Quickstart

This quickstart takes you from a simple setup to a fully functional AI agent in just a few minutes.

LangChain Docs MCP server

If you’re using an AI coding assistant or IDE (e.g. Claude Code or Cursor), you should install the LangChain Docs MCP server to get the most out of it. This ensures your agent has access to up-to-date LangChain documentation and examples.

Requirements

For these examples, you will need to:

• Install the LangChain package
• Set up a Claude (Anthropic) account and get an API key
• Set the ANTHROPIC_API_KEY environment variable in your terminal

Although these examples use Claude, you can use any supported model by changing the model name in the code and setting up the appropriate API key.

Build a basic agent

Start by creating a simple agent that can answer questions and call tools. The agent will use Claude Sonnet 4.5 as its language model, a basic weather function as a tool, and a simple prompt to guide its behavior.

from langchain.agents import create_agent

def get_weather(city: str) -> str:
    """Get weather for a given city."""
    return f"It's always sunny in {city}!"

agent = create_agent(
    model="claude-sonnet-4-5-20250929",
    tools=[get_weather],
    system_prompt="You are a helpful assistant",
)

# Run the agent
agent.invoke(
    {"messages": [{"role": "user", "content": "what is the weather in sf"}]}
)

To learn how to trace your agent with LangSmith, see the LangSmith documentation.

Build a real-world agent

Next, build a practical weather forecasting agent that demonstrates key production concepts:

1. Detailed system prompts for better agent behavior
2. Create tools that integrate with external data
3. Model configuration for consistent responses
4. Structured output for predictable results
5. Conversational memory for chat-like interactions
6. Create and run the agent to test the fully functional agent

Let’s walk through each step:

1. Define the system prompt

   The system prompt defines your agent’s role and behavior. Keep it specific and actionable:

   SYSTEM_PROMPT = """You are an expert weather forecaster, who speaks in puns.
   
   You have access to two tools:
   
   - get_weather_for_location: use this to get the weather for a specific location
   - get_user_location: use this to get the user's location
   
   If a user asks you for the weather, make sure you know the location. If you can tell from the question that they mean wherever they are, use the get_user_location tool to find their location."""
   
2. Create tools

   Tools let a model interact with external systems by calling functions you define. Tools can depend on runtime context and also interact with agent memory.

   Notice below how the get_user_location tool uses runtime context:

   from dataclasses import dataclass
   from langchain.tools import tool, ToolRuntime
   
   @tool
   def get_weather_for_location(city: str) -> str:
       """Get weather for a given city."""
       return f"It's always sunny in {city}!"
   
   @dataclass
   class Context:
       """Custom runtime context schema."""
       user_id: str
   
   @tool
   def get_user_location(runtime: ToolRuntime[Context]) -> str:
       """Retrieve user information based on user ID."""
       user_id = runtime.context.user_id
       return "Florida" if user_id == "1" else "SF"
   

   Tools should be well-documented: their name, description, and argument names become part of the model’s prompt. LangChain’s @tool decorator adds metadata and enables runtime injection with the ToolRuntime parameter.

3. Configure your model

   Set up your language model with the right parameters for your use case:

   from langchain.chat_models import init_chat_model
   
   model = init_chat_model(
       "claude-sonnet-4-5-20250929",
       temperature=0.5,
       timeout=10,
       max_tokens=1000
   )
   

   Depending on the model and provider chosen, initialization parameters may vary; refer to their reference pages for details.

4. Define response format

   Optionally, define a structured response format if you need the agent responses to match a specific schema.

   from dataclasses import dataclass
   
   # We use a dataclass here, but Pydantic models are also supported.
   @dataclass
   class ResponseFormat:
       """Response schema for the agent."""
       # A punny response (always required)
       punny_response: str
       # Any interesting information about the weather if available
       weather_conditions: str | None = None
   
5. Add memory

   Add memory to your agent to maintain state across interactions. This allows the agent to remember previous conversations and context.

   from langgraph.checkpoint.memory import InMemorySaver
   
   checkpointer = InMemorySaver()
   

   In production, use a persistent checkpointer that saves message history to a database. See Add and manage memory for more details.

