Research note
What duplicate file checks miss inside Codex session files
We looked at 1,467 Codex session files on a Mac we use every day. A whole-file duplicate check found nothing, because no two files were identical. Inside the files it was a different picture, and 76.9% of the decoded image bytes were copies of an image that had already appeared.
What we measured
In September 2026 we read every Codex session file on one actively used Mac, 1,467 files and 16.509 GB in total. We only read them. Nothing was rewritten, moved or deleted during the study.
Codex keeps screenshots and other images inside the session file as encoded text. We pulled out every image payload we could recognize, decoded it, and took a SHA-256 hash of the image bytes. Two occurrences with the same hash are the same image, whichever file they sit in.
What we found
The files held 62,014 image occurrences but only 12,104 distinct hashes. Decoded, the images came to 5.987 GB. Keeping the first occurrence of each image and counting everything after it as a copy, the copies added up to 4.605 GB, which is 76.9% of the decoded image data.
A duplicate file finder compares whole files, so on this Mac it would have reported no duplicates at all. The repetition lives inside the conversations. We did not try to work out why each copy was written, which would be a separate study.
Limits of this study
This is one Mac and the way it happens to be used, so the share on another machine could be much higher or much lower. Payloads we could not recognize as images were left out of the image count. It is a study of file contents and says nothing about how accurate Mint’s cleanup is.
The 76.9% is measured against decoded image bytes only. Session files also hold text, and disk usage is counted differently again, so please don’t read the figure as an estimate of space you could get back.
What this means for cleaning up Codex
Finding the same image twice does not make it safe to rewrite a conversation. Another conversation may be built on top of this one, you may still want the screenshots, and Codex only supports certain changes to its own files. Those questions come before any hash.
In Mint, your Codex conversations have their own group on the Disk tab. Optimize swaps the screenshots in a conversation for small previews, turns audio and video into placeholders and leaves the text as it was, while Delete removes the whole conversation through Codex. Both are permanent. Optimize skips any conversation that another conversation is built on.
Frequently asked questions
Does 76.9% mean I can free 76.9% of my Codex storage?
No. The figure is a share of the decoded image bytes in this one sample. Session files also contain text, and the study made no estimate of recoverable space.
Why didn’t a duplicate file finder catch this?
Duplicate finders compare whole files, and no two of the 1,467 session files were identical. The repeated images sit inside different files.
Does this study measure how accurate Mint’s cleanup is?
No. It is a content study on a single Mac. Measuring accuracy would take labeled samples from many machines and app versions.
See what Codex keeps on your Mac
Start a Mint trial and open the Disk tab to see what your AI tools have stored before you decide what to change.