6.4.0-alpha Firmware for Prusa MK3.5

Summary

• New features
  • Belt tuning wizard (C1)
  • Faster and more responsive Prusa Connect experience
  • Auto-retract (MK4, alpha for XL)
  • Selective factory reset
  • Chamber support for backwards G-Code compatibility (C1)
• Changes and improvements
  • Improved print status messages
  • Lazier homing, more cautious calibration
  • Autoload improvements
  • Unified XL & CORE One chamber filtration
  • Option to continue after nozzle cleaning failure
  • Print sheet detection (MK4)
  • Respect maximum printed Z position
  • Gearbox alignment calibration (XL)
  • Filament change from Tune menu in any tool (XL)
  • Support cancelling more objects
  • Wi-Fi signal strength indicator
  • BSOD knob press behavior
  • Reworked filament sensor evaluation
  • Ukrainian language support for all printers
• Fixes
  • Purge option on nozzle cleaning failure
  • Disable print fan during filament change
  • Various layer shift fixes
  • All the fixes from the 6.3.X releases have been expanded to other printers.
  • Bunch of more fixes.

This is the alpha release for the upcoming firmware 6.4.0 for the Prusa CORE One, MK4S, MK4, MK3.9S, MK3.9, MK3.5S, MK3.5, MINI & XL. It reunifies the firmware development branch after the temporary split during the CORE One launch, where 6.3.x was released exclusively for CORE One and other printers remained on 6.2.x – along with bringing new features, improvements, and fixes across all platforms. While marked as an alpha, this release maintains the stability and security of existing printer features.

Belt tuning wizard (C1)

A new guided wizard has been added to the Prusa CORE One to help users properly tension the X/Y belts. The printer uses controlled belt excitation combined with stroboscopic feedback to visualize belt resonance, making the tuning process more intuitive and reliable.

The wizard, accessible via Settings → Manual Belt Tuning, begins by prompting the user to check that the gantry is properly aligned, as misalignment can affect measurement accuracy. A quick manual alignment can be done by loosening both tensioner bolts, gently straightening the gantry by hand, and re-tightening the bolts evenly while keeping the gantry square. More information about gantry alignment can be found in this article.

The wizard then measures the vibration frequency of both belts and recommends whether to tighten or loosen them. The target frequencies are 96 Hz for the upper belt and 92 Hz for the lower belt.

NOTE: A difference of up to 8 Hz between the belts is acceptable, as long as both values remain within the 90–98 Hz range. If values cannot be reached, the wizard will suggest manual squaring before continuing.
If your gantry is out of alignment, you may not be able to reach these exact values. In some cases, the lower belt can even show a slightly higher frequency than the upper belt.

NOTE2: Since the in-depth article on using the Belt Tune Wizard is still in preparation and will be published together with the stable or RC release of this firmware, all QR codes currently point to these release notes.

Faster and more responsive Prusa Connect experience

Communication between the printer and Prusa Connect is now handled using WebSockets, replacing the previous polling-based method.
While this change does not affect raw data transfer speed, it significantly improves overall responsiveness and interaction with the cloud service. Users can expect (via Prusa Connect):

• Lower latency when starting prints
• Faster execution of remote commands
• Smoother and more fluid control during cloud-based workflows

This update also lays the groundwork for future enhancements to remote monitoring and control by the user, allowing for continued improvements to the connected experience in future releases.

Auto-retract (MK4, alpha on XL)

The auto-retract feature is now enabled on MK4 and XL printers, after its first introduction on CORE One in firmware 6.3.X. Filament is automatically retracted from the nozzle at the end of each print, which helps reduce oozing during bed leveling and removes the need to preheat the nozzle when unloading.

On the XL, the feature is currently available only as an alpha preview - at this stage, the printer retracts only the active tool after a print. We are working on further improvements, with the final version planned for the 6.4 release.

NOTE: Auto-retraction is not applied to flexible filaments; to handle these, we have added a new “Is flexible” parameter, configurable in filament presets via the printer’s GUI or using the M865 G-code.

Selective factory reset

We have introduced more granular control over the factory reset process. Instead of wiping all data at once, settings are now divided into categories, allowing you to decide what to reset and what to keep. These categories include:

• Calibrations: All the calibrations data and selftest results
• Common misconfigurations: Quirky settings that might cause unexpected behavior if you don’t know about them, for example disabling filament sensors, changing steps per mm
• Network: Wi-Fi, Ethernet, Prusa Connect, PrusaLink related items
• User interface: Items that affect user interaction with the printer – language, sound settings, footer settings, touchscreen, LEDs, and so on
• Stats: Print & error statistics
• HW configuration: Items that are linked to the hardware installed on the printer: nozzle diameters, extruder type, (also) steps per mm, printer type,...
• Printer functions: Configuration of various print functions, such as MMU, chamber filtration, HW checks,...
• User profiles: Custom filaments, print sheets (MINI and MK3.5)

To simplify the process, we have also prepared several reset presets for the most common scenarios:

• Ownership transfer: Useful when you’re giving the printer to someone else. Will reset all your personal settings such like network and filament profiles, but keep things like hardware configuration, calibrations and statistics.
• Fix common misconfigurations: This is what the support team will likely advise you to do during troubleshooting. Intended to fix things like filament sensor not working because it was accidentally turned off from the menu.
• Keep HW config: Keeps only the hardware configuration (what nozzles, fans, extruders the printer has), nothing else. This reset is performed before shipping assembled units to the customers.
• Full reset: Wipes everything, except for the firmware itself
• Hard reset: Wipes everything, including the firmware. A USB drive with a new firmware will be needed.

