Agent
Codex (OpenAI coding agent)
Codex is OpenAI's software-engineering agent. It started in 2025 as an open-source terminal tool and a cloud agent inside ChatGPT, and later grew into a desktop app with millions of weekly users.
- Release date
- Developer
- OpenAI
What it is
Codex is OpenAI's software-engineering agent: a system that reads code, makes changes and runs tasks on a developer's behalf. The name was first used for a 2021 code model and was revived in 2025 for the agent.[1]
Codex comes in several forms. Codex CLI is an open-source coding agent that runs locally in a terminal. Codex Cloud is a web-based agent inside ChatGPT. A desktop Codex app followed in 2026.[1][2][4]
The difference between an agent and a chat model is that an agent can take actions. It reads files in a project, edits them, runs commands and reports the result, rather than only answering in text. The developer then reviews what the agent produced and decides whether to keep it.
Origins
Codex CLI was released on 16 April 2025, alongside the o3 and o4-mini models, under the Apache 2.0 license.[1][2] On 16 May 2025 OpenAI launched Codex Cloud as a research preview. It was powered by codex-1, a version of the o3 reasoning model.[1][3] Codex reached ChatGPT Plus users in June 2025.[1]
GPT-5-Codex became Codex's default model on 15 September 2025. Codex became generally available at DevDay on 6 October 2025.[7]
Growth
A desktop Codex app launched in February 2026. Reuters reported that more than 1 million developers had used Codex in the preceding month.[4] Fortune reported 1.6 million weekly active users on 4 March 2026, and weekly users reached about 2 million by mid-March 2026.[1][5]
A low-latency research preview, GPT-5.3-Codex-Spark, ran on Cerebras hardware from February 2026. A Windows version of the app followed on 5 March 2026.[1]
In March 2026 OpenAI reported on Codex Security, a vulnerability scanner. Its testing covered 1.2 million commits over 30 days and found nearly 800 critical and more than 10,000 high-severity issues.[6]
Product directions
On 19 March 2026 OpenAI announced a desktop superapp that would combine ChatGPT, the Atlas browser and Codex, and agreed to acquire Astral, the maker of Python tools.[1] On 9 July 2026 the Codex app was merged into the ChatGPT desktop app.[1][7] In September 2026 OpenAI's Agents API public beta exposed the managed Codex harness to developers.[7]
Models over time
The models used by Codex have changed over time. They include codex-1, GPT-5-Codex, GPT-5.1-Codex-Max, GPT-5.3-Codex, GPT-5.5 and GPT-5.6.[1][7]
20264
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ReleaseOpenAI releases GPT-5.6 and ChatGPT Work
On 9 July 2026 OpenAI released GPT-5.6, a three-model family named Sol, Terra and Luna, together with ChatGPT Work for enterprise teams, and merged its Codex app into the ChatGPT desktop app.
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ReleaseOpenAI releases GPT-5.5
OpenAI released GPT-5.5 Thinking and GPT-5.5 Pro to paid ChatGPT users on 23 April 2026, then extended the models to Codex and the API, reporting strong benchmark results and later making a faster GPT-5.5 Instant the default.
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ReleaseOpenAI releases GPT-5.4 with built-in computer use
On 5 March 2026 OpenAI released GPT-5.4 in Thinking and Pro versions, its first general-purpose model with native computer use; it scored 75% on OSWorld-Verified, above the 72.4% human average.
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ReleaseOpenAI releases GPT-5.3-Codex
On 5 February 2026 OpenAI released GPT-5.3-Codex for its Codex coding agent, describing it as the most capable agentic coding model to date and saying it had helped build itself. A faster research preview, GPT-5.3-Codex-Spark, followed a week later.
20255
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ReleaseOpenAI releases GPT-5.1-Codex-Max
On 19 November 2025 OpenAI released GPT-5.1-Codex-Max, an agentic coding model added to its Codex agent, and alongside it GPT-5.1 Pro, which replaced GPT-5 Pro.
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ReleaseOpenAI DevDay 2025: AgentKit, Apps SDK and Codex general availability
OpenAI held DevDay 2025 in San Francisco on 6 October 2025. It announced AgentKit, the Apps SDK and general availability for Codex, and disclosed usage figures for ChatGPT and its API.
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ReleaseOpenAI releases GPT-5-Codex for its Codex agent
On 15 September 2025 OpenAI released GPT-5-Codex, a version of GPT-5 tuned for agentic software engineering, as the default model for its Codex coding agent, with API access following later that month at GPT-5 prices.
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ReleaseOpenAI launches the Codex software-engineering agent
OpenAI's Codex, launched in May 2025 as a research preview, put a software-engineering agent inside ChatGPT: powered by codex-1 and trained with reinforcement learning, it ran each coding task in its own cloud sandbox.
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ReleaseOpenAI releases o3 and o4-mini
On 16 April 2025 OpenAI released o3 and o4-mini, billed as its smartest models to date, alongside the open-source Codex CLI; o3 scored 71.7% on SWE-bench Verified and 2727 Elo on Codeforces.
Developed by1
Notes and sources
- OpenAI Codexhttps://en.wikipedia.org/wiki/OpenAI_Codex
- OpenAI debuts Codex CLI, an open source coding tool for terminalshttps://techcrunch.com/2025/04/16/openai-debuts-codex-cli-an-open-source-coding-tool-for-terminals/
- Introducing Codexhttps://openai.com/index/introducing-codex/
- OpenAI launches Codex app to gain ground in AI coding racehttps://www.reuters.com/business/media-telecom/openai-launches-codex-app-gain-ground-ai-coding-race-2026-02-02/
- OpenAI sees Codex users spike to 1.6 million, positions coding tool as gateway to AI agents for businesshttps://fortune.com/2026/03/04/openai-codex-growth-enterprise-ai-agents/
- OpenAI Rolls Out Codex Security Vulnerability Scannerhttps://www.securityweek.com/openai-rolls-out-codex-security-vulnerability-scanner/
- OpenAI GPT Model Release Timelinehttps://hidekazu-konishi.com/entry/openai_gpt_model_release_timeline.html
Cite this page: Codex (OpenAI coding agent). TellWorld, https://www.tellworld.net/agents/openai-codex. Updated 2026-11-08