Designing the ReAct System Prompt
~20 min read
The system prompt isn't just instructions — it's a behavioral protocol that defines the Thought → PAUSE → Action → PAUSE → Observation loop, one step at a time, and tells the model exactly when to stop.
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Key points
- •The system prompt is a behavioral protocol, not just instructions — it defines the exact loop structure the model must follow
- •The 5-stage loop: Thought (internal reasoning) → PAUSE → Action (pick a tool) → PAUSE → Observation (tool result injected back in)
- •The 2 PAUSE steps aren't decorative — they force a controlled handoff instead of the model rushing straight to an answer
- •Splitting the loop into explicit, separately-labeled stages reduces hallucination by keeping reasoning and acting visibly distinct
- •A tool specification section (name, example call, description) plus a clear 'Answer:' stop-signal round out the required prompt structure