Why it matters

ReAct is the most-used agent pattern. Understanding it enables most agent frameworks.

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The architecture

Format: 'Thought: I need to X. Action: search(query). Observation: ...'. Continues until 'Final Answer: ...'.

ReAct stepThoughtreasonActioncall toolObservationresultThought/Action/Observation format parseable; enables structured agent execution
ReAct step format.
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How it works end to end

Prompt shows examples of the format. Model imitates.

Parser extracts action + args, invokes tool, appends observation.

Loop until final answer or max steps.