diff --git a/core/agent/loop.py b/core/agent/loop.py
index c6a8d53..405292d 100644
--- a/core/agent/loop.py
+++ b/core/agent/loop.py
@@ -73,7 +73,8 @@ class AgentLoop:
         max_steps: 最大有效步数（每次成功工具调用计一步）。
         max_retries: JSON 解析连续失败的最大容忍次数。
         step_retries: LLM 瞬时异常的步级重试次数（不含首次调用）。
-        step_retry_delays: 步级重试的退避秒数序列，超出部分取末值。
+        step_retry_delays: 步级重试的退避秒数序列，超出部分取末值；
+            不得为空（空序列构造时抛 ValueError，fail-fast）。
         retryable_exceptions: 可重试异常元组，默认 (TimeoutError, OSError)——
             ssl.SSLError/ConnectionError 均为 OSError 子类，覆盖穿透
             GovernedLLMClient 内部重试栈的瞬时异常；openai API 类异常由
@@ -90,6 +91,8 @@ class AgentLoop:
         step_retry_delays: tuple[float, ...] = (20.0, 40.0),
         retryable_exceptions: tuple[type[BaseException], ...] = (TimeoutError, OSError),
     ) -> None:
+        if not step_retry_delays:
+            raise ValueError("step_retry_delays 不得为空")
         self._llm = llm
         self._max_steps = max_steps
         self._max_retries = max_retries
@@ -138,7 +141,7 @@ class AgentLoop:
                     messages, token_usage, session_id=session_id
                 )
             except Exception as e:
-                logger.error("LLM API 调用失败: {}", e)
+                logger.error("LLM API 调用失败（{}）: {}", type(e).__name__, e)
                 result = LoopResult(
                     steps=steps,
                     steps_used=step_count,
diff --git a/tests/unit/test_agent_loop.py b/tests/unit/test_agent_loop.py
index fe00b06..b7e3c5b 100644
--- a/tests/unit/test_agent_loop.py
+++ b/tests/unit/test_agent_loop.py
@@ -363,3 +363,23 @@ class TestStepLevelRetry:
         assert result.stop_reason == "error"
         sleep_mock.assert_not_awaited()
         assert loop._llm.chat.await_count == 1
+
+    @pytest.mark.asyncio
+    async def test_exhaustion_raises_last_original_exception(self, monkeypatch) -> None:
+        """耗尽语义窄测试：_call_llm_with_step_retry 耗尽后原样抛出最后一次异常。"""
+
+        async def _fake_sleep(seconds: float) -> None:
+            pass
+
+        monkeypatch.setattr("core.agent.loop.asyncio.sleep", _fake_sleep)
+        last_error = TimeoutError("t3-last")
+        loop = self._make_loop([TimeoutError("t1"), TimeoutError("t2"), last_error])
+        token_usage = {"prompt_tokens": 0, "completion_tokens": 0}
+        with pytest.raises(TimeoutError) as exc_info:
+            await loop._call_llm_with_step_retry([], token_usage, session_id=None)
+        assert exc_info.value is last_error
+
+    def test_empty_retry_delays_rejected_at_init(self) -> None:
+        """空 step_retry_delays 在构造时 fail-fast 抛 ValueError。"""
+        with pytest.raises(ValueError, match="step_retry_delays"):
+            AgentLoop(llm=AsyncMock(), max_steps=10, step_retry_delays=())