MCU Protocol error caused by running an older version of the firmware


As explained in Klipper Architecture / Ecosystem, the “Klipper core” actually consists of 3 different components:

Component Function Typical Way to Update
Host Process Running on the SBC. Core management functions Update Manager of the web interfaces
MCU Firmware running on the printer board make menuconfig followed by make flash ...
Host MCU MCU-like functions on the SBC make menuconfig followed by make flash ...

As shown in the table above, only the Host Process can be easily updated from the web interfaces, but the other critical components require manual intervention.

Depending on the changes or updates to the Host Process, it may be necessary to update the other components as well.
Since this is rarely necessary, Klipper will notify this with an error message. In the klippy.log file and in the web interfaces, the error will look similar to the following:

Klipper reports: ERROR

MCU Protocol error

This is frequently caused by running an older version of the  
firmware on the MCU(s). Fix by recompiling and flashing the  

Your Klipper version is: v0.11.0-271-g5f990f93 
MCU(s) which should be updated:  
mcu: Current version v0.10.0-478-g78454dd3  
rpi: Current version v0.10.0-364-g4d4c8d8f  
Up-to-date MCU(s):
  • The line Your Klipper version is shows the version of the Host Process
  • The mcu line(s) lists all printer-boards and their current versions that need to be updated. Since Klipper supports multiple concurrent printer-boards, e.g. a main MCU and various additional (CAN) MCUs, there might be multiple entries.
  • rpi corresponds to the Host MCU running on the SBC
  • Up-to-date MCU(s) would list these that do not require an update


One or multiple MCUs or the SBC’s Host MCU are outdated compared to the currently running Host Process and lack support for some of its features.

Potential solutions