diff --git a/docs/design/15-runaway-guard.md b/docs/design/15-runaway-guard.md
new file mode 100644
index 0000000..421b00f
--- /dev/null
+++ b/docs/design/15-runaway-guard.md
@@ -0,0 +1,61 @@
+# §15 Runaway Guard — Per-Task Dispatch 上限
+
+> 设计文档 v1.0 | 2026-06-16
+
+## 问题
+
+mail/toolchain task 走 handler auto-working（跳过 claim 阶段），不受 claim_timeout 的 3 次重试兜底保护。如果一个 auto-working task 反复 spawn 但永远到不了 done/failed，会无限循环消耗资源。
+
+### 实际案例
+
+2026-06-15 mention 重复投递事件：`spawn_full_agent` 在 `use_main_session=True` 时返回 `None`，ticker `_process_mentions` 误判为失败，每次 tick（30s）都重试。同一 mention 投递了 4 次，直到 retry_count 达到 mention_queue 的 5 次上限才停止。
+
+直接根因已由 PR #80 修复，但如果类似 bug 再次出现，当前没有任何机制阻止 task 层面的无限循环。
+
+## 设计
+
+### 机制
+
+tasks 表新增 `dispatch_count` 字段，每次 ticker 成功 dispatch 一个 task 时递增。当 `dispatch_count >= 10`（全局默认）时，自动标 failed。
+
+### 默认值选择
+
+全局默认 10 次。参考 Hermes v0.13 Best Practices §3 "Per-Task 重试上限"：
+
+- 简单任务重试 1 次
+- 复杂任务重试 3 次
+- crash recovery（3 次）+ api_retry（3 次）余量 = ~10 次
+
+### 适用范围
+
+所有 task 类型（task/mail/toolchain），所有非终态（pending/working/claimed）。
+
+### 检查时机
+
+在 `_check_timeouts` 方法开头，先于现有的 claimed/working 超时检查执行。
+
+### 与现有机制的关系
+
+| 机制 | 覆盖场景 | 触发动作 |
+|------|---------|---------|
+| claim_timeout retry_count >= 3 | 广播任务无人认领 | 升级庞统 |
+| crash_limit 3/30min | working 状态 crash | 标 failed |
+| api_retry_count | API 连续失败 | 标 failed |
+| 续杯 max_retries 3 | 续杯耗尽 | 标 failed |
+| working timeout | working 超时 | 标 failed 或 done |
+| **runaway_guard 10 次** | **任何状态的无限循环** | **标 failed** |
+
+runaway_guard 是最后一道防线，覆盖所有其他机制遗漏的循环场景。
+
+## 改动文件
+
+| 文件 | 改动 |
+|------|------|
+| `src/blackboard/db.py` | `_safe_add_column(conn, "tasks", "dispatch_count", "INTEGER DEFAULT 0")` |
+| `src/blackboard/models.py` | Task dataclass 加 `dispatch_count: int = 0` |
+| `src/daemon/ticker.py` | `_dispatch_pending` / `_dispatch_reviews` 递增 dispatch_count；`_check_timeouts` 加 runaway guard 检查 |
+
+## 参考
+
+- Hermes v0.13 Kanban Best Practices §3 "Per-Task 重试上限"
+- 实际案例：2026-06-15 mention 重复投递事件（PR #80 修复了直接根因，runaway guard 作为兜底）
diff --git a/src/blackboard/db.py b/src/blackboard/db.py
index f2dc012..7563f4e 100644
--- a/src/blackboard/db.py
+++ b/src/blackboard/db.py
@@ -117,6 +117,7 @@ def _migrate_v28(conn: sqlite3.Connection) -> None:
 
     _safe_add_column(conn, "tasks", "round_count", "INTEGER DEFAULT 0")
     _safe_add_column(conn, "tasks", "resumed_from", "TEXT")
+    _safe_add_column(conn, "tasks", "dispatch_count", "INTEGER DEFAULT 0")
 
