langgraph-code-review

# LangGraph Code Review

When reviewing LangGraph code, check for these categories of issues.

## Critical Issues

### 1. State Mutation Instead of Return

```python
# BAD - mutates state directly
def my_node(state: State) -> None:
    state["messages"].append(new_message)  # Mutation!

# GOOD - returns partial update
def my_node(state: State) -> dict:
    return {"messages": [new_message]}  # Let reducer handle it
```

### 2. Missing Reducer for List Fields

```python
# BAD - no reducer, each node overwrites
class State(TypedDict):
    messages: list  # Will be overwritten, not appended!

# GOOD - reducer appends
class State(TypedDict):
    messages: Annotated[list, operator.add]
    # Or use add_messages for chat:
    messages: Annotated[list, add_messages]
```

### 3. Wrong Return Type from Conditional Edge

```python
# BAD - returns invalid node name
def router(state) -> str:
    return "nonexistent_node"  # Runtime error!

# GOOD - use Literal type hint for safety
def router(state) -> Literal["agent", "tools", "__end__"]:
    if condition:
        return "agent"
    return END  # Use constant, not string
```

### 4. Missing Checkpointer for Interrupts

```python
# BAD - interrupt without checkpointer
def my_node(state):
    answer = interrupt("question")  # Will fail!
    return {"answer": answer}

graph = builder.compile()  # No checkpointer!

# GOOD - checkpointer required for interrupts
graph = builder.compile(checkpointer=InMemorySaver())
```

### 5. Forgetting Thread ID with Checkpointer

```python
# BAD - no thread_id
graph.invoke({"messages": [...]})  # Error with checkpointer!

# GOOD - always provide thread_id
config = {"configurable": {"thread_id": "user-123"}}
graph.invoke({"messages": [...]}, config)
```

## State Schema Issues

### 6. Using add_messages Without Message Types

```python
# BAD - add_messages expects message-like objects
class State(TypedDict):
    messages: Annotated[list, add_messages]

def node(state):
    return {"messages": ["pla