diff --git a/server/internal/handler/chat_input_ownership_test.go b/server/internal/handler/chat_input_ownership_test.go
index 7d52d53de4..93cfe4a344 100644
--- a/server/internal/handler/chat_input_ownership_test.go
+++ b/server/internal/handler/chat_input_ownership_test.go
@@ -332,3 +332,55 @@ func assertNoRetryChild(t *testing.T, ctx context.Context, taskID string) {
 		t.Fatalf("expected no retry child for a completed no_response turn, got %d", n)
 	}
 }
+
+// insertChannelChatTask creates a running chat task with chat_input_task_id NULL
+// — the legacy/channel (Slack/Lark) shape — directly, bypassing the task-owned
+// direct-send path.
+func insertChannelChatTask(t *testing.T, ctx context.Context, agentID, runtimeID, sessionID string) string {
+	t.Helper()
+	var taskID string
+	if err := testPool.QueryRow(ctx, `
+		INSERT INTO agent_task_queue (agent_id, runtime_id, chat_session_id, status, priority, started_at, dispatched_at)
+		VALUES ($1, $2, $3, 'running', 2, now(), now())
+		RETURNING id
+	`, agentID, runtimeID, sessionID).Scan(&taskID); err != nil {
+		t.Fatalf("setup: create channel chat task: %v", err)
+	}
+	return taskID
+}
+
+// TestCompleteTask_ChannelEmptyOutputWritesNoRow pins the MUL-4351 review fix:
+// a legacy/channel task (chat_input_task_id NULL) that completes with empty
+// output must NOT write an assistant row — so chat:done carries empty content
+// and the Slack/Lark outbound keeps silently dropping it. The no_response
+// fallback body must never reach an external channel. A non-empty channel
+// completion still writes an ordinary message.
+func TestCompleteTask_ChannelEmptyOutputWritesNoRow(t *testing.T) {
+	if testHandler == nil {
+		t.Skip("database not available")
+	}
+	ctx := context.Background()
+	agentID, sessionID, runtimeID, _ := setupDirectChatSession(t, ctx, "channel-like chat")
+
+	// Empty output → no row at all.
+	emptyTask := insertChannelChatTask(t, ctx, agentID, runtimeID, sessionID)
+	if _, err := testHandler.TaskService.CompleteTask(ctx, parseUUID(emptyTask), completeResult(t, "   "), "", ""); err != nil {
+		t.Fatalf("complete channel task (empty): %v", err)
+	}
+	if rows := assistantRows(t, ctx, sessionID); len(rows) != 0 {
+		t.Fatalf("channel empty completion must write NO assistant row (Slack/Lark silent-drop), got %d", len(rows))
+	}
+
+	// Non-empty output → one ordinary message (kind 'message', not no_response).
+	textTask := insertChannelChatTask(t, ctx, agentID, runtimeID, sessionID)
+	if _, err := testHandler.TaskService.CompleteTask(ctx, parseUUID(textTask), completeResult(t, "channel reply"), "", ""); err != nil {
+		t.Fatalf("complete channel task (text): %v", err)
+	}
+	rows := assistantRows(t, ctx, sessionID)
+	if len(rows) != 1 {
+		t.Fatalf("channel non-empty completion must write exactly one message, got %d", len(rows))
+	}
+	if rows[0].MessageKind != protocol.ChatMessageKindMessage || rows[0].Content != "channel reply" {
+		t.Fatalf("channel message = kind %q content %q, want message/'channel reply'", rows[0].MessageKind, rows[0].Content)
+	}
+}
diff --git a/server/internal/handler/daemon.go b/server/internal/handler/daemon.go
index 25711c9af8..51c72bc9ae 100644
--- a/server/internal/handler/daemon.go
+++ b/server/internal/handler/daemon.go
@@ -1774,38 +1774,38 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 					}
 				}
 			}
-			// Build the chat prompt from EVERY user message that has arrived
-			// since the agent's last reply — not just the most recent one. A
-			// short-window debounce (MUL-2968) can land several user messages
-			// before a single run fires; the agent resumes its prior session
-			// and only learns of new input through resp.ChatMessage, so
-			// delivering just the latest message would silently drop the
-			// earlier ones (e.g. "看上海天气" then "还有青岛" → only Qingdao
-			// answered). The unanswered set is the trailing run of user
-			// messages after the last assistant message (every completed or
-			// failed run writes an assistant row, so that anchor advances each
-			// turn). Attachments are collected from each included message so
-			// the agent can `multica attachment download <id>` — the markdown
-			// URL alone is signed and 30-min expiring on the private CDN.
 			// Resolve the user-message input batch for this run. A task-owned
 			// direct-chat task (chat_input_task_id set, MUL-4351) reads exactly
 			// the user messages tagged with its own input owner, so a message
 			// that arrived after this turn was sealed can never be absorbed here.
 			// Legacy and channel (Slack/Lark) tasks carry a NULL owner and keep
-			// the trailing-message selector, so a rolling deploy never replays
-			// their history.
+			// the trailing-message selector — the run of user messages after the
+			// last assistant row, which also covers a debounced burst (MUL-2968:
+			// "看上海天气" then "还有青岛" must both be delivered) — so a rolling
+			// deploy never replays their history. Attachments are collected per
+			// included message so the agent can `multica attachment download <id>`
+			// (the inline markdown URL is signed + 30-min expiring on the CDN).
 			var unanswered []db.ChatMessage
+			var inputLoadErr error
 			if task.ChatInputTaskID.Valid {
-				if owned, err := h.Queries.ListChatInputMessages(r.Context(), task.ChatInputTaskID); err == nil {
-					unanswered = owned
-				} else {
-					slog.Warn("chat claim: load owned input messages failed",
-						"task_id", uuidToString(task.ID),
-						"chat_input_task_id", uuidToString(task.ChatInputTaskID),
-						"error", err)
-				}
-			} else if msgs, err := h.Queries.ListChatMessages(r.Context(), cs.ID); err == nil && len(msgs) > 0 {
+				unanswered, inputLoadErr = h.Queries.ListChatInputMessages(r.Context(), task.ChatInputTaskID)
+			} else if msgs, err := h.Queries.ListChatMessages(r.Context(), cs.ID); err == nil {
 				unanswered = trailingUserMessages(msgs)
+			} else {
+				inputLoadErr = err
+			}
+			// A read failure must NOT masquerade as "zero input". Preserve the
+			// just-dispatched task (the stale-dispatched reclaim redelivers it)
+			// and reject the claim with 5xx, rather than cancelling a valid direct
+			// task on a transient DB error (MUL-4351 review).
+			if inputLoadErr != nil {
+				outcome = "error_chat_input_load"
+				slog.Error("chat claim: load chat input messages failed; preserving task for redelivery",
+					"task_id", uuidToString(task.ID),
+					"chat_session_id", uuidToString(cs.ID),
+					"error", inputLoadErr)
+				writeError(w, http.StatusInternalServerError, "failed to load chat input")
+				return
 			}
 
