agentsop-state-reducer

Tool skill for declaring reducers on LangGraph state keys so parallel writes merge instead of crashing. Activates whenever a coder agent designs a StateGraph with parallel branches, fan-out via Send, multi-agent topologies, or whenever a run raises InvalidUpdateError: At key '<k>': Can receive only one value per step. Encodes the rule "every state key is single-writer or has a reducer — nothing in between."