GPT stands for Generative Pre-trained Transformer. Each version showed that scaling up the same basic idea — predict the next token — unlocks new abilities. (How that works: Lesson 1 of our LLM course.)
The timeline
| Model | When | Why it mattered |
|---|---|---|
| GPT-1 | 2018 | ~117M parameters. Proved “pre-train on lots of text, then fine-tune” works across tasks. |
| GPT-2 | 2019 | 1.5B parameters. Wrote surprisingly fluent paragraphs; released in stages over misuse concerns. |
| GPT-3 | 2020 | 175B parameters. Could follow instructions from a few examples in the prompt (“few-shot”) — no fine-tuning needed. |
| ChatGPT (GPT-3.5) | Nov 2022 | Chat format plus training from human feedback made it helpful for ordinary people; reached an estimated 100 million users in about two months. |
| GPT-4 | Mar 2023 | Big jump in reasoning and exam-style tests; could read images. |
| GPT-4 Turbo | Nov 2023 | 128K context, cheaper. |
| GPT-4o | May 2024 | “Omni”: text, images and audio in one fast model; real-time voice conversations. |
| o1 and the o-series | From Sep 2024 | Reasoning models that “think” (spend extra computation) before answering — much stronger at maths and code. |
| GPT-4.1 | Apr 2025 | Developer-focused: better coding and instruction following, very long context. |
| gpt-oss-20b / 120b | Aug 2025 | OpenAI’s first open-weight models since GPT-2 — downloadable and free to run. Both are available in this site’s AI Helper. |
| GPT-5 | Aug 2025 | Unified fast answers and deeper reasoning, routing between them automatically. |
OpenAI continues to release updated versions; check the official model list for the latest names, prices and limits.
Three ideas that explain the progress
- Scale: more parameters, data and compute kept improving results.
- Alignment: training with human (and AI) feedback turned a text predictor into an assistant.
- Test-time reasoning: letting a model think longer on hard questions improves accuracy — at the cost of speed.
Free and paid access
| Option | Cost | Notes |
|---|---|---|
| ChatGPT free tier | Free | Usage limits on the strongest models; limits change over time. |
| ChatGPT Plus / Pro | Monthly | Higher limits and newest features. |
| API | Pay per token | For building your own apps. |
| gpt-oss | Free to run | On your own hardware or free cloud tiers. |
⚠️ Whatever the version, verify facts, numbers and code. Newer models hallucinate less, not never.
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