     # 3. checkpoints 表（M3）
     conn.execute("""CREATE TABLE IF NOT EXISTS checkpoints (
diff --git a/src/blackboard/models.py b/src/blackboard/models.py
index b6a2dbc..23b8e9a 100644
--- a/src/blackboard/models.py
+++ b/src/blackboard/models.py
@@ -41,6 +41,8 @@ class Task:
     resumed_from: Optional[str] = None       # 暂停前状态，恢复时回到原状态
     # v2.9 四相循环
     round_count: int = 0                     # 庞统 review 轮次计数
+    # §15 Runaway Guard
+    dispatch_count: int = 0                  # 被 ticker dispatch 的总次数
     # v2.8 归档
     archived: bool = False
     archived_at: Optional[str] = None
diff --git a/src/daemon/ticker.py b/src/daemon/ticker.py
index 2bbda1a..dec6b06 100644
--- a/src/daemon/ticker.py
+++ b/src/daemon/ticker.py
@@ -1084,6 +1084,19 @@ Parent Task ID: {parent_task.id}
             broadcast_ids = await self._broadcast_claim(broadcast_tasks, db_path, project_id)
             dispatched.extend(broadcast_ids)
 
+        # §15 Runaway Guard: 统一递增 dispatch_count
+        if dispatched:
+            conn = get_connection(db_path)
+            try:
+                for tid in dispatched:
+                    conn.execute(
+                        "UPDATE tasks SET dispatch_count = COALESCE(dispatch_count, 0) + 1 WHERE id=?",
+                        (tid,),
+                    )
+                conn.commit()
+            finally:
+                conn.close()
+
         return dispatched
 
     async def _broadcast_claim(self, tasks: list, db_path: Path,
@@ -1376,6 +1389,19 @@ Parent Task ID: {parent_task.id}
             except Exception:
                 logger.exception("Review dispatch failed for %s", task.id)
 
+        # §15 Runaway Guard: 统一递增 dispatch_count (review)
+        if dispatched:
+            conn = get_connection(db_path)
+            try:
+                for tid in dispatched:
+                    conn.execute(
+                        "UPDATE tasks SET dispatch_count = COALESCE(dispatch_count, 0) + 1 WHERE id=?",
+                        (tid,),
+                    )
+                conn.commit()
+            finally:
+                conn.close()
+
         return dispatched
 
     # ------------------------------------------------------------------
@@ -1388,6 +1414,31 @@ Parent Task ID: {parent_task.id}
         reclaimed: List[str] = []
         now = datetime.utcnow()  # UTC，与 SQLite datetime('now') 一致
 
+        # §15 Runaway Guard: per-task dispatch_count 上限检查
+        # 覆盖所有状态，防止无限循环 dispatch
+        MAX_DISPATCH_COUNT = 10
+        for status_to_check in ("pending", "working", "claimed"):
+            tasks_to_check = queries.tasks_by_status(status_to_check)
+            for task in tasks_to_check:
+                dispatch_count = getattr(task, 'dispatch_count', 0) or 0
+                if dispatch_count >= MAX_DISPATCH_COUNT:
+                    conn = get_connection(db_path)
+                    try:
+                        ok = self._transition_status(
+                            conn, task.id, "failed",
+                            agent="daemon",
+                            detail={"reason": "runaway_guard",
+                                    "dispatch_count": dispatch_count,
+                                    "message": f"dispatch {dispatch_count} 次仍未完成，自动标 failed"},
+                        )
+                        if ok:
+                            reclaimed.append(task.id)
+                            logger.error(
+                                "Task %s: runaway guard triggered (dispatch_count=%d, status=%s), marking failed",
+                                task.id, dispatch_count, status_to_check)
+                    finally:
+                        conn.close()
+
         # claimed 超时 → 重置为 pending（如果 retry_count >= 3 则升级庞统）
         claimed = queries.tasks_by_status("claimed")
         for task in claimed:
diff --git a/tests/integration/test_ticker_integration.py b/tests/integration/test_ticker_integration.py
index d4822a9..72ba960 100644
--- a/tests/integration/test_ticker_integration.py
+++ b/tests/integration/test_ticker_integration.py
@@ -543,3 +543,94 @@ class TestCheckTimeoutsUnified:
         reclaimed = ticker._check_timeouts(db_path)
 
