LangGraph State Values Not Persisting Between Workflow Nodes

[jess_brown]

I’m building a LangGraph workflow with multiple connected nodes and having trouble with state persistence. My workflow has several nodes linked with conditional routing, but the state data isn’t carrying over properly.

Here’s my setup with the first two nodes:

def message_processor(state: TypedDict) -> None:
    result, state = fetch_messages(state)
    if result == "Messages Found":
        state["current_step"] = "category_handler"
    else:
        state["error_msg"] = "No Messages Available"
        state["current_step"] = "final_output"

def get_current_step(state: TypedDict) -> str:
    logging.debug(f"Current Step: {state["current_step"]}")
    return state["current_step"]

I’m using a conditional edge to route between nodes:

flow.add_conditional_edges(
    "message_processor",
    get_current_step,
    {
        "category_handler": "category_handler",
        "final_output": "final_output",
    },
)

The problem is that my debug logging shows an empty string for the state value, which means the state isn’t being passed correctly between nodes. My complete workflow looks like this:

flow = StateGraph(MyGraphState)
flow.add_node("message_processor", message_processor)
flow.add_node("category_handler", category_handler)
flow.add_node("priority_handler", priority_handler)
flow.add_node("task_handler", task_handler)
flow.add_node("final_output", final_output_node)

flow.set_entry_point("message_processor")

flow.add_conditional_edges(
    "message_processor",
    get_current_step,
    {
        "category_handler": "category_handler",
        "final_output": "final_output",
    },
)

flow.add_conditional_edges(
    "category_handler",
    get_current_step,
    {
        "priority_handler": "priority_handler",
        "final_output": "final_output",
    },
)

flow.add_edge("task_handler", "final_output")
flow.add_edge("final_output", END)

I’m compiling this in one class and invoking it from another function. Could the separation across multiple files be causing the state to not persist? What am I missing here?

[ryanl]

Classic LangGraph gotcha right there. Your node functions need to return the modified state, not None. LangGraph merges whatever you return back into the main state.

Change your message_processor to:

def message_processor(state: TypedDict) -> TypedDict:
    result, state = fetch_messages(state)
    if result == "Messages Found":
        state["current_step"] = "category_handler"
    else:
        state["error_msg"] = "No Messages Available"
        state["current_step"] = "final_output"
    return state  # This is what you're missing

I hit this exact issue 6 months ago on a document processing workflow. Spent hours debugging why my state kept resetting. LangGraph doesn’t modify state in place - it expects you to return what should be merged back.

Your conditional routing function looks fine, but make sure all your other nodes (category_handler, priority_handler, etc.) are also returning their state modifications.

The file separation isn’t your problem. I’ve got workflows split across multiple modules and they work fine as long as the graph compilation and invocation are handled properly.

[CreatingStone]

Had the same issue when I started with langgraph. Your functions aren’t returning the state changes - that’s why everything’s empty. Langgraph ignores your modifications without a return statement. Also double-check that fetch_messages is actually updating the state correctly.

[Finn_Mystery]

Had the exact same issue when I started with LangGraph! Your message_processor function is modifying the state but not returning it - that’s why LangGraph doesn’t see your changes. Each node needs to return the updated state or it won’t carry over to the next node. Just add return state at the end instead of return None. Also double-check that your MyGraphState TypedDict includes all the fields you’re using like current_step and error_msg. I got those same empty strings in my debug logs until I fixed the return statements. The file separation is fine as long as you’re passing the compiled graph correctly between modules.

[JackWolf69]

Your node functions are missing return statements - that’s the problem. I hit this exact issue last year while building a customer service workflow and wasted hours debugging it. LangGraph handles state updates functionally, so you’ve got to explicitly return whatever changes you want to keep. Your message_processor function changes the state dictionary but returns None, which means LangGraph has nothing to merge back. Double-check that fetch_messages is returning the updated state too - that could be causing issues as well. The conditional routing looks fine though. Add return statements to all your node functions and those debug logs should start showing what you expect.

[Hermione_Book]

Your function signatures are the problem. message_processor is typed to return None but needs to return the updated state dictionary. LangGraph won’t persist state changes between nodes without explicit returns. I hit this exact issue building a multi-stage data processing workflow - state modifications just vanished without proper returns. Fix the return statements first, then double-check your MyGraphState TypedDict includes all the fields you’re accessing. Missing field definitions cause silent failures where updates seem to work but don’t stick. Your conditional routing looks fine, so adding returns to all node functions should get state flowing properly through your workflow.