Order the four phases of a single iteration of a ReAct (Reason + Act) loop in the correct sequence.

**ReAct** stands for Reason + Act. Each iteration of the loop has four moves and they happen in a fixed order. First, the model emits a **Thought**, a short reasoning step that says what it wants to do next. Then it emits an Action, which is a tool call (function name plus arguments). Then the orchestrator (the code wrapping the model, not the model itself) runs the tool and captures the result. Finally, that result is appended to the model's context as an **Observation**, and the next iteration starts. The model emits only the Thought and the Action. The orchestrator executes the tool and supplies the Observation. The cycle continues until the model decides it has enough information and emits a final answer instead of another Action.

**ReAct** (Yao et al. 2023) interleaves reasoning with tool use. One iteration goes in this order: Thought, Action, Tool execution, Observation. The model writes a Thought line first, a short verbalized reasoning step about what it needs next. It then emits an Action, a structured tool call (function name plus arguments). The orchestrator, which is the runtime around the model, parses the action, calls the actual tool, and captures the return value. That return value is then appended to the model's context as an **Observation**, and the next iteration begins. The split matters because the model only ever emits text. It never executes a tool itself. The orchestrator is responsible for parsing the action, running it, and feeding back the result. Modern hosted runtimes (LangGraph, OpenAI Assistants, Claude tool-use loops) implement exactly this pattern. The Thought sometimes collapses into native tool-call message types, but the four phases are still there in some form. The loop terminates when the model emits a final answer instead of another Action, which is why the Thought phase is where the agent decides to stop.