Fix STATUS_MP_PROCESSOR_MISMATCH 0X40000029 error

You plug in a new processor or swap motherboards, hit the power button, and instead of Windows loading you get a blue screen with 0X40000029. That's frustrating. Here's the fix, plain and direct.

The fix: force single-core boot, then update BIOS

Create a Windows installation USB (8GB or bigger, use Media Creation Tool from Microsoft).
Boot from the USB — press F12, F2, or Del during startup to pick the USB drive.
On the setup screen, press Shift + F10 to open a command prompt.
Type this and hit Enter:
```
diskpart
list volume
```
Find the volume where Windows is installed. Usually it's C: but could be D: or E:. Note the drive letter.
Exit diskpart:
```
exit
```

Now run (replace C: with your actual drive letter):

bcdedit /store C:\boot\bcd /set {default} numproc 1

Reboot without the USB. Windows will start with only one CPU core active.
Update your BIOS — go to your motherboard manufacturer's site, download the latest BIOS, flash it. This is critical.
After BIOS update, boot back into the command prompt from USB and remove the core limit:
```
bcdedit /store C:\boot\bcd /deletevalue {default} numproc
```
Reboot normally. The error should be gone.

Why this works

What's actually happening here is that Windows checks the CPU revision level at boot. If you have two physical CPUs (or a multicore processor with different stepping revisions), Windows sees the mismatch and throws 0X40000029 to protect system stability. It's a safety check — Windows won't load drivers if it thinks the CPUs might behave differently.

The reason step 6 works is that bcdedit /set numproc 1 tells Windows to only use one logical processor, bypassing the cross-CPU check entirely. Once you're in the OS, the real fix is updating the BIOS. Old BIOS versions often don't properly report the CPU stepping version, or they mix up cache topology between cores. A BIOS update fixes this — manufacturers add microcode patches that normalize the revision reporting across all cores.

If you're running a server board with two actual CPU chips (dual-socket), the mismatch might be real: one CPU is a newer stepping than the other. In that case, updating the BIOS won't help. You'll need to match both CPUs to the same stepping. But for 99% of home users who see this error, it's a BIOS reporting bug after a CPU swap.

Less common variations

Virtual machine hosts — if you moved a VM from one host to another with different CPU models, you can get this error. The fix is to shut down the VM, enable CPU compatibility mode in the hypervisor settings, then boot and install integration services.
Windows 11 on unsupported hardware — some workarounds that disable TPM or Secure Boot can trigger this error. Re-enable those in BIOS first, then try the bcdedit fix.
After a power failure during BIOS update — this corrupts the SMBIOS table. You'll need to clear CMOS (remove the battery for 30 seconds) and reflash the BIOS from a USB stick with a recovery file from the manufacturer.

Prevention

Before swapping CPUs, write down your current BIOS version and check the manufacturer's site for updates. Flash the new BIOS before installing the new processor. If you're buying a used CPU, ask the seller for the stepping version (e.g., "Stepping B0" or "Rev. C0") — matching steppings avoids this headache.

Also, keep a Windows installation USB handy. You never know when you'll need the command prompt to undo a bad boot config.

If after all this the error still appears, your motherboard might not support the new CPU at all — check the CPU support list on the manufacturer's website. Sometimes they silently drop support for older chips on new BIOS versions.