Hey, thanks for the report.
This is the second report of the same bug in the last month (the previous one is here: Linux Sandbox: Error: Failed to apply sandbox: IO error: Step 4/7 (mount denies) failed: Mount count limit reached (1000)), but yours is the most detailed. The root cause is now clearly identified. It is file-glob patterns from ~/.cursorignore that are limited to $HOME in ignoreMapping and end up creating bind mounts for individual files instead of tmpfs mounts for directories.
I’ve passed this to the team. I don’t have an ETA yet, but having such a clear reproduction case really helps us prioritize it.
Your workaround with CURSOR_SANDBOX_MOUNT_CAP=30000 is the best option available right now. The extra ~3 s per sandboxed command is annoying, but expected, since it still has to walk through $HOME.
We’ll update this thread as soon as there’s progress on a fix.