LLMilestonesA milestone timeline of AI and large language models

Act V · Reasoning and agents

Closed modelOpenAI

GPT-5

Unifies quick answers and deep reasoning in one system that decides when to think.

Why it matters

Reasoning models and general models merged; users no longer have to pick a model.

Costs and controversy

Early auto-routing and the retirement of older models upset users.

Built on

GPT-4oo1

Led to

GPT-5.5

Source: openai.com ↗

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