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Install LangChain

pip

pip install -U langchain

uv

uv add langchain

Quickstart

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.

INFO

For this example, you will need to set up a Claude (Anthropic) account and get an API key. Then, set the ANTHROPIC_API_KEY environment variable in your terminal.

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"}]}
)

Build a real-world agent

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

Let's walk through each step:

1. Detailed system prompts for better agent behavior 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 that integrate with external data

   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"

   TIP

   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 via the ToolRuntime parameter.

3. Model configuration for consistent responses

   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
   )

4. Structured output for predictable results

   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. Conversational memory for chat-like interactions

   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 to a database. See Add and manage memory for more details.

6. Create and run the agent create a fully functional agent

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

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

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


# 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=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
# )