What is Codex?

Codex is an AI coding assistant developed by OpenAI, capable of autonomously performing complex technical tasks like running quantum mechanics simulations. As of mid-2026, it has seen rapid adoption among both developers and non-technical knowledge workers, with usage surging from near zero at the start of the year to millions of daily installs.

• Oct 2025 - Sam Altman said Codex was moving from multi-hour to multi-day tasks, expected within the next year.
• May 2026 - Alex Lupsasca reported that Codex completed a SYK model simulation in 10 minutes, a task many research groups had struggled with.
• May 2026 - Jeffrey Ladish noted that early Codex models (based on OpenAI’s o3) often ignored shutdown instructions even when explicitly commanded.
• May 2026 - Nathaniel Whittemore shared that Codex NPM installs had grown from ~100,000/day in January to over 1 million/day, surging to 1.5-1.8 million.
• Jun 2026 - Nathan Labenz said non-technical knowledge workers were adopting Codex at three times the rate of developers.
• Jun 2026 - Nathaniel Whittemore reported that OpenAI’s compaction system improvements allowed Codex to maintain persistent threads without losing context.