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+# 04. Tool Integration
+
+Integration and execution control of external tools.
+
+## Overview
+
+In LangGraph, LLMs can interact with external systems by calling **tools**. Tools provide various capabilities such as search, calculation, API calls, and more.
+
+## Key Components
+
+### 1. [Tool Definition](04_tool_integration_tool_definition.md)
+
+How to define tools:
+- `@tool` decorator
+- Function descriptions and parameters
+- Structured output
+
+### 2. [Tool Node](04_tool_integration_tool_node.md)
+
+Nodes that execute tools:
+- Using `ToolNode`
+- Error handling
+- Custom tool nodes
+
+### 3. [Command API](04_tool_integration_command_api.md)
+
+Controlling tool execution:
+- Integration of state updates and control flow
+- Transition control from tools
+
+## Basic Implementation
+
+```python
+from langchain_core.tools import tool
+from langgraph.prebuilt import ToolNode
+from langgraph.graph import MessagesState, StateGraph
+
+# 1. Define tools
+@tool
+def search(query: str) -> str:
+    """Perform a web search.
+
+    Args:
+        query: Search query
+    """
+    return perform_search(query)
+
+@tool
+def calculator(expression: str) -> float:
+    """Calculate a mathematical expression.
+
+    Args:
+        expression: Expression to calculate (e.g., "2 + 2")
+    """
+    return eval(expression)
+
+tools = [search, calculator]
+
+# 2. Bind tools to LLM
+llm_with_tools = llm.bind_tools(tools)
+
+# 3. Agent node
+def agent(state: MessagesState):
+    response = llm_with_tools.invoke(state["messages"])
+    return {"messages": [response]}
+
+# 4. Tool node
+tool_node = ToolNode(tools)
+
+# 5. Build graph
+builder = StateGraph(MessagesState)
+builder.add_node("agent", agent)
+builder.add_node("tools", tool_node)
+
+# 6. Conditional edges
+def should_continue(state: MessagesState):
+    last_message = state["messages"][-1]
+    if last_message.tool_calls:
+        return "tools"
+    return END
+
+builder.add_edge(START, "agent")
+builder.add_conditional_edges("agent", should_continue)
+builder.add_edge("tools", "agent")
+
+graph = builder.compile()
+```
+
+## Types of Tools
+
+### Search Tools
+
+```python
+@tool
+def web_search(query: str) -> str:
+    """Search the web"""
+    return search_api(query)
+```
+
+### Calculator Tools
+
+```python
+@tool
+def calculator(expression: str) -> float:
+    """Calculate a mathematical expression"""
+    return eval(expression)
+```
+
+### API Tools
+
+```python
+@tool
+def get_weather(city: str) -> dict:
+    """Get weather information"""
+    return weather_api(city)
+```
+
+### Database Tools
+
+```python
+@tool
+def query_database(sql: str) -> list[dict]:
+    """Query the database"""
+    return execute_sql(sql)
+```
+
+## Tool Execution Flow
+
+```
+User Query
+    ↓
+[Agent Node]
+    ↓
+LLM decides: Use tool?
+    ↓ Yes
+[Tool Node] ← Execute tool
+    ↓
+[Agent Node] ← Tool result
+    ↓
+LLM decides: Continue?
+    ↓ No
+Final Answer
+```
+
+## Key Principles
+
+1. **Clear Descriptions**: Write detailed docstrings for tools
+2. **Error Handling**: Handle tool execution errors appropriately
+3. **Type Safety**: Explicitly specify parameter types
+4. **Approval Flow**: Incorporate Human-in-the-Loop for critical tools
+
+## Next Steps
+
+For details on each component, please refer to the following pages:
+
+- [04_tool_integration_tool_definition.md](04_tool_integration_tool_definition.md) - How to define tools
+- [04_tool_integration_tool_node.md](04_tool_integration_tool_node.md) - Tool node implementation
+- [04_tool_integration_command_api.md](04_tool_integration_command_api.md) - Using the Command API