LangGraph Code Review

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

Review gates (sequenced)

Complete in order. Each step has an objective pass condition before moving on.

1. Locate graph code — Search the review scope for StateGraph, compile(, invoke, ainvoke, add_node, add_edge, add_conditional_edges. Pass: a short list of file paths (or explicit “none in scope” after searching).

2. Map state schema — For each graph state type (TypedDict, BaseModel, etc.), list fields that hold lists, dicts, or messages and whether Annotated + reducers (add_messages, operator.add, …) are present. Pass: every such field is either covered by a reducer pattern below or explicitly flagged as intentional overwrite.

3. Trace persistence — If interrupts, thread_id, or checkpoint APIs appear, follow them to compile(..., checkpointer=...) and invocation config. Pass: behavior matches the interrupt/checkpointer/thread_id guidance below—or you document a concrete mismatch with file:line.

4. Report with evidence — For each finding you will deliver, record file path and line number(s) (or a minimal quoted snippet). Pass: no critical or high-severity issue is stated without that citation.

5. Run the checklist — Use the checklist at the end of this skill; each item is satisfied, not applicable (with reason), or open with evidence. Pass: no item left silently unchecked.

Critical Issues

1. State Mutation Instead of Return

# 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

# 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

# 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

# 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