ERROR_PARTIAL_COPY (0X0000012B): ReadProcessMemory/WriteProcessMemory fails partially

Cause 1: Game mods or third-party overlays interfering with memory access

This is by far the most common trigger for error 0x0000012B. I've seen it dozens of times when someone's running a game with mods—especially memory editors like Cheat Engine, or custom DLL injectors for older games. The game's anti-cheat (like Easy Anti-Cheat or BattlEye) tries to read or write memory, but a mod or overlay is already holding a partial lock on that memory region. Windows returns ERROR_PARTIAL_COPY because only part of the requested memory could be accessed before the system gave up.

What you'll see: The error pops up right after launching a game, or when a mod tool tries to attach to a process. It might also show in event logs with source "Application Error" and code 0x0000012B.

Fix it:

1. Close every program that isn't essential—especially overlays like Discord, MSI Afterburner, RivaTuner, or Steam overlay. Right-click their icons in the system tray and choose "Exit."
2. If you're using mods, remove them temporarily. Move the mod folder (like "mods" or "scripts") to your desktop. Don't just disable them in the game's menu—actually move them out of the game directory.
3. Restart your PC. This clears any stale memory handles left by those tools.
4. Launch the game or application again. If the error goes away, you've found the culprit. Re-enable mods one at a time until it breaks again.

After restarting, you shouldn't see the error anymore. If you still get it, move to Cause 2.

Cause 2: Corrupted system files or driver