ChatGPT is OpenAI's flagship conversational AI assistant and the most widely used AI chatbot in the world. As of 2026 it is powered by the GPT-5.5 model family, with GPT-5.5 Instant serving as the fast default model and GPT-5.5 Pro available for higher-accuracy professional work. The platform features a 1 million token context window, enabling it to process long documents, large codebases, and extended conversations. Beyond text chat, ChatGPT offers Advanced Voice mode built on GPT-Realtime-2, real-time web search for current information, Deep Research for multi-source report generation, and an Agent mode that can autonomously plan and complete multi-step tasks across tools. Users can generate and edit images, create videos with Sora, write and debug code with Codex, and collaborate in Canvas. Memory carries context across sessions, custom GPTs let users build tailored assistants, and file uploads support document and data analysis. ChatGPT is available through a web interface, iOS, Android, and desktop apps, as well as OpenAI's developer API. Subscription tiers range from a free plan to Go, Plus, Pro, Business, and Enterprise, with team and enterprise plans adding shared workspaces, admin controls, SSO/SCIM, and audit logs.