ModelRefs / ReAct — Prompt Pattern
ReAct — Prompt Pattern
Interleave Reasoning and Acting (tool calls) in a single loop. Model alternates Thought → Action → Observation until it decides to answer.
Overview
Model alternates Thought → Action → Observation until it decides to answer. Foundational tool-use loop.
When to use it: You need an agent that can use tools and reason about their outputs.
Pattern details
| Pattern class | agent |
|---|---|
| Difficulty | intermediate |
| Also known as | react agent, reasoning + acting |
| Last reviewed | 2026-06-07 |
Known failure modes
- Runaway loops — Agent keeps calling tools forever. Mitigation: Set max steps; require Final Answer trigger.
- Format drift — Agent stops emitting Action/Observation tokens. Mitigation: Validate format each step; retry on drift.
When not to use it
- Using ReAct when a single function call would do.
Continue your research
Use these connected ModelRefs sections to compare alternatives, inspect implementation paths, and review the evidence and governance boundaries relevant to ReAct — Prompt Pattern.
Frequently asked questions
When should I use the ReAct prompt pattern?
You need an agent that can use tools and reason about their outputs.
What are common failure modes of ReAct?
Runaway loops • Format drift
Does ReAct work across models?
Yes when the anatomy is preserved — see model compatibility matrix on the pattern page.