• New features

  • Automatic chamber vent opening/closing (C1/+)
  • Belt tuning wizard (C1/+)
  • Faster and more responsive Prusa Connect experience
  • Nozzle cleaning failed wizard
  • Auto-retract (MK4 family, XL)
  • Auto-retract improvements (C1/+)
  • Auto-retract disable option (C1/+)
  • 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 family)
  • 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

  • All the fixes from the 6.3.X releases have been expanded to other printers.
  • Various fixes and stability improvements

This is the stable release of firmware 6.4.0 for the CORE One+, CORE One, MK4S, MK4, MK3.9S, MK3.9, MK3.5S, MK3.5, MINI, and 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, quality-of-life improvements, and fixes across all platforms.

Automatic chamber vent opening/closing (C1/+)

The automatic chamber vent mechanism introduced with the CORE One L is now also available on the CORE One. After updating, the feature can be enabled via Settings → Hardware → Chamber Vent Control by selecting the Auto option.

A Manual mode is also available, which keeps the vents closed by default and only shows an informational reminder before each print.

Vents can be controlled manually using the new M870 G-code ("M870 O" = open, "M870 C" = close), although we do not recommend including these commands in slicer profiles, as the printer handles vent control automatically.

The printed parts required to retrofit this feature on existing CORE One printers will be published on Printables in the coming days. A link will be added here once available.

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.
Important: Always tighten both tensioner bolts evenly and alternately. This ensures that the gantry stays square during adjustment.

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.

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.

Nozzle cleaning failed wizard

The recovery process for failed nozzle cleaning has been reworked into a new step-by-step wizard.

The wizard also includes a filament retraction step to prevent oozing before retrying the automatic cleaning sequence, increasing the chance of successful recovery.

Auto-retract (MK4 family, 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.

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.

Auto-retract improvements

The auto-retract system has been improved across the board, with particularly noticeable benefits for multi-tool prints on the XL. It now respects slicer-issued retractions and avoids adding redundant movements.

On the MK4, the print head now parks at a higher Z-height after print completion to make it easier to remove any ooze left by the auto-retract ramming process.

Auto-retract disable option

Auto-retract can now be disabled globally in the Settings menu, although this is not recommended for standard use.

It is also possible to disable auto-retract per filament type. In the filament detail menu, the former "Flexible" option has been renamed to "Auto Retract", with its behavior adjusted accordingly.

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 family)

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.

Release notes

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

• New features

  • Automatic chamber vent opening/closing
  • Auto-retract improvements
  • Auto-retract disable option

• Fixes

  • Fixed XL toolcrashes
  • Fixed nozzle heating for unload when auto-retracted
  • Fixed XL enclosure not reporting temperature (#4800, #4829)
  • Fixed sensor info showing only zeroes (#4808)
  • Fixed stack overflow when downloading from Prusa Connect and opening Prusa Link at the same time
  • Fixed getting stuck in tool mapping after homing calibration gets recommended (#4799)
  • Fixed BSOD on M501 (#4912)
  • Fixed absorbing heat status showing wrong value (#4754)
  • Fixed BSOD when opening the door during nozzle cleaning

• Known issues 

This is the second release candidate 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.

For a complete overview of features introduced in firmware 6.4.0, users can refer to the 6.4.0-alpha release notes and 6.4.0-RC release notes published earlier.

Automatic chamber vent opening/closing

The automatic chamber vent mechanism introduced with the CORE One L is now also available on the CORE One. After updating, the feature can be enabled via Settings → Hardware → Chamber Vent Control by selecting the Auto option.

A Manual mode is also available, which keeps the vents closed by default and only shows an informational reminder before each print.

Vents can be controlled manually using the new M870 G-code ("M870 O" = open, "M870 C" = close), although we do not recommend including these commands in slicer profiles, as the printer handles vent control automatically.

The printed parts required to retrofit this feature on existing CORE One printers will be published on Printables in the coming days. A link will be added here once available.

Auto-retract improvements

The auto-retract system has been improved across the board, with particularly noticeable benefits for multi-tool prints on the XL. It now respects slicer-issued retractions and avoids adding redundant movements.

