agent: restore metadata-only prompt injection

pkg/agent/loop.go

@@ -1649,7 +1649,7 @@ func (al *AgentLoop) runTurn(ctx context.Context, ts *turnState) (turnResult, er
 		ts.opts.SenderDisplayName,
 		activeSkillNames(ts.agent, ts.opts)...,
 	)
-	annotateCurrentUserMessageForLLM(messages, ts.userMessage, annotatedUserMessage)
+	messages = annotateCurrentUserMessageForLLM(messages, ts.userMessage, annotatedUserMessage)
 
 	cfg := al.GetConfig()
 	maxMediaSize := cfg.Agents.Defaults.GetMaxMediaSize()
@@ -1680,7 +1680,7 @@ func (al *AgentLoop) runTurn(ctx context.Context, ts *turnState) (turnResult, er
 				ts.opts.SenderID, ts.opts.SenderDisplayName,
 				activeSkillNames(ts.agent, ts.opts)...,
 			)
-			annotateCurrentUserMessageForLLM(messages, ts.userMessage, annotatedUserMessage)
+			messages = annotateCurrentUserMessageForLLM(messages, ts.userMessage, annotatedUserMessage)
 			messages = resolveMediaRefs(messages, al.mediaStore, maxMediaSize)
 		}
 	}
@@ -2053,7 +2053,7 @@ turnLoop:
 					nil, ts.channel, ts.chatID, ts.opts.SenderID, ts.opts.SenderDisplayName,
 					activeSkillNames(ts.agent, ts.opts)...,
 				)
-				annotateCurrentUserMessageForLLM(messages, ts.userMessage, annotatedUserMessage)
+				messages = annotateCurrentUserMessageForLLM(messages, ts.userMessage, annotatedUserMessage)
 				callMessages = messages
 				if gracefulTerminal {
 					callMessages = append(append([]providers.Message(nil), messages...), ts.interruptHintMessage())
@@ -3561,9 +3561,9 @@ func annotateCurrentUserMessageForLLM(
 	messages []providers.Message,
 	rawUserMessage string,
 	annotatedUserMessage string,
-) {
+) []providers.Message {
 	if rawUserMessage == annotatedUserMessage {
-		return
+		return messages
 	}
 	for i := len(messages) - 1; i >= 0; i-- {
 		if messages[i].Role != "user" {
@@ -3573,8 +3573,10 @@ func annotateCurrentUserMessageForLLM(
 			continue
 		}
 		messages[i].Content = annotatedUserMessage
-		return
+		return messages
 	}
+	// Keep session history raw, but ensure metadata-only current turns still reach the LLM.
+	return append(messages, providers.Message{Role: "user", Content: annotatedUserMessage})
 }
 
 func formatUserMessageWithThreadMetadata(

pkg/agent/loop_test.go

@@ -268,6 +268,80 @@ func TestProcessMessage_AnnotatesThreadMetadataInPromptOnly(t *testing.T) {
 	}
 }
 
+func TestProcessMessage_AnnotatesThreadMetadataOnlyPromptWhenContentEmpty(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "agent-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,
+				ModelName:         "test-model",
+				MaxTokens:         4096,
+				MaxToolIterations: 10,
+			},
+		},
+	}
+
+	msgBus := bus.NewMessageBus()
+	provider := &recordingProvider{}
+	al := NewAgentLoop(cfg, msgBus, provider)
+
+	msg := bus.InboundMessage{
+		Channel:   "discord",
+		SenderID:  "discord:123",
+		ChatID:    "group-1",
+		Content:   "",
+		MessageID: "123",
+		Sender: bus.SenderInfo{
+			DisplayName: "Alice",
+			Username:    "@alice",
+		},
+		Peer: bus.Peer{
+			Kind: "direct",
+			ID:   "discord:123",
+		},
+		Metadata: map[string]string{
+			metadataKeyReplyToMessage: "120",
+		},
+	}
+
+	response, err := al.processMessage(context.Background(), msg)
+	if err != nil {
+		t.Fatalf("processMessage() error = %v", err)
+	}
+	if response != "Mock response" {
+		t.Fatalf("processMessage() response = %q, want %q", response, "Mock response")
+	}
+	if len(provider.lastMessages) == 0 {
+		t.Fatal("provider did not receive any messages")
+	}
+
+	wantAnnotated := "[from:Alice (@alice); msgs:#123, reply_to:#120]"
+	lastMessage := provider.lastMessages[len(provider.lastMessages)-1]
+	if lastMessage.Role != "user" || lastMessage.Content != wantAnnotated {
+		t.Fatalf("last provider message = %+v, want user annotation %q", lastMessage, wantAnnotated)
+	}
+
+	route := al.registry.ResolveRoute(routing.RouteInput{
+		Channel: msg.Channel,
+		Peer:    extractPeer(msg),
+	})
+	defaultAgent := al.registry.GetDefaultAgent()
+	if defaultAgent == nil {
+		t.Fatal("No default agent found")
+	}
+	history := defaultAgent.Sessions.GetHistory(route.SessionKey)
+	for _, h := range history {
+		if h.Role == "user" && h.Content == wantAnnotated {
+			t.Fatalf("history contains annotated prompt message: %+v", h)
+		}
+	}
+}
+
 func TestProcessMessage_UseCommandLoadsRequestedSkill(t *testing.T) {
 	tmpDir := t.TempDir()
 	skillDir := filepath.Join(tmpDir, "skills", "shell")