293,12 +293,12 @@ func TestConcurrentRegistrationAndTrigger(t *testing.T) {
 	// Goroutines registering hooks.
 	for i := range 10 {
 		wg.Add(1)
-		go func() {
+		go func(idx int) {
 			defer wg.Done()
-			r.OnMessageReceived("reg-hook", i, func(_ context.Context, _ *MessageReceivedEvent) error {
+			r.OnMessageReceived(fmt.Sprintf("reg-hook-%d", idx), idx, func(_ context.Context, _ *MessageReceivedEvent) error {
 				return nil
 			})
-		}()
+		}(i)
 	}
 
 	// Goroutines triggering hooks concurrently.
@@ -322,7 +322,7 @@ func TestInsertSorted(t *testing.T) {
 	// Register with priorities: 50, 10, 30, 20, 40
 	priorities := []int{50, 10, 30, 20, 40}
 	for _, p := range priorities {
-		r.OnBeforeToolCall("p-"+string(rune('0'+p)), p, func(_ context.Context, _ *BeforeToolCallEvent) error {
+		r.OnBeforeToolCall(fmt.Sprintf("p-%d", p), p, func(_ context.Context, _ *BeforeToolCallEvent) error {
 			order = append(order, p)
 			return nil
 		})