D'oh! GPIO45 was pulled up. Removing the pullup made flashing work as expected. Thank you very much for the help - I knew it was something an expert would spot instantly but I was baffled. Adding that pullup was my last design change and I think when checking before fabrication I read GPIO45 as GPIO...

This is all happening when the S2 is booted into download mode.

When not booted into download mode, the device is not detected as a USB device at all, does not appear in Device Manager, does not appear when using dfu-util -l, etc.

I have an ESP32-S2-SOLO module on a custom PCB, with my USB data lines connected directly to the ESP32-S2 D+ and D- pins. I can see the ESP32-S2 in the Device Manager (and its associated COM port) when the chip is booted in download mode, and I have the drivers installed (Windows 11). I can compile ...