LangGraph isn't cheaper than LangChain — unless you opt out of its defaults

LangGraph isn't cheaper than LangChain — unless you opt out of its defaults Cost-audit series, episode 4. This series began with an AI agent that burned 136M tokens overnight → . When LangChain deprecated ConversationBufferMemory (the subject of episode 1 in this series), the official migration path was LangGraph. The pitch: explicit state management, you control exactly what flows where. More expressive, more controllable. It is — but only if you reach for the controls. The default state model in LangGraph has the same unbounded-growth problem as the memory it replaced. Teams migrating to escape ConversationBufferMemory's cost curve often land on an identical curve, with new graph complexity on top. This audit shows exactly where the default grows, what it costs, and what opt-outs exist. The default: MessagesState + add_messages The quickstart in LangGraph's own docs uses this pattern: from langgraph.graph import StateGraph , MessagesState def my_node ( state : MessagesState ): messages = state [ " messages " ] response = llm . invoke ( messages ) # sends ALL messages to the LLM return { " messages " : [ response ]} graph = StateGraph ( MessagesState ) graph . add_node ( " agent " , my_node ) MessagesState is a TypedDict with a single key, messages , backed by the add_messages reducer. Here's what that reducer does: # langgraph/graph/message.py — add_messages (def at line 18; merge loop below) def add_messages ( left : Messages , right : Messages ) -> Messages : # ... (coerces left/right to lists of BaseMessage) ... left_idx_by_id = { m . id : i for i , m in enumerate ( left )} merged = left . copy () ids_to_remove = set () for m in right : if ( existing_idx : = left_idx_by_id . get ( m . id )) is not None : if isinstance ( m , RemoveMessage ): ids_to_remove . add ( m . id ) else : merged [ existing_idx ] = m # same id → update in place else : merged . append ( m ) # new id → APPEND (the list grows) merged = [ m for m in merged if m . id not in ids_to_remove ] retu