ESP8266 Community Forum

The Xtensa core used in the ESP8266 has support in OpenOCD. OpenOCD works with common ARM JTAG units. JTAG is the same everywhere so these 'ARM' JTAGs should work on the ESP8266 too.

JTAG has several advantages. It can use several different schemes to program the flash chip. JTAG can also provide source level debugging and single step. OpenOCD is already integrated with gdb.

As soon as someone get some hardware it would interesting to see what you can do with OpenOCD.

wouldn't wrtiting to the flash require a small application running on the controller transferring the received bytes to the connected flash controller?
aka flash loader?

Mind you: it's not an arm; it's an Xtensa architecture proc. There also seems to be an openocd-variant for that, though.

on the leaked VM there is XT-OCD (C:\usr\xtensa\XtDevTools\downloads\RC-2010.1\tools\xt-ocd-8.0.1-windows-installer.exe).
One can use then usbwiggler from Macraigor, or even LPT Wiggler.

Regards flash, in the leaked VM there is as well XTCOM_UTIL.exe tool. It seems to be kind of flasher over UART i think.
I don't have (yet) any hardware so can't test these things.