ReAct Pattern (Reasoning + Acting) 2026: Deep Turkish Technical Guide — From Academia to Production

## 1. Introduction ReAct Pattern - LLMs generate Thoughts, take Actions (tool calls), receive Observations. Yao et al. 2022 ICLR paper. Foundation of modern agentic AI. ## 2. CoT vs ReAct CoT - internal reasoning only. ReAct - reasoning + external world interaction. ## 3. T-A-O Loop Thought - Action - Observation iterative cycle until final answer. ## 4. 5 Variants Vanilla ReAct, MRKL, Self-Ask, ReWOO, Plan-and-Execute. ## 5. Modern Implementation LangChain AgentExecutor, LangGraph state machines, OpenAI Function Calling. ## 6. Tool Design 8 principles - atomic, descriptive, strict schema, deterministic, error handling, idempotent, bounded, auditable. ## 7. Cost Optimization 3-10x CoT - use ReWOO, prompt caching, smaller models for simple tasks. ## 8. Conclusion ReAct foundational for agentic AI. LangGraph state machine modern best practice.