diff --git a/src/daemon/spawner.py b/src/daemon/spawner.py
index d8acc6a..7fe9138 100644
--- a/src/daemon/spawner.py
+++ b/src/daemon/spawner.py
@@ -474,6 +474,16 @@ curl -X POST http://{self.api_host}:{self.api_port}/api/projects/{project_id}/ta
             session_id = str(uuid.uuid4())
         _sid_key = session_id or "main"  # counter 用的 key
 
+        # Phase -1: Gateway 存活检查（#08）
+        # Gateway 不可达 → AgentBusyError(reason="gateway_down") → 任务留 pending
+        # → ticker 30s 后自动重试。仅检查 main session spawn（subagent 走 API）。
+        if use_main_session and not skip_counter:
+            gw_alive = await self._probe_gateway()
+            if not gw_alive:
+                logger.warning("Phase -1: Gateway unreachable (%s:%d), deferring %s",
+                               self.gateway_host, self.gateway_port, agent_id)
+                raise AgentBusyError(agent_id, reason="gateway_down")
+
         # Phase 0: Pre-acquire 修复（无锁）
         # timeout/failed 状态先修复再 acquire。revive 只改 running→idle，幂等安全。
         # asyncio 协作式并发保证同一时刻只有一个协程在执行，revive 的 sessions.json