 			parts := make([]string, 0, len(unanswered))
diff --git a/server/internal/service/task.go b/server/internal/service/task.go
index eb15a0b5e5..3e8cbcebdb 100644
--- a/server/internal/service/task.go
+++ b/server/internal/service/task.go
@@ -1769,18 +1769,18 @@ func (s *TaskService) CompleteTask(ctx context.Context, taskID pgtype.UUID, resu
 				return fmt.Errorf("update chat session resume pointer: %w", err)
 			}
 
-			// Write exactly one assistant outcome in the SAME transaction as the
-			// status flip and resume-pointer update (MUL-4351). A non-empty final
-			// output becomes an ordinary assistant message; an empty/whitespace
-			// output becomes a visible no_response outcome. Failing here rolls the
-			// whole completion back so the daemon retries the terminal callback —
-			// we never leave a completed task without its single outcome row, and
-			// the status CAS above guarantees a replay can't write a second one.
+			// Write the assistant outcome in the SAME transaction as the status
+			// flip and resume-pointer update (MUL-4351). For a task-owned direct
+			// task this is exactly one row (message or no_response); for a
+			// legacy/channel task an empty output writes no row (see
+			// writeChatCompletionOutcome). Failing here rolls the whole completion
+			// back so the daemon retries the terminal callback, and the status CAS
+			// above guarantees a replay can't write a second row.
 			msg, err := s.writeChatCompletionOutcome(ctx, qtx, t, result)
 			if err != nil {
 				return fmt.Errorf("write chat assistant outcome: %w", err)
 			}
-			chatAssistantMsg = &msg
+			chatAssistantMsg = msg
 		}
 		return nil
 	}); err != nil {
@@ -1898,14 +1898,24 @@ func (s *TaskService) CompleteTask(ctx context.Context, taskID pgtype.UUID, resu
 // message_kind still show this text instead of an empty bubble (MUL-4351).
 const chatNoResponseFallback = "The agent finished this turn without a text reply."
 
