New in Mender: Zephyr 4.4 support and reliability improvements

Three new releases are now available: Mender MCU Client 1.1.0, Mender Client 5.0.6, and Mender Client 6.0.1.

Highlights | The MCU Client now supports Zephyr 4.4, delivering a set of features that make over-the-air (OTA) software updates on constrained devices more robust. The two Linux client patch releases focus on reliability — addressing issues reported from the field.

Note: Mender Client 5.0.x is end-of-life (EOL) as of September 2026. Upgrading to a supported version soon is highly recommended as 5.0.6 is likely the last release in this series.

Mender MCU Client 1.1.0: OS support and optimizations

Zephyr 4.4 support

The Mender MCU Client now builds and runs on Zephyr 4.4, the latest stable Zephyr release. Teams tracking upstream don't have to pin an older version of the client — or an older version of Zephyr — to keep their update path working!

Simultaneously, the Mender MCU Client adds support for Mbed TLS 4.x. Following the significant API breakage in Mbed TLS 4.0, Zephyr's move to the newer series allows projects that add Mbed TLS to do so without losing the TLS-backed authentication of Mender. Builds on Mbed TLS 3.x continue to work, and the dependency was also bumped to 3.6.6.

Chunked artifact download

Artifacts can now be downloaded in smaller chunks, reducing network buffer sizes. On microcontrollers, where RAM is measured in tens or hundreds of kilobytes, chunked artifacts can meaningfully reduce the memory footprint of a download since the chunk size is completely configurable.

Note: Smaller chunks require longer download times; however, this is typically not a major concern.

Fallback server URL

A secondary Mender server URL can now be configured. If the primary server is unreachable, the client falls back to the secondary one. When migrating between Mender Servers or for fleets that need a second route home, a secondary Mender service ensures a device is never stranded without a way to receive updates.

Note: Only one Tenant Token can currently be specified; both Servers must accept the same token or ignore it. Specifying a Tenant Token per Server will be added in a future release.

Important bug fixes

• A crash in the Zephyr inventory code when IPv4 information was unavailable.
• A use-after-free in the clears_provides loop in mender_filter_provides.
• A null check that dereferenced the deployment_data double pointer incorrectly.

Mender Client 5.0.6 and 6.0.1: Enhancing reliability

Mender Client 5.0.6 and 6.0.1 are patch releases. No new configuration is required, and no behavior changes are expected beyond the fixes themselves. Both releases are safe upgrades within their respective series.

Both releases enhance client reliability and address cases where a device could become unmanageable until it is restarted:

• The client no longer hangs indefinitely when a server accepts a request but never sends a response. Stalled transactions now time out, without breaking mender-connect's WebSocket at the protocol switch.
• Deployments no longer hang when a network outage during artifact download lasts long enough for the download resumer to give up. The failure is now reported, and the client returns to its polling loop.
• mender-connect no longer deadlocks its entire daemon when a pong arrives while a timed-out remote terminal session is being torn down.

Deployment correctness and reporting also improved in both new releases:

• The directory and single-file Update Modules no longer report a deployment as successful when the destination could not be backed up. Previously, the install was recorded as a new software version while the destination was left untouched.
• Large deployment logs are trimmed (the end is retained). The server now accepts large deployment logs, rather than consuming bandwidth on an upload that is rejected or leaving you with no logs at all for a failed deployment.
• Deployment log uploads are no longer rejected when the deployment keeps writing to the log while the upload is in flight.
• The artifact generators verify that the file exists before reporting success and clean up their temporary files when they exit early due to an error.

A segfault caused by the initialization order of global variables is also addressed — particularly important on Ubuntu 26.04 when building with Link-Time Optimization.

Mender Client 6.0.1 also includes updates to mender-container-modules, including correct image retagging on rollback and starting compositions from their final location so that bind-mount paths resolve correctly.

Share your feedback

Share your feedback about the new Mender updates and suggestions for future improvements in the Mender Hub General Discussions forum. If you encounter any issues with the new release, please report them in the Mender JIRA issue tracker with detailed reproduction steps and system information.