ChatGPT

OpenAI was incorporated in December 2015 as a nonprofit AI research organization, co-founded by Sam Altman, Greg Brockman, Ilya Sutskever, Elon Musk, and others, backed by an initial $1 billion pledge. Its earliest published models — GPT-1 (2018), GPT-2 (2019, staged release due to dual-use concerns), and GPT-3 (May 2020, 175 billion parameters) — established the generative pre-trained transformer paradigm. In 2019 Microsoft invested $1 billion and became OpenAI's primary cloud provider; in January 2022 OpenAI published InstructGPT research on aligning language models via human feedback, the direct technical predecessor to ChatGPT.

ChatGPT launched as a free research preview on November 30, 2022, built on GPT-3.5 with a simple chat interface. It reached 100 million monthly active users within two months — the fastest consumer-app adoption in internet history at the time. OpenAI introduced the ChatGPT Plus subscription ($20/month) on February 1, 2023, and followed with GPT-4 on March 14, 2023: a multimodal model accepting text and image input, simultaneously deployed in Microsoft Bing. ChatGPT plugins launched March 23, 2023, offering early tool-use and browsing. Italy banned the service in March 2023 over GDPR and age-verification shortcomings; OpenAI implemented the requested safeguards and the ban was lifted in April 2023.

November 2023 brought OpenAI's most dramatic episode: the board fired Sam Altman on November 17, citing a failure to be 'consistently candid.' Five days later, following intense pressure from Microsoft and a near-total employee revolt, Altman was reinstated and the board was reconstituted. That same month, OpenAI held its first DevDay, unveiling GPT-4 Turbo with a 128K-token context window and the Assistants API. The GPT Store launched in January 2024 with over 3 million user-built GPTs. May 2024 brought GPT-4o ('omni'), folding text, audio, and vision into a single real-time model — and also brought the resignation of Chief Scientist Ilya Sutskever (succeeded by Jakub Pachocki) and safety co-leader Jan Leike, who cited the company's deprioritization of safety. By late 2024, roughly half of OpenAI's AI safety researchers had left the company.

In September 2024, OpenAI released o1-preview, introducing a new 'reasoning' model family that produces internal chain-of-thought before answering. The full o1 model and the $200/month ChatGPT Pro tier launched in December 2024 alongside the Sora text-to-video model. ChatGPT Search went live October 31, 2024, embedding real-time cited web results directly in chat. By August 2024, ChatGPT had 200 million weekly active users. Early 2025 saw the Operator autonomous web agent (January), Deep Research multi-source report synthesis (February), and the Codex cloud-based software engineering agent research preview (May 2025). On August 7, 2025, GPT-5 launched as OpenAI's first 'unified' model — merging the fast GPT-series and the o-series reasoning models behind a real-time router — and became the default for all ChatGPT users, though the rollout was bumpy, requiring OpenAI to restore legacy model access and adjust the model's personality following user complaints. In October 2025, OpenAI completed its conversion to a Public Benefit Corporation (OpenAI Group PBC), with the OpenAI Foundation retaining a 26% governance equity stake.

The GPT-5 generation iterated at a sub-60-day cadence: GPT-5.1 (November 2025), GPT-5.2 (December 2025), GPT-5.4 (March 2026, integrating Codex-trained coding capabilities and native computer use), and GPT-5.5 (April 23, 2026), the current flagship. GPT Image 2 (gpt-image-2), launched April 21, 2026 as ChatGPT Images 2.0, replaced DALL-E 3 and GPT Image 1.5 with a reasoning-powered image model achieving ~99% multilingual text accuracy and up to 4K resolution. As of April 2026, ChatGPT reported 900 million+ weekly active users, 50 million+ subscribers, and 4 million active Codex users; the company closed a record $122 billion funding round at an $852 billion valuation led by Amazon, SoftBank, and Nvidia, filed for an IPO on June 8, 2026, and restructured its Microsoft partnership from exclusive cloud arrangement to a non-exclusive agreement allowing multi-cloud deployment.