diff --git a/mini_agent/__init__.py b/mini_agent/__init__.py
index 3c8742a..1d7b1c5 100644
--- a/mini_agent/__init__.py
+++ b/mini_agent/__init__.py
@@ -3,18 +3,20 @@
 """
 
 from .agent import MiniAgent
-from .llm import SimpleLLM
+from .llm import SimpleLLM, LLMResponse
 from .tools import ToolCollection, PythonExecutor, FileEditor, BashExecutor
-from .schema import Message, Memory, AgentState
+from .schema import Message, Memory, AgentState, Role
 
 __all__ = [
     "MiniAgent",
-    "SimpleLLM", 
+    "SimpleLLM",
+    "LLMResponse",
     "ToolCollection",
     "PythonExecutor",
     "FileEditor", 
     "BashExecutor",
     "Message",
     "Memory",
-    "AgentState"
+    "AgentState",
+    "Role",
 ]
\ No newline at end of file
diff --git a/mini_agent/agent.py b/mini_agent/agent.py
index e59d534..373eb83 100644
--- a/mini_agent/agent.py
+++ b/mini_agent/agent.py
@@ -19,6 +19,8 @@ def __init__(
         system_prompt: Optional[str] = None,
         max_steps: int = 10
     ):
+        if max_steps < 1:
+            raise ValueError(f"max_steps must be >= 1, got {max_steps}")
         self.name = name
         self.llm = llm
         self.tools = ToolCollection()
@@ -128,7 +130,8 @@ async def act(self) -> None:
                 # 准备结果消息
                 if result.success:
                     result_content = result.output
-                    print(f"✅ 工具执行成功: {result_content[:100]}...")
+                    preview = result_content[:100] if result_content else "(empty)"
+                    print(f"✅ 工具执行成功: {preview}...")
                 else:
                     result_content = f"错误: {result.error}"
                     print(f"❌ 工具执行失败: {result.error}")
diff --git a/mini_agent/schema.py b/mini_agent/schema.py
index 150415a..a1d10eb 100644
--- a/mini_agent/schema.py
+++ b/mini_agent/schema.py
@@ -3,7 +3,7 @@
 """
 from enum import Enum
 from typing import List, Optional, Dict, Any
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 
 class Role(str, Enum):
@@ -39,7 +39,7 @@ def tool_message(cls, content: str, tool_call_id: str) -> "Message":
 
 
 class Memory(BaseModel):
-    messages: List[Message] = []
+    messages: List[Message] = Field(default_factory=list)
     
     def add_message(self, message: Message):
         self.messages.append(message)
diff --git a/mini_agent/tools.py b/mini_agent/tools.py
index 0aae704..fb08e2d 100644
--- a/mini_agent/tools.py
+++ b/mini_agent/tools.py
@@ -141,6 +141,12 @@ class BashExecutor(BaseTool):
     
     async def execute(self, command: str, **kwargs) -> ToolResult:
         try:
+            import warnings
+            warnings.warn(
+                "BashExecutor uses shell=True which is a security risk. "
+                "Only use with trusted input.",
+                stacklevel=2,
+            )
             result = subprocess.run(
                 command,
                 shell=True,