OpenAI Codex case studies: 7 sourced stories
Stories using OpenAI Codex, in the app, the CLI or the cloud agent. Of the 7 stories, 4 were published by a tool vendor, 2 by the people who did the work and 1 by independent researchers or press. 3 involve an existing codebase and 3 started from scratch. Every number links to where it was published.
- Practitioner (own blog/paper) New product greenfield
Three people build Loft with coding agents over three months, then ship its open format 53 hours after the first commit
Loft · 2026-10-16
- 3 people Team
- ~3 months (July 10 – Oct 16) V1 duration
- Nearly 2,000 (merges excluded) V1 commits
Claude Code, OpenAI Codex
- Vendor-published Test migration brownfield
Asana finishes a five-year Enzyme → React Testing Library migration in two weeks with Codex
Asana · 2026-08-19
- ~2 weeks Calendar time
- ~1.5 weeks Engineering effort
- Up to 4 Parallel agents
OpenAI Codex
- Independent Research / benchmark
NBER study of 500,000+ GitHub developers: coding agents multiply commits and code, but releases rise only ~20%
NBER working paper (Demirer, Musolff, Yang) · 2026-05
- 500,000+ Developers studied
- +30% autocomplete, ~+180% with sync agents, ~+240% with async agents (cumulative) Weekly commits
- +958% Sync agents: lines of code
GitHub Copilot, Claude Code, OpenAI Codex
- Vendor-published Research / benchmark brownfield
Legacy-Bench: frontier agents pass only 16.9–42.5% of COBOL, Java 7 and Fortran tasks
Factory · 2026-04-01
- 16.9%–42.5% Pass rate range (12 model–agent combos)
- >70% Same models on Terminal-Bench 2 / SWE-bench Verified
- 42.5% Best overall (GPT-5.3-Codex)
Factory Droid, OpenAI Codex, Gemini CLI
- Vendor-published New product greenfield
OpenAI team ships an internal product with zero hand-written code: ~1M lines and ~1,500 merged PRs in five months with Codex
OpenAI · 2026-02-11
- ~5 months Duration
- On the order of 1 million lines (incl. infra, tooling, docs) Codebase size
- ~1,500 PRs merged
OpenAI Codex
- Vendor-published Refactor / maintenance brownfield
Cisco uses Codex across 15+ repos to cut build times ~20% and speed up C/C++ defect fixing
Cisco · 2026-01
- 1,500+ per month Engineering hours saved
- ~20% reduction Build time
- 10–15× increase Defect resolution throughput
OpenAI Codex
- Practitioner (own blog/paper) Language port greenfield
Simon Willison ports an HTML5 parser from Python to JavaScript with Codex CLI in about 4.5 hours
Simon Willison (independent developer) · 2025-12-15
- ~4–4.5 hours Idea to finished library
- 8 Prompts
- 9,000 lines of JavaScript, 43 commits Output
OpenAI Codex