Langchain

LangChain

Quick start

from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser
from langchain_openai import ChatOpenAI

prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a supply chain planner."),
    ("human", "Plan production for {product} in {quarter}"),
])
model = ChatOpenAI(model="gpt-4o")
chain = prompt | model | StrOutputParser()

result = chain.invoke({"product": "Widget-A", "quarter": "Q3"})

Core abstractions

Runnable interface

All LangChain components implement Runnable:

runnable.invoke(input)        # Single call
runnable.batch([a, b, c])     # Parallel batch
async for chunk in runnable.astream(input):  # Streaming
    ...

LCEL (LangChain Expression Language)

Chain components with | (pipe):

chain = prompt | model | output_parser
chain = RunnableParallel({"summary": chain1, "detail": chain2})
chain = RunnablePassthrough.assign(extra=lambda x: compute(x))

Chat models

from langchain_openai import ChatOpenAI
from langchain_anthropic import ChatAnthropic

model = ChatOpenAI(model="gpt-4o", temperature=0)
model = ChatAnthropic(model="claude-3-5-sonnet-20241022")

Tool calling with LangChain

from langchain_core.tools import tool

@tool
def get_inventory(item_id: str) -> int:
    """Return current inventory for an item."""
    return 42

llm_with_tools = model.bind_tools([get_inventory])
result = llm_with_tools.invoke("Check inventory for item X")
# result.tool_calls contains the parsed tool call

Output parsers

from langchain_core.output_parsers import PydanticOutputParser
from pydantic import BaseModel

class Plan(BaseModel):
    steps: list[str]
    deadline: str

parser = PydanticOutputParser(pydantic_object=Plan)
chain = prompt | model | parser

Prompt templates

from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder

prompt = ChatPromptTemplate.from_messages([
    ("system", "{system_prompt}"),
    MessagesPlaceholder("history"),
    ("human", "{input}"),
])

When to use LangChain vs LangGraph

Rule of thumb: if you need a loop, branching, or persistence, use LangGraph.

Message format

from langchain_core.messages import HumanMessage, AIMessage, ToolMessage, SystemMessage

messages = [
    SystemMessage("You are helpful."),
    HumanMessage("Hello"),
    AIMessage("Hi!"),
    HumanMessage("What is 2+2?"),
]

Common pitfalls

• LCEL functions must be serializable: Don't use lambdas with closures over external state. Use RunnableLambda with a named function instead.
• Tool binding vs. tool calling: bind_tools() enables the model to request tool calls but doesn't execute them. Use ToolNode in LangGraph for execution.
• Output parser errors: When the model produces malformed JSON, PydanticOutputParser raises. Wrap with OutputFixingParser or use result_type in PydanticAI instead.

References

• For graph-based orchestrators, see the langgraph skill.
• For type-safe agents with structured results, see the pydantic-ai skill.

Source: nsanguan/axon — distributed by TomeVault.