Langgraph

from typing import Annotated, TypedDict
from langgraph.graph import StateGraph, START, END
from langgraph.graph.message import add_messages
from langgraph.prebuilt import ToolNode
from langchain_openai import ChatOpenAI
from langchain_core.tools import tool

# 1. Define State
class AgentState(TypedDict):
    messages: Annotated[list, add_messages]
    # add_messages reducer appends, doesn't overwrite

# 2. Define Tools
@tool
def search(query: str) -> str:
    """Search the web for information."""
    # Implementation here
    return f"Results for: {query}"

@tool
def calculator(expression: str) -> str:
    """Evaluate a math expression."""
    return str(eval(expression))

tools = [search, calculator]

# 3. Create LLM with tools
llm = ChatOpenAI(model="gpt-4o").bind_tools(tools)

# 4. Define Nodes
def agent(state: AgentState) -> dict:
    """The agent node - calls LLM."""
    response = llm.invoke(state["messages"])
    return {"messages": [response]}

# Tool node handles tool execution
tool_node = ToolNode(tools)

# 5. Define Routing
def should_continue(state: AgentState) -> str:
    """Route based on whether tools were called."""
    last_message = state["messages"][-1]
    if last_message.tool_calls:
        return "tools"
    return END

# 6. Build Graph
graph = StateGraph(AgentState)

# Add nodes
graph.add_node("agent", agent)
graph.add_node("tools", tool_node)

# Add edges
graph.add_edge(START, "agent")
graph.add_conditional_edges("agent", should_continue, ["tools", END])
graph.add_edge("tools", "agent")  # Loop back

# Compile
app = graph.compile()

# 7. Run
result = app.invoke({
    "messages": [("user", "What is 25 * 4?")]
})

from typing import Annotated, TypedDict
from operator import add
from langgraph.graph import StateGraph

# Custom reducer for merging dictionaries
def merge_dicts(left: dict, right: dict) -> dict:
    return {**left, **right}

# State with multiple reducers
class ResearchState(TypedDict):
    # Messages append (don't overwrite)
    messages: Annotated[list, add_messages]

    # Research findings merge
    findings: Annotated[dict, merge_dicts]

    # Sources accumulate
    sources: Annotated[list[str], add]

    # Current step (overwrites - no reducer)
    current_step: str

    # Error count (custom reducer)
    errors: Annotated[int, lambda a, b: a + b]

# Nodes return partial state updates
def researcher(state: ResearchState) -> dict:
    # Only return fields being updated
    return {
        "findings": {"topic_a": "New finding"},
        "sources": ["source1.com"],
        "current_step": "researching"
    }

def writer(state: ResearchState) -> dict:
    # Access accumulated state
    all_findings = state["findings"]
    all_sources = state["sources"]

    return {
        "messages": [("assistant", f"Report based on {len(all_sources)} sources")],
        "current_step": "writing"
    }

# Build graph
graph = StateGraph(ResearchState)
graph.add_node("researcher", rese