         assert "t-review-dead" not in reclaimed
+
+
+# ---------------------------------------------------------------------------
+# E13: §15 Runaway Guard — per-task dispatch_count 上限
+# ---------------------------------------------------------------------------
+
+class TestRunawayGuard:
+    """E13: dispatch_count >= 10 → 自动标 failed（覆盖所有非终态）"""
+
+    @pytest.fixture
+    def guard_project(self, tmp_path):
+        """创建项目 + 任务"""
+        data_root = tmp_path / "projects"
+        registry = ProjectRegistry(data_root)
+        registry.create_project("guard-proj", "Guard Test", agents=["agent-a"])
+        db_path = data_root / "guard-proj" / "blackboard.db"
+        bb = Blackboard(db_path)
+        return registry, db_path, bb
+
+    def test_runaway_guard_triggers_working(self, guard_project):
+        """E13.1: working 状态 dispatch_count >= 10 → 标 failed"""
+        registry, db_path, bb = guard_project
+
+        bb.create_task(Task(
+            id="t-runaway", title="Runaway Task", status="working",
+            assigned_by="daemon", current_agent="agent-a",
+        ))
+
+        conn = bb._conn()
+        try:
+            conn.execute(
+                "UPDATE tasks SET dispatch_count = 10 WHERE id = ?", ("t-runaway",))
+            conn.commit()
+        finally:
+            conn.close()
+
+        ticker = Ticker(registry, tick_interval=30)
+        reclaimed = ticker._check_timeouts(db_path)
+
+        assert "t-runaway" in reclaimed
+        task = Queries(db_path).task_by_id("t-runaway")
+        assert task.status == "failed"
+
+    def test_runaway_guard_triggers_pending(self, guard_project):
+        """E13.2: pending 状态 dispatch_count >= 10 → 标 failed"""
+        registry, db_path, bb = guard_project
+
+        bb.create_task(Task(
+            id="t-pending-runaway", title="Pending Runaway", status="pending",
+            assigned_by="daemon",
+        ))
+
+        conn = bb._conn()
+        try:
+            conn.execute(
+                "UPDATE tasks SET dispatch_count = 10 WHERE id = ?",
+                ("t-pending-runaway",))
+            conn.commit()
+        finally:
+            conn.close()
+
+        ticker = Ticker(registry, tick_interval=30)
+        reclaimed = ticker._check_timeouts(db_path)
+
+        assert "t-pending-runaway" in reclaimed
+        task = Queries(db_path).task_by_id("t-pending-runaway")
+        assert task.status == "failed"
+
+    def test_runaway_guard_not_triggered(self, guard_project):
+        """E13.3: dispatch_count < 10 → 正常流程不受影响"""
+        registry, db_path, bb = guard_project
+
+        bb.create_task(Task(
+            id="t-normal", title="Normal Task", status="working",
+            assigned_by="daemon", current_agent="agent-a",
+        ))
+
+        conn = bb._conn()
+        try:
+            conn.execute(
+                "UPDATE tasks SET dispatch_count = 5 WHERE id = ?", ("t-normal",))
+            conn.commit()
+        finally:
+            conn.close()
+
+        ticker = Ticker(registry, tick_interval=30)
+        reclaimed = ticker._check_timeouts(db_path)
+
+        assert "t-normal" not in reclaimed
+        task = Queries(db_path).task_by_id("t-normal")
+        assert task.status == "working"