-// writeChatCompletionOutcome writes exactly one assistant chat_message for a
-// completed direct-chat task, inside the caller's completion transaction. A
-// non-empty final output becomes an ordinary assistant message; an empty or
-// whitespace-only output becomes a visible no_response outcome carrying a
-// non-empty English fallback body. It never auto-retries: an empty output is a
-// legitimate terminal result (a tool-only turn), and re-running it would repeat
-// any external side effects the agent already performed.
-func (s *TaskService) writeChatCompletionOutcome(ctx context.Context, qtx *db.Queries, task db.AgentTaskQueue, result []byte) (db.ChatMessage, error) {
+// writeChatCompletionOutcome writes the assistant chat_message outcome for a
+// completed chat task inside the caller's completion transaction, returning the
+// row (nil when none is written).
+//
+// Task-owned direct (web/mobile) tasks — chat_input_task_id set — get the
+// explicit single-outcome contract: a non-empty final output becomes an ordinary
+// assistant message, and an empty/whitespace output becomes a visible
+// no_response outcome carrying a non-empty English fallback body. It never
+// auto-retries: an empty output is a legitimate terminal result (a tool-only
+// turn) and re-running it would repeat side effects already performed.
+//
+// Legacy and channel (Slack/Lark) tasks — chat_input_task_id NULL — keep the
+// prior behavior: a non-empty output writes an ordinary assistant message, but
+// an EMPTY output writes NO row, so chat:done carries empty content and the
+// channel outbound silently drops it. This preserves Slack/Lark empty-completion
+// semantics unchanged (MUL-4351 review): the no_response fallback body must never
+// be pushed to an external channel.
+func (s *TaskService) writeChatCompletionOutcome(ctx context.Context, qtx *db.Queries, task db.AgentTaskQueue, result []byte) (*db.ChatMessage, error) {
 	// result is the daemon request re-marshalled by the handler, so it is always
 	// valid JSON; an empty Output is the only case this branch cares about.
 	var payload protocol.TaskCompletedPayload
@@ -1913,6 +1923,13 @@ func (s *TaskService) writeChatCompletionOutcome(ctx context.Context, qtx *db.Qu
 	// Same unescape as the issue-comment path: literal `\n` from agent stdout
 	// becomes a real newline so the chat panel renders paragraph breaks.
 	body := util.UnescapeBackslashEscapes(payload.Output)
+	isEmpty := strings.TrimSpace(body) == ""
+
+	// Channel/legacy empty completion: emit no assistant row, only an empty
+	// chat:done for typing/lifecycle. Keeps the Slack/Lark silent-drop path.
+	if isEmpty && !task.ChatInputTaskID.Valid {
+		return nil, nil
+	}
 
 	params := db.CreateChatMessageParams{
 		ChatSessionID: task.ChatSessionID,
@@ -1920,14 +1937,19 @@ func (s *TaskService) writeChatCompletionOutcome(ctx context.Context, qtx *db.Qu
 		TaskID:        task.ID,
 		ElapsedMs:     computeChatElapsedMs(task),
 	}
-	if strings.TrimSpace(body) == "" {
+	if isEmpty {
+		// Task-owned direct task: explicit, visible no_response outcome.
 		params.Content = chatNoResponseFallback
 		params.MessageKind = pgtype.Text{String: protocol.ChatMessageKindNoResponse, Valid: true}
 	} else {
 		params.Content = redact.Text(body)
 		// message_kind left NULL → COALESCE defaults to 'message'.
 	}
-	return qtx.CreateChatMessage(ctx, params)
+	row, err := qtx.CreateChatMessage(ctx, params)
+	if err != nil {
+		return nil, err
+	}
+	return &row, nil
 }
 
 // FailTask marks a task as failed.