I Used REA to Clone OpenAI's Codex

I Used REA to Clone OpenAI's Codex

I gave Claude Code only the files that OpenAI's Codex installs on a computer, with no source code at all, and asked it to rebuild the program in Python while a hidden test I wrote beforehand decided if the copy was good enough. REA, the open source reverse engineering tool, is the one it was supposed to use.

The first copy scored 99% on my easy test and only 75.4% on a much harder one, so I made REA and Ghidra mandatory. The second copy passed 416 of 422 checks (98.6%) and the whole AI bill for both rounds was about $16.80. Everything on screen is replayed from the real logs and results.

0:00 Two terminals, one secret 1:08 The one rule that makes it fair 1:44 Step 1: what is inside the file 2:29 Step 2: making the real Codex talk 3:31 Step 3: the hidden test 4:12 Step 4: the first copy 4:50 Step 5: a much harder test 5:32 Step 6: Ghidra and REA get their turn 6:43 The verdict 7:40 What this really means

Links Test kit from this video (the hidden test, the fake AI server and the AI's notes): https://github.com/softsweb/codex-clone-hidden-test-kit REA on GitHub: https://github.com/morluto/rea Codex on GitHub (open source, Apache-2.0): https://github.com/openai/codex

Notes and honesty

• Codex is open source (Apache-2.0), that is why I picked it. I did not do this to Cursor or Claude Code because their terms forbid it.
• I only copied Codex's non-interactive mode (codex exec). The copy has no screen and no model, it is only the engine, and I used a fake AI server so the test cost nothing.
• Not tested: the real sandbox (it does not start on my computer), the interactive screen, resume and review commands, and a real model.
• The full Ghidra analysis of the 294 MB program never finished (every call timed out after 330 seconds), so I show exactly which parts worked: a carved 15 MB piece of the file that REA could open.
• Remaining misses (6 checks of 422): the real program rejects a tiny image that the copy accepts, the copy does not count tokens used by a goal, it lacks the codex-path folder in PATH, and it does not print the applied patch.
• Only analyze software you are allowed to test. I am not a lawyer and this is not legal advice.
• This video is not sponsored.

#REA #Codex #ClaudeCode #ReverseEngineering #OpenSource