0x80070643
Windows
Medium severity
Error 0x80070643 — Fatal Error During Installation
An MSI install failed — recently very common with the Defender/WinRE security update failing to resize the recovery partition.
Last reviewed 2026-08-29 by the Tech Issue Guide team.
Symptoms
- “Security Update for Windows (KB50xxxxx)” repeatedly fails with 0x80070643.
- A .NET Framework update or repair ends with the same code.
- The update reappears after every reboot.
What causes 0x80070643
- The Windows Recovery Environment (WinRE) partition is too small for the update and has no room to grow.
- A corrupt .NET Framework installation.
- A pending reboot or a stuck installer service.
How to fix it — 3 steps
- For the WinRE update: disable ReAgentC, shrink the OS partition by ~250 MB, extend the recovery partition to about 750 MB, then re-enable ReAgentC (command below is the check/enable part; use Disk Management or diskpart for the resize).
- For .NET failures: download the offline .NET Framework Repair Tool and the latest runtime installer from Microsoft and reinstall.
- Reboot to clear any pending operations, then retry the update once.
Terminal / CLI command
reagentc /disable && reagentc /enable && reagentc /infoElevated Command Prompt. Microsoft publishes an official PowerShell script for the KB5034441 partition resize — use it if you are unsure about manual diskpart steps.
How to confirm the fix: The update installs once and no longer returns in Windows Update.
Prevent it from coming back
Leave the recovery partition at its default size, and reboot before installing large updates.
Frequently asked questions
Can I just hide the KB update instead?
You can with the wushowhide tool, but you would be skipping a BitLocker-related security fix. Resizing the partition is the better long-term answer.
How much should I shrink the OS partition?
About 250 MB is enough to grow WinRE to 750 MB. You will not notice the difference in free space.