If none of the presets fits your needs, you can start with the closest one and then manually adjust each category to reset or keep it according to your preference.

Chamber support for backwards G-Code compatibility (C1)

When printing on the CORE One in compatibility mode using files sliced for the MK4 or other supported printers, the firmware now automatically handles chamber temperature control. Since these G-code files do not contain the required commands for chamber heating or cooling, the printer applies chamber management heuristically. Previously, no chamber control was performed in this mode.

Improved print status messages

Status messages have been revised to provide clearer feedback during printer operations. They now give more detail about what is happening, show progress where applicable, and remain visible until the operation is completed instead of disappearing after just a few seconds.
NOTE: They also do not disappear when the print is paused, which addresses the issue #3143

Lazier homing, more cautious calibration

Homing behavior on the XL and CORE One has been refined to avoid unnecessary movements. In cases such as filament load or unload, the printer now performs only basic homing, skipping the extra diagonal movements. If a precise position is required later, only the refinement step is executed. For unload operations with auto-retraction, homing is still performed to ensure the correct filament removal angle.

This change applies only to specific situations—G-code G28 without parameters, which is placed at the start of every print, will still trigger a full homing sequence. We are also considering updating slicer profiles in the future to allow optional homing.

In addition, the long and noisy precise homing calibration is no longer run automatically. Instead, the printer now checks reliability and suggests the calibration only when needed, and you can always choose to skip it. If required, the precise homing calibration can be started manually at any time from the Calibrations menu.

Autoload improvements

The autoload process has been refined to make filament changes faster and easier. Printers now begin preheating the nozzle to 170 °C as soon as the load procedure starts, without waiting for the user to confirm the filament type. On printers equipped with a side filament sensor (CORE One, XL), the procedure also starts automatically when filament is inserted into the sensor. In this case, the toolhead moves to an optimal position for feeding filament through the PTFE tube, and the extruder begins spinning to assist insertion.

NOTE: On the XL, the relevant tool still needs to be selected before autoload is triggered.

Additionally, the printer now checks whether the extruder filament sensor is already active when filament is inserted into the side sensor, warning the user in case debris or leftover filament is present in the printhead.

Unified XL & CORE One chamber filtration

We have unified the code for chamber filtration between the XL and CORE One, so both printers now share the same features. As a result, the CORE One gains all functionality previously available on the XL, and vice versa.

In addition, several new options have been introduced. You can enable Filter all materials to keep filtration running regardless of filament type, and the Filter usage counter can now be adjusted manually for your own convenience.

We have also added two new G-codes: M147 (set) and M148 (unset), which let you explicitly mark whether a print requires filtration or not, overriding the automatic behavior based on filament parameters. (#4480)

Option to continue after nozzle cleaning failure

If nozzle cleaning fails, the printer now offers an Ignore option, allowing you to continue with the print instead of being forced to abort the job.

Print sheet detection (MK4)

The print sheet detection feature, originally introduced with the CORE One, is now available on MK4 printers. Before starting a print, the printer checks whether a sheet is installed and notifies the user if it is missing.

Respect maximum printed Z position

The firmware now tracks the highest extruding Z position reached during a print and ensures that toolchanges, filament changes, and parking moves are performed above this level. This prevents the toolhead from colliding with already finished parts during sequential printing (complete individual objects).

Gearbox alignment calibration

Gearbox alignment procedure is now available for the XL. We recommend running the procedure in the case that you needed to disassemble and reassemble the extruder.

Filament change from Tune menu in any tool (XL)

You can now change filament in any tool from the Tune menu, not just the currently picked one.

Support cancelling more objects

Formerly, our printers were only able to control cancelling of the first 64 objects in the print file. This limit has now been increased to 1024.
NOTE: In the printer UI, you will still only see the names of the first 16 objects. The rest will be displayed as “Object X”.

Wi-Fi signal strength indicator

The Wi-Fi icon in the header now displays signal strength, with individual bars lighting up according to the connection quality.

BSOD knob press behavior

Pressing the knob during a BSOD no longer restarts the printer - it now only silences the beeper. To restart the printer, use the dedicated Restart button.

Reworked filament sensor evaluation

The evaluation method for the Nextruder filament sensor has been redesigned. This update improves reliability by reducing false runout detections and ensuring that actual runouts are correctly recognized.

Ukrainian language support for all printers

Ukrainian language support, first introduced on the CORE One in firmware 6.3.3, is now available across all printers.

Purge option on nozzle cleaning failure

When nozzle cleaning fails, the Retry button now includes a Purge option. In this mode, the nozzle is heated to extrusion temperature, a small amount of filament is purged, and then an auto-retract is performed to prevent oozing during cleaning and bed leveling. The user is prompted to remove the purged filament, after which the nozzle cools back down to its cleaning temperature (typically 170 °C).

Various layer shift fixes

Multiple bugs related to layer shifts have been addressed. While some edge cases may still exist, layer shifts should now occur less frequently and affect fewer models.

Disable print fan during filament change

Release notes

https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.4.0-alpha