Agent Architecture Patterns

A single-agent architecture assigns one agent full responsibility for a task from start to finish, using tools without delegating to other agents or routing through an orchestrator. This pattern keeps coordination overhead at zero and makes debugging straightforward: you have one reasoning chain to inspect, one set of tool calls to trace, and one system prompt to tune.

The Reasoning and Acting (ReAct) pattern is the foundational architecture where agents alternate between thinking steps and tool calls in a loop, combining chain-of-thought reasoning with grounded actions. Each cycle produces a thought (the model's reasoning about what to do next), an action (a tool call or output), and an observation (the result of the action) that feeds the next iteration; most modern agent frameworks use this pattern because the visible reasoning chain lets you understand why the agent chose a particular action without needing to instrument the model internals.

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.