On the MK4, the print head now parks at a higher Z-height after print completion to make it easier to remove any ooze left by the auto-retract ramming process.

Auto-retract disable option

Auto-retract can now be disabled globally in the Settings menu, although this is not recommended for standard use.

It is also possible to disable auto-retract per filament type. In the filament detail menu, the former "Flexible" option has been renamed to "Auto Retract", with its behavior adjusted accordingly.

Fixed XL toolcrashes

Toolcrashes on multi-tool XL printers introduced in the alpha firmware have been fixed. The 6.4.0 firmware should now be fully functional on the XL printers.

Fixed nozzle heating for unload when auto-retracted

Printers no longer heat the nozzle for unload if the filament is auto-retracted.

Known issues

Prusa XL – Tool recovery: In some cases, triggering a tool recovery causes the print to stop responding. The nozzle and bed remain heated, but the print does not resume and cannot be recovered.

MK4S, MK4, MK3.9S, MK3.9 – The filament sensor only reacts to the first runout event during a print. Subsequent runouts are not detected until the print is restarted.

Both issues will be addressed in final release version of 6.4.0.

Release notes:

https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.4.0-RC2

• New features
  • Nozzle cleaning failed wizard
• Fixes
  • Fix #4816

This is the release candidate for the upcoming firmware 6.4.0 for the Prusa CORE One, MK4S, MK4, MK3.9S and MK3.9. 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.

Builds for XL, MINI, MK3.5S and MK3.5 are temporarily not included due to identified issues currently under investigation.

For a complete overview of features introduced in firmware 6.4.0, users can refer to the 6.4.0-alpha release notes published earlier.

Nozzle cleaning failed wizard

The recovery process for failed nozzle cleaning has been reworked into a new step-by-step wizard. It replaces the previous purge option introduced in 6.4.0-alpha.

The wizard also includes a filament retraction step to prevent oozing before retrying the automatic cleaning sequence, increasing the chance of successful recovery.

Release notes: 

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

• 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

• Display refresh rate fallback to improve stability
• Reduced sensitivity to bed heating fluctuations on XL
• Fixes for rare BSOD crashes during filament handling

This is the stable release of firmware 6.2.6 for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL, bringing targeted stability improvements and fixes.

The improvements to Prusa Connect communication introduced in the 6.2.6-alpha build (using WebSockets) are not included in this stable release. Based on data collected during the alpha phase, we are continuing to evaluate their potential inclusion in a future firmware update.

Automatic display refresh rate fallback

If repeated display anomalies (such as flickering) are detected, the printer now shows a prompt offering to switch to a lower display refresh rate. To ensure the message remains readable even on unstable screens, the firmware temporarily applies the lower refresh rate before showing the prompt. If the user chooses "No", the original (higher) rate is restored.

The refresh rate can also be adjusted manually at any time via Settings → Hardware → Display Refresh Speed.

Reduced modular bed temperature peak sensitivity (XL only)

On the Original Prusa XL, we've reduced the sensitivity to short-term temperature spikes on the modular bed. This should significantly reduce the occurrence of Unexpected temperature peak errors caused by nozzle heat affecting individual bed segments.

Release notes

https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.2.6

• Display refresh rate fallback to improve stability
• Reduced sensitivity to bed heating fluctuations on XL
• Fixes for rare BSOD crashes during filament handling

This is the release candidate for the upcoming firmware 6.2.6 for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL, bringing targeted stability improvements and fixes.

While the 6.2.6-alpha build included early improvements to Prusa Connect communication using websockets, these changes are not part of this release candidate. We are currently evaluating the data gathered during the alpha phase and may consider introducing the updated communication model in a future firmware release.

Automatic display refresh rate fallback

If repeated display anomalies (such as flickering) are detected, the printer now shows a prompt offering to switch to a lower display refresh rate. To ensure the message remains readable even on unstable screens, the firmware temporarily applies the lower refresh rate before showing the prompt. If the user chooses "No", the original (higher) rate is restored.

The refresh rate can also be adjusted manually at any time via Settings → Hardware → Display Refresh Speed.

