State Machines vs Pure ReAct

State machine agents enforce explicit transitions between well-defined phases (gather requirements, plan, implement, test, review), while pure ReAct (Reasoning and Acting) agents let the model freely choose the next action at each step based on its own reasoning, representing a trade-off between predictability and flexibility. State machines excel when the workflow has a known structure and auditability matters, because every transition is explicit and observable; pure ReAct excels when the task is open-ended and the optimal action sequence cannot be predetermined upfront. The practical tradeoff a production team faces is that state machines give you auditability and predictability at the cost of requiring you to enumerate every state in advance, which fails for open-ended tasks where the necessary states only become apparent once work begins.