Shelly firmware update fails or is stuck at 99% (Gen3/Gen4): what helps and what doesn't

What you see: In the Shelly app or web interface the firmware update aborts, shows an error, never starts (the automatic midnight update never completes) or the progress stays at 99% or "in progress". In the forums this was reported after versions 1.7 and 2.0 for, among others, Plug S Gen3, Plug M Gen3, Pro 3EM, Pro 1 and 2PM Gen4.

What it means: There is no single cause. Official and semi-official sources mainly name a weak Wi-Fi signal, too many simultaneous updates, and memory and download problems. For the hang at 99% on Pro 3EM and Plug S Gen3 there is a report in the Shelly Community but no confirmed fix (as of this page).

Cost: €0 / CHF 0. Everything here is settings and reboots.

Step 1: Reboot the device and start the update again 5 min

1. Wait until it's clear the update is no longer running. Never cut power during a running update.
2. Reboot the device from the app or web interface (Settings › Reboot), not by unplugging it.
3. Start the update again.

Rebooting and retrying is the first step named by both shelly.guide and an administrator of the Shelly Community.

Step 2: Update from the local web interface instead of the app 5 min

1. Look up the device's IP address in the app (device information) or in your router.
2. Open http://<IP address> in a browser on the same network as the device.
3. Start the update under Settings › Firmware and keep the page open until the device has restarted.

According to shelly.guide, updating via the web interface fixes most cases. Plug M Gen3 owners in the German-language Shelly forum also report that the update only worked for them via the browser.

Step 3: Check the Wi-Fi signal and briefly turn off Eco mode 10 min

• Read the signal strength in the web interface (Wi-Fi status). shelly.guide names a signal weaker than −70 dBm as a likely cause. Remedies: move the router or access point closer, reposition the device, or retry once the network is improved (see Shelly offline and Wi-Fi problems).
• Turn Eco mode (Settings) off for the update. A Shelly Community administrator names disabling Eco mode, alongside rebooting, as a remedy for failing updates. You can turn it back on afterwards.

Step 4: Update devices one at a time depends on how many

Don't start all devices at once ("Update all"); update them one after another. shelly.guide explicitly recommends this.

Step 5: Don't run the automatic update at midnight 2 min

If the automatic update never completes but a manual one works:

• In an owner's blog post, the automatic midnight update did not complete; after moving the update time to after 04:00 it worked. The German-language Shelly forum also advises moving the update time away from midnight.
• Background per the official changelog: from 2.0.0, schedule-triggered downloads are randomly delayed ("Jitter schedule-triggered downloads").

This is an owner report, not official guidance, but it's risk-free to try.

Step 6: Home Assistant: check the Bluetooth proxy script 5 min

If you use the device as a Bluetooth proxy for Home Assistant, the blog post mentioned above says the installed script (aioshelly_ble_integration) can use so much memory that the update stalls at a few percent. The post describes the update completing after the script was stopped. Restart the script afterwards.

Step 7: Write down the error message 2 min

Note the exact message from the web interface or diagnostic log. The sources mention, among others:

• "Checksum does not match after writing partition-table.bin" (Pro 1, update from 1.3.3 to 2.0.0, Shelly Community)
• "DATA_LOSS: ZIP flush error : premature end of data" (blog post)
• "Bad HTTP response: 404" during download (Plug M Gen3 with factory firmware, Shelly Community)

The sources give no official explanation for these messages, but they are useful for support.

Note for older devices: according to the official changelog, 1.3.3 was a mandatory update before 1.4.0; devices on older versions were not offered 1.4.0 directly.

What doesn't help

• Factory reset: according to shelly.guide, not a fix for update problems.
• Third-party firmware files via URL: according to shelly.guide, /ota?url= is meant for Gen1. For Gen2 and newer, contact support; a wrong file can brick the device.
• Plug M Gen3 owners with factory firmware report in the Shelly Community that they could update the device with a beta firmware via "Update from URL" (run twice); afterwards AP and range extender were off. This is an owner report with risk and not a recommendation of this page.

When to contact support

• The update still fails after steps 1–5 via the web interface and with a good signal.
• Progress stays at 99% and rebooting changes nothing.
• The device no longer boots or is unreachable after an update.

Give device type, device ID, current firmware version and the exact error message: Shelly support (official).

Sources

• Shelly Gen2+ API documentation: changelog – current versions, gradual rollout of 2.0.0, jitter for scheduled downloads, 1.3.3 mandatory before 1.4.0, "not all devices receive the same updates"
• shelly.guide: Update problems – reboot, update via web interface, −70 dBm, never cut power, update one at a time, factory reset doesn't help, /ota only for Gen1 (guide project by Shelly staff)
• Shelly Community: Failing firmware updates since 1.7.0 through most recent 2.0.1 – administrator reply: reboot or disable Eco mode and retry
• Shelly Community: OTA update to 2.0.0 (and 2.0.1-beta2) hangs forever at 99% on two different Gen2+ devices – "in_progress 99%", device stays reachable, reboot doesn't help, no fix
• Shelly Community: Firmware update 1.3.3 to 2.0.0 fails with a Shelly Pro 1 – "Checksum does not match after writing partition-table.bin"
• Shelly Community: Firmware update setting – gradual rollout explained
• Shelly Community: Update M Plug to v 2.0.0 fails and Shelly Plug M Gen3 cannot update from the factory firmware – 404 during download, risky beta workaround
• Shelly Forum (German): Update- und Funktionsprobleme – Plug M Gen3 only updatable via browser
• Shelly Forum (German): Automatisches Firmware-Update geht nicht – move update time away from midnight
• Blog post (German): Shelly 2.0.0 auto-update and web GUI don't reach 2.0.1 – update after 04:00, "DATA_LOSS: ZIP flush error", Home Assistant BLE script uses memory
• Reddit r/ShellyUSA: 2.0.0 update failed for 2PM Gen4 – update aborts at about 75%