langgraph-implementation

# LangGraph Implementation

## Core Concepts

LangGraph builds stateful, multi-actor agent applications using a graph-based architecture:

- **StateGraph**: Builder class for defining graphs with shared state
- **Nodes**: Functions that read state and return partial updates
- **Edges**: Define execution flow (static or conditional)
- **Channels**: Internal state management (LastValue, BinaryOperatorAggregate)
- **Checkpointer**: Persistence for pause/resume capabilities

## Essential Imports

```python
from langgraph.graph import StateGraph, START, END
from langgraph.graph.message import MessagesState, add_messages
from langgraph.checkpoint.memory import InMemorySaver
from langgraph.types import Command, Send, interrupt, RetryPolicy
from typing import Annotated
from typing_extensions import TypedDict
```

## State Schema Patterns

### Basic State with TypedDict

```python
class State(TypedDict):
    counter: int                                    # LastValue - stores last value
    messages: Annotated[list, operator.add]         # Reducer - appends lists
    items: Annotated[list, lambda a, b: a + [b] if b else a]  # Custom reducer
```

### MessagesState for Chat Applications

```python
from langgraph.graph.message import MessagesState

class State(MessagesState):
    # Inherits: messages: Annotated[list[AnyMessage], add_messages]
    user_id: str
    context: dict
```

### Pydantic State (for validation)

```python
from pydantic import BaseModel

class State(BaseModel):
    messages: Annotated[list, add_messages]
    validated_field: str  # Pydantic validates on assignment
```

## Building Graphs

### Basic Pattern

```python
builder = StateGraph(State)

# Add nodes - functions that take state, return partial updates
builder.add_node("process", process_fn)
builder.add_node("decide", decide_fn)

# Add edges
builder.add_edge(START, "process")
builder.add_edge("process", "decide")
builder.add_edge("decide", END)

# Compile
graph = builder.compile()
```

### Node Funct