6. Create and run the agent

   Now assemble your agent with all the components and run it!

   from langchain.agents.structured_output import ToolStrategy
   
   agent = create_agent(
       model=model,
       system_prompt=SYSTEM_PROMPT,
       tools=[get_user_location, get_weather_for_location],
       context_schema=Context,
       response_format=ToolStrategy(ResponseFormat),
       checkpointer=checkpointer
   )
   
   # `thread_id` is a unique identifier for a given conversation.
   config = {"configurable": {"thread_id": "1"}}
   
   response = agent.invoke(
       {"messages": [{"role": "user", "content": "what is the weather outside?"}]},
       config=config,
       context=Context(user_id="1")
   )
   
   print(response['structured_response'])
   # ResponseFormat(
   #     punny_response="Florida is still having a 'sun-derful' day! The sunshine is playing 'ray-dio' hits all day long! I'd say it's the perfect weather for some 'solar-bration'! If you were hoping for rain, I'm afraid that idea is all 'washed up' - the forecast remains 'clear-ly' brilliant!",
   #     weather_conditions="It's always sunny in Florida!"
   # )
   
   
   # Note that we can continue the conversation using the same `thread_id`.
   response = agent.invoke(
       {"messages": [{"role": "user", "content": "thank you!"}]},
       config=config,
       context=Context(user_id="1")
   )
   
   print(response['structured_response'])
   # ResponseFormat(
   #     punny_response="You're 'thund-erfully' welcome! It's always a 'breeze' to help you stay 'current' with the weather. I'm just 'cloud'-ing around waiting to 'shower' you with more forecasts whenever you need them. Have a 'sun-sational' day in the Florida sunshine!",
   #     weather_conditions=None
   # )
   

Show Full example code

from dataclasses import dataclass

from langchain.agents import create_agent
from langchain.chat_models import init_chat_model
from langchain.tools import tool, ToolRuntime
from langgraph.checkpoint.memory import InMemorySaver
from langchain.agents.structured_output import ToolStrategy


# Define system prompt
SYSTEM_PROMPT = """You are an expert weather forecaster, who speaks in puns.

You have access to two tools:

- get_weather_for_location: use this to get the weather for a specific location
- get_user_location: use this to get the user's location

If a user asks you for the weather, make sure you know the location. If you can tell from the question that they mean wherever they are, use the get_user_location tool to find their location."""

# Define context schema
@dataclass
class Context:
    """Custom runtime context schema."""
    user_id: str

# Define tools
@tool
def get_weather_for_location(city: str) -> str:
    """Get weather for a given city."""
    return f"It's always sunny in {city}!"

@tool
def get_user_location(runtime: ToolRuntime[Context]) -> str:
    """Retrieve user information based on user ID."""
    user_id = runtime.context.user_id
    return "Florida" if user_id == "1" else "SF"

# Configure model
model = init_chat_model(
    "claude-sonnet-4-5-20250929",
    temperature=0
)

# Define response format
@dataclass
class ResponseFormat:
    """Response schema for the agent."""
    # A punny response (always required)
    punny_response: str
    # Any interesting information about the weather if available
    weather_conditions: str | None = None

# Set up memory
checkpointer = InMemorySaver()

# Create agent
agent = create_agent(
    model=model,
    system_prompt=SYSTEM_PROMPT,
    tools=[get_user_location, get_weather_for_location],
    context_schema=Context,
    response_format=ToolStrategy(ResponseFormat),
    checkpointer=checkpointer
)

# Run agent
# `thread_id` is a unique identifier for a given conversation.
config = {"configurable": {"thread_id": "1"}}

response = agent.invoke(
    {"messages": [{"role": "user", "content": "what is the weather outside?"}]},
    config=config,
    context=Context(user_id="1")
)

print(response['structured_response'])
# ResponseFormat(
#     punny_response="Florida is still having a 'sun-derful' day! The sunshine is playing 'ray-dio' hits all day long! I'd say it's the perfect weather for some 'solar-bration'! If you were hoping for rain, I'm afraid that idea is all 'washed up' - the forecast remains 'clear-ly' brilliant!",
#     weather_conditions="It's always sunny in Florida!"
# )


# Note that we can continue the conversation using the same `thread_id`.
response = agent.invoke(
    {"messages": [{"role": "user", "content": "thank you!"}]},
    config=config,
    context=Context(user_id="1")
)

print(response['structured_response'])
# ResponseFormat(
#     punny_response="You're 'thund-erfully' welcome! It's always a 'breeze' to help you stay 'current' with the weather. I'm just 'cloud'-ing around waiting to 'shower' you with more forecasts whenever you need them. Have a 'sun-sational' day in the Florida sunshine!",
#     weather_conditions=None
# )

To learn how to trace your agent with LangSmith, see the LangSmith documentation.

Congratulations! You now have an AI agent that can:

• Understand context and remember conversations
• Use multiple tools intelligently
• Provide structured responses in a consistent format
• Handle user-specific information through context
• Maintain conversation state across interactions

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