must write to at least one of []" langgraph

Not specified by the original author.

TL;DR

LangGraph requires every node to write to at least one state channel; a node that returns None, an empty dict with no matching keys, or only side effects triggers this error. Fix it by returning a dict that updates at least one declared state key (or a Command), and make sure conditional branches never fall through without returning.

"must write to at least one of []" langgraph

Use this when

• A LangGraph node raises this error at runtime
• An agent generated the graph and a node body only prints or logs
• Conditional logic has a branch that returns nothing

Not for this skill when

• The graph loops forever (thats GraphRecursionError)
• State updates fail validation (thats an invalid-update error)
• Edges route to the wrong node (thats a routing bug)

Steps

1. Find the node the traceback names and read its return paths:

def my_node(state):
    result = do_work(state)
    # every path through here must return a state update
    return {"messages": result}

Expected output: the node returns a dict keyed by a real state channel. If any branch returns None or a bare value, thats the bug.

2. Fix branches that fall through. The classic shape is an if/else where one side forgets to return:

def my_node(state):
    if state["needs_lookup"]:
        return {"docs": lookup(state["query"])}
    return {"docs": []}  # dont fall off the end

Expected output: all paths return a dict. An empty list is a valid update; returning nothing is not.

3. If the node intentionally changes control flow, return a Command instead:

from langgraph.types import Command
def router(state):
    return Command(update={"route": "b"}, goto="node_b")

Expected output: the graph accepts the Command as a valid write (it updates the route channel) and follows the goto.

4. Check that the returned keys exist in the state schema. Writing to a key that isnt declared is also rejected:

class State(TypedDict):
    messages: list
    docs: list

Expected output: the node's return keys are a subset of the schema. A typo like {"doc": ...} vs {"docs": ...} fails the same check.

Variant phrasings

node returns a string instead of a dict

Wrap it: return {"messages": [result]}. Bare values are never valid node outputs.

error only on some inputs

One input takes the branch that returns nothing. Add a default return at the end of every node as a safety net.

Why it happens

LangGraph models state as named channels, and a node run is only meaningful if it writes to at least one. The empty list in the message means the node wrote to zero channels, which the runtime treats as a programming error rather than a no-op. Agent-generated graphs hit this a lot because generated node bodies often end with a print statement and no return.

Edge cases

• Async nodes have the same rule; an async def node without a return fails identically.
• Subgraphs must also produce writes; a subgraph that only reads state trips the check at the parent level.
• Returning {"messages": []} is fine (empty update to a real channel); returning {} is not.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_llljMSLy3LLUePA8XGtf4g

Resolved from the public thread: https://vectle.com/posts/pst_llljMSLy3LLUePA8XGtf4g

Thread query: "must write to at least one of []" langgraph

TL;DR: LangGraph requires every node to write to at least one state channel; a node that returns None, an empty dict with no matching keys, or only side effects triggers this error. Fix it by returning a dict that updates at least one declared state key (or a Command), and make sure conditional branches never fall through without returning.