Reduced modular bed temperature peak sensitivity (XL only)

On the Original Prusa XL, we've reduced the sensitivity to short-term temperature spikes on the modular bed. This should significantly reduce the occurrence of Unexpected temperature peak errors caused by nozzle heat affecting individual bed segments.
GitHub reference → #3484

Fixes for rare blue screen of death (BSOD) crashes

We have resolved several edge cases where the printer could crash into a blue error screen, most notably during filament selection and preheat operations.

Release notes: https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.2.6-RC

• Faster and more responsive communication with Prusa Connect

This is the alpha release for the upcoming firmware 6.2.6 for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL, introducing a major backend upgrade for printers connected to Prusa Connect. While marked as an alpha, this release maintains the stability and security of existing features of the printer.

Note: The MINI firmware build with Japanese language support is not included in this alpha release.

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.

Release notes

https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.2.6-alpha

• Reduced USB errors
• CORE One printer detection
• Fixed returning to wrong Z position after pause
• New print fan type
• Improved extruder error detection

This is the stable release of firmware 6.2.4 for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL.

Reduced USB errors

We fixed a rare condition that could occasionally trigger a “USB Error” pause during printing.

CORE One printer detection

The firmware brings a new bootloader version 2.4.1, which is able to recognize MK4S from CORE One (based on whether the door sensor is connected).

NOTE: Before you convert your MK4S to CORE One you need to update the FW to 6.2.4. If xBuddy is already connected in CORE One and the door sensor is attached, the bootloader refuses to update. We will remind you of this in the MK4S to CORE One Conversion Guide.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.
This was already fixed in 6.2.3, but 6.2.3 was only released for MK4 and MK3.5. 6.2.4 brings this fix to the XL as well.

New print fan type

We have extended our list of suppliers for print fans on the XL. The new fans have slightly different properties. Since the printer cannot automatically detect which fan is installed, we have added a settings option (Settings -> Tools Settings -> Print Type Fan) for selecting the correct fan type the printer is equipped with.

Improved extruder error detection

Extruder errors (extruder not spinning) should no longer trigger for parked tools.

Release notes: https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.2.4

• Reduced USB errors
• CORE One printer detection
• Fixed returning to wrong Z position after pause
• New print fan type
• Improved extruder error detection

This is a public release of the 6.2.4-RC firmware for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL.

This is a release candidate intended for experienced users. We’ve not been translating changed/newly added texts yet. The features in this firmware are still under development and may not perform as expected. Should you encounter any bugs or issues with the printer’s performance, please report them via GitHub or in our forums.

Reduced USB errors

We fixed a rare condition that could occasionally trigger a “USB Error” pause during printing.

CORE One printer detection

The firmware brings a new bootloader version 2.4.1, which is able to recognize MK4S from CORE One (based on whether the door sensor is connected).

NOTE: Before you convert your MK4S to CORE One you need to update the FW to 6.2.4. If xBuddy is already connected in CORE One and the door sensor is attached, the bootloader refuses to update. We will remind you of this in the MK4S to CORE One Conversion Guide.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.
This was already fixed in 6.2.3, but 6.2.3 was only released for MK4 and MK3.5. 6.2.4 brings this fix to the XL as well.

New print fan type

We have extended our list of suppliers for print fans on the XL. The new fans have slightly different properties. Since the printer cannot automatically detect which fan is installed, we have added a settings option (Settings -> Tools Settings -> Print Type Fan) for selecting the correct fan type the printer is equipped with.

Improved extruder error detection

Extruder errors (extruder not spinning) should no longer trigger for parked tools.

• Hotfix for the MMU overcurrent
• Fixed returning to wrong Z position after pause

This is a stable release of firmware version 6.2.3 firmware for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5 & MK3.5S.

Hotfix for the MMU overcurrent

A recent supply change of a few electrical components has caused issues during the MMU power-on procedure due to the new components having different tolerances.

We have adjusted the MMU power-on procedure to accommodate this change. The current solution is a hotfix that introduces a small (~1s) delay when powering on the MMU. A more permanent solution will be released soon.

We would like to thank you, the community, for your efforts regarding this issue. Your observations and discoveries have made it much easier for us to locate the problem and focus more on how we could fix it.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.

Release notes: https://github.com/prusa3d/Prusa-Firmware-Buddy/releases/tag/v6.2.3