Flasher Stub

esptool.py is a serial flasher utility. It communicates with the ROM bootloader in Espressif SoCs in order to load user applications or read chip data via serial port.

The ROM bootloader is burned into the ESP chip during manufacturing and cannot be updated. A new version is issued only when a new chip revision is released.

esptool.py works around the limitations imposed by a fixed ROM bootloader by implementing a flasher stub (also known as “stub loader” or just “stub”). It is a small application used as a temporary substitute or extension for the ROM.

When esptool.py connects to a chip, it first uploads the flasher stub, which basically replaces the original bootloader. All following operations are then handled by the stub.


The flasher stub behaves the same as the original bootloader, but uses more heavily optimized UART routines.

The main benefit is improved performance of flashing and some other operations (like reading flash). Additionally, there are a few commands which are only available when using the stub loader (such as erase_flash and erase_region). It also allows to work around any bugs in ROM bootloaders.

Disabling the Stub Loader

There might be cases where it is necessary to disable the stub loader (e.g. debugging). To do that, run esptool.py with the --no-stub argument. All operations will then be handled by the original ROM bootloader. See the related advanced options page.