The Codex usage allowance was back. Time to open a fresh thread and get something done.
Except the new thread refused to start.
failed to open thread writer lock
No space left on device (os error 28)
The AI had plenty of room in its usage budget. The Mac had almost no room on its SSD. A powerful coding assistant was stopped by a file too small to be worth worrying about. Brilliant brain, no desk.
The error was about the Mac, not the model
No space left on device is an operating-system error indicating that the destination cannot accept the write because space is unavailable. Error 28 is commonly called ENOSPC.
A desktop AI tool may send demanding work to servers, but the app still needs local files for session state and coordination. A writer lock is a small sign that helps prevent two processes from writing over the same information.
You can have enough cloud allowance to generate thousands of lines of code and still lack enough disk space to create that sign. The failure happened before the interesting work even began.
Two budgets, one very awkward day
A Codex usage reset restores a particular usage allowance. It does not clear your Downloads folder, expand your Mac's SSD, or fix every local error.
Think of getting a ticket to a concert only to find the venue entrance blocked by moving boxes. The ticket is valid. The doorway is still blocked.
In the incident described here, clearing unneeded files freed disk space and Codex started working again. That sequence strongly supports storage exhaustion as the immediate cause in this case—not a failed usage reset.
Check storage before you buy anything
On a current Mac, open System Settings → General → Storage to see what is using the drive. Apple suggests reviewing unneeded downloads, apps, backups and files in Trash, or moving suitable large files elsewhere.
Start with material you can identify and safely remove, such as outdated installers or genuine duplicates. Check your backups before touching irreplaceable documents or photos.
Developers may also have large build outputs and duplicate project folders. But a mysterious folder full of credentials or app state is not free real estate. Decluttering is not demolition.
An external SSD may be worthwhile if essential files genuinely exceed the space you have. If a cleanup creates enough headroom, you can postpone the purchase. Either way, leave spare capacity for temporary files and everyday work.
If you have ample free space and still see the same error, investigate the specific filesystem, write location, quotas or another local issue instead of deleting files at random.
The wonderfully boring fix
This wasn't a story about AI becoming less intelligent. The cloud budget was refreshed; the local disk was full. Making space restored the app.
Codex: “Ready to think.”
Mac: “Nowhere to write.”
Human: “Before we invent the future, can we clear the desk?”
Sometimes the smartest AI troubleshooting tool is the empty space you made five minutes ago.
