# GPT Model Timeline (GPT-1 to GPT-5.x)

> Source: https://aiwiki.ai/wiki/gpt_model_timeline
> Updated: 2026-07-07
> Categories: AI Models, Generative AI, Large Language Models
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
> From AI Wiki (https://aiwiki.ai), the free encyclopedia of artificial intelligence. Reuse freely with attribution to "AI Wiki (aiwiki.ai)".

[OpenAI](/wiki/openai)'s GPT lineage runs from [GPT-1](/wiki/gpt-1) (June 2018, 117 million parameters, a 512-token context window) to the GPT-5.6 "Sol, Terra, Luna" family that entered limited preview on June 26, 2026. As of July 2026 the newest generally available model is [GPT-5.5](/wiki/gpt-5.5) (released April 23, 2026), while GPT-5.6 remains restricted to preview partners. The arc has three phases: a pure scaling race (GPT-1 to [GPT-3](/wiki/gpt-3), 117M to 175B parameters), a product and multimodal era ([ChatGPT](/wiki/chatgpt), [GPT-4](/wiki/gpt-4), [GPT-4o](/wiki/gpt-4o)), and, since [o1](/wiki/o1) in September 2024, a "reasoning" era that [GPT-5](/wiki/gpt-5) unified into one model that automatically routes between fast answers and deliberate thinking.

Short version, if you just want the answer: OpenAI has publicly disclosed a parameter count for exactly three models (GPT-1, GPT-2, GPT-3). Every model from GPT-4 onward is marked "undisclosed" below, and any specific number you see for them (for example the widely repeated 1.8-trillion figure for GPT-4) is an unofficial third-party estimate, never confirmed by OpenAI. For the current default assistant use GPT-5.5; the o-series (o1, [o3](/wiki/o3), [o4-mini](/wiki/o4-mini)) was the reasoning-specialist line that GPT-5 folded in. This page tracks OpenAI's GPT and o-series specifically; for a cross-lab index of every major flagship, see the [AI Model Release Timeline](/wiki/ai_model_release_timeline).

## The GPT and o-series timeline at a glance

All models are developed by OpenAI and are proprietary (accessible via [ChatGPT](/wiki/chatgpt) and the OpenAI API). "Undisclosed" means OpenAI has never published an official parameter count. Context is the maximum context window; where the API and ChatGPT differ, the larger API figure is shown. Last verified: July 2026.

| Date | Model | Params | Context window | Headline change |
| --- | --- | --- | --- | --- |
| 2018-06 | [GPT-1](/wiki/gpt-1) | 117M | 512 | First generative pre-trained transformer: unsupervised pre-training plus supervised fine-tuning [1] |
| 2019-02 | [GPT-2](/wiki/gpt-2) | 1.5B | 1K | About 10x the scale; coherent long-form text; staged release over misuse fears [2] |
| 2020-05 | [GPT-3](/wiki/gpt-3) | 175B | 2K | About 100x the scale; few-shot in-context learning; launched the OpenAI API [3] |
| 2022-11 | [GPT-3.5](/wiki/gpt-3.5) / [ChatGPT](/wiki/chatgpt) | Undisclosed | 4K | RLHF-tuned chat model; ChatGPT reached about 100M users in two months [4] |
| 2023-03 | [GPT-4](/wiki/gpt-4) | Undisclosed | 8K / 32K | Multimodal (image input); roughly 90th-percentile bar-exam performance [5] |
| 2023-11 | [GPT-4 Turbo](/wiki/gpt-4-turbo) | Undisclosed | 128K | Cheaper, 128K context, fresher knowledge cutoff, JSON mode (DevDay) [6] |
| 2024-05 | [GPT-4o](/wiki/gpt-4o) | Undisclosed | 128K | Natively multimodal "omni" (text, vision, audio); real-time voice; free-tier flagship [7] |
| 2024-09 to 12 | [o1](/wiki/o1) | Undisclosed | 128K to 200K | First reasoning model; scales test-time compute via long chains of thought [8] |
| 2025-02 | [GPT-4.5](/wiki/gpt-4.5) | Undisclosed | 128K | Largest non-reasoning GPT (codenamed Orion); shipped as a research preview [9] |
| 2025-01 to 04 | [o3](/wiki/o3) / [o4-mini](/wiki/o4-mini) | Undisclosed | 200K | Tool-using reasoning: models call tools inside the reasoning chain [10] |
| 2025-08 | [GPT-5](/wiki/gpt-5) | Undisclosed | 400K | First "unified" model: an auto-router picks fast vs thinking per query [11] |
| 2025-11 | [GPT-5.1](/wiki/gpt-5.1) | Undisclosed | 400K | Warmer, steerable personalities (8 presets); adaptive reasoning [12] |
| 2025-12 | [GPT-5.2](/wiki/gpt-5.2) | Undisclosed | 400K | Split into Instant, Thinking, and Pro; accuracy and coding gains; shipped early [13] |
| 2026-02 | [GPT-5.3](/wiki/gpt-5.3) | Undisclosed | 400K | Unified the Codex and general branches; first OpenAI model on non-NVIDIA silicon [14] |
| 2026-03 | [GPT-5.4](/wiki/gpt-5.4) | Undisclosed | 1M | 1M-token context; built-in computer use; lower error rate [15] |
| 2026-04 | [GPT-5.5](/wiki/gpt-5.5) | Undisclosed | 1M | Agentic-coding and scientific-reasoning gains; became the ChatGPT default [16] |
| 2026-06 | GPT-5.6 (Sol, Terra, Luna) | Undisclosed | n/r (preview) | Next-generation family in limited, access-restricted preview [17] |

## What is the GPT lineage, in order?

The full order is GPT-1 (2018), [GPT-2](/wiki/gpt-2) (2019), GPT-3 (2020), GPT-3.5 and ChatGPT (2022), GPT-4 (2023), GPT-4 Turbo (2023), GPT-4o (2024), o1 (2024), GPT-4.5 (early 2025), o3 and o4-mini (2025), GPT-5 (2025), then the rapid point releases GPT-5.1, GPT-5.2, GPT-5.3, GPT-5.4, GPT-5.5, and GPT-5.6 across late 2025 and the first half of 2026. GPT stands for [generative pre-trained transformer](/wiki/generative_pre-trained_transformer), the architecture introduced with GPT-1 and built on the [Transformer](/wiki/transformer). The cadence compressed sharply over time: seven years separated GPT-1 from GPT-5, but OpenAI then shipped GPT-5.1 through GPT-5.6 in under a year.

## When did GPT stop disclosing parameter counts?

GPT-3, at 175 billion parameters, was the last OpenAI model with an officially published size [3]. Starting with GPT-4, OpenAI has declined to disclose parameter counts, training-data size, or architecture details, citing competitive and safety reasons [5]. The number most often attached to GPT-4, roughly 1.8 trillion parameters in a mixture-of-experts configuration, comes from third-party industry reporting and leaks; OpenAI has never confirmed it, so it should be treated as an unofficial estimate rather than a specification. For every model from GPT-4 onward, the honest answer is "undisclosed," which is why the table above avoids putting a specific figure in those cells. GPT-3.5 is a borderline case: it derives from the 175B [GPT-3](/wiki/gpt-3) series, but the exact size of the deployed gpt-3.5-turbo model was never confirmed, so it is listed as undisclosed too.

## How did context windows and model scale grow?

Context windows expanded far faster than disclosed parameter counts. GPT-1 handled 512 tokens and GPT-2 handled 1,024; GPT-3 reached 2,048 and the first ChatGPT ran at 4,096 [3][4]. GPT-4 launched with 8K and 32K variants, then GPT-4 Turbo jumped to 128K in late 2023, a level GPT-4o and GPT-4.5 kept [6][7][9]. The reasoning models pushed to 200K (o1's general-availability release and the o3 line) [8][10], GPT-5 opened at 400,000 tokens in the API [11], and GPT-5.4 and GPT-5.5 reached a full 1 million tokens in the API in 2026 [15][16]. In practice ChatGPT surfaces a smaller window than the API for the same underlying model, and advertised maximums include both input and output budgets (GPT-5-class models expose roughly 272K input tokens within a 400K total) [11].

## What are the o-series reasoning models?

The o-series was OpenAI's separate line of "reasoning" models that spend extra compute at inference time to think through a problem before answering. It began with o1, previewed on September 12, 2024 and released in full on December 5, 2024 with a 200K context window [8]. [o3-mini](/wiki/o3-mini) followed on January 31, 2025, and the full [o3](/wiki/o3) shipped alongside [o4-mini](/wiki/o4-mini) on April 16, 2025, adding agentic tool use inside the chain of thought [10]. The reasoning turn added a second axis of progress beyond raw pre-training scale, and every major lab raced on it. With GPT-5, OpenAI merged this line back into the main GPT branch: rather than choosing a separate model, users get one system that decides when to think longer. For the dedicated hub, see [OpenAI o-series](/wiki/openai_o-series).

## What changed across GPT-5 and the 5.x point releases?

[GPT-5](/wiki/gpt-5), released August 7, 2025, was OpenAI's first "unified" model, combining the fast responses of the GPT line with the deliberate reasoning of the o-series behind an automatic router [11]. The point releases then arrived roughly every one to two months. [GPT-5.1](/wiki/gpt-5.1) (November 12, 2025) focused on a warmer, more steerable personality with selectable presets and adaptive reasoning [12]. [GPT-5.2](/wiki/gpt-5.2) (December 11, 2025) split into Instant, Thinking, and Pro tiers and was reportedly accelerated by an internal "Code Red" after Google's Gemini 3 briefly took the lead [13]. [GPT-5.3](/wiki/gpt-5.3) (Codex on February 5, 2026; Instant on March 3, 2026) unified the coding and general branches and, via its Codex-Spark variant, became the first OpenAI model served on non-NVIDIA silicon [14]. [GPT-5.4](/wiki/gpt-5.4) (March 5, 2026) added a 1-million-token API context and built-in computer use while cutting error rates [15]. [GPT-5.5](/wiki/gpt-5.5) (April 23, 2026) improved agentic coding and scientific reasoning and became the ChatGPT default [16]. GPT-5.6, a three-model family named Sol (flagship), Terra (balanced), and Luna (fast), entered a limited, access-restricted preview on June 26, 2026 [17].

## Which GPT model should you use now?

As of July 2026, GPT-5.5 is the strongest generally available OpenAI model and the ChatGPT default, so it is the safe pick for most work [16]. GPT-5.6 Sol is more capable on paper but is limited to preview partners, so it is not a practical choice for general users yet [17]. GPT-4o remains the ubiquitous, low-latency multimodal workhorse where full frontier reasoning is not required [7], and the standalone o-series models have largely been superseded by GPT-5's built-in thinking mode. For a broader comparison across labs (Anthropic, Google, xAI, and open-weight models), use the cross-lab [AI Model Release Timeline](/wiki/ai_model_release_timeline).

## References

1. OpenAI, "Improving Language Understanding by Generative Pre-Training" (GPT-1), 2018. https://cdn.openai.com/research-covers/language-unsupervised/language_understanding_paper.pdf
2. OpenAI, "Better Language Models and Their Implications" (GPT-2), February 14, 2019. https://openai.com/research/better-language-models
3. T. Brown et al., "Language Models are Few-Shot Learners" (GPT-3, 175B), May 28, 2020. https://arxiv.org/abs/2005.14165
4. OpenAI, "Introducing ChatGPT," November 30, 2022. https://openai.com/blog/chatgpt
5. OpenAI, "GPT-4" and GPT-4 Technical Report, March 14, 2023. https://openai.com/index/gpt-4-research/
6. OpenAI, "New models and developer products announced at DevDay" (GPT-4 Turbo), November 6, 2023. https://openai.com/blog/new-models-and-developer-products-announced-at-devday
7. OpenAI, "Hello GPT-4o," May 13, 2024. https://openai.com/index/hello-gpt-4o/
8. OpenAI, "Introducing OpenAI o1-preview," September 12, 2024; o1 general availability, December 5, 2024. https://openai.com/index/introducing-openai-o1-preview/
9. OpenAI, "Introducing GPT-4.5," February 27, 2025. https://openai.com/index/introducing-gpt-4-5/
10. OpenAI, "Introducing OpenAI o3 and o4-mini," April 16, 2025 (o3-mini, January 31, 2025). https://openai.com/index/introducing-o3-and-o4-mini/
11. OpenAI, "Introducing GPT-5," August 7, 2025. https://openai.com/index/introducing-gpt-5/
12. OpenAI, "GPT-5.1: A smarter, more conversational ChatGPT," November 12, 2025. https://openai.com/index/gpt-5-1/
13. OpenAI, "Introducing GPT-5.2," December 11, 2025. https://openai.com/index/introducing-gpt-5-2/
14. OpenAI, "Introducing GPT-5.3-Codex," February 5, 2026; GPT-5.3 Instant, March 3, 2026. https://openai.com/index/introducing-gpt-5-3-codex/
15. TechCrunch, "OpenAI launches GPT-5.4 with Pro and Thinking versions," March 5, 2026. https://techcrunch.com/2026/03/05/openai-launches-gpt-5-4-with-pro-and-thinking-versions/
16. OpenAI, "Introducing GPT-5.5," April 23, 2026; Fortune coverage, April 23, 2026. https://openai.com/index/introducing-gpt-5-5/
17. OpenAI, "Previewing GPT-5.6 Sol: a next-generation model," June 26, 2026; Axios and VentureBeat coverage, June 26, 2026. https://openai.com/index/previewing-gpt-5-6-sol/
18. OpenAI Help Center, "Model Release Notes." https://help.openai.com/en/articles/9624314-model-release-notes
19. AI Wiki, "AI Model Release Timeline (2022-2026)." https://aiwiki.ai/wiki/ai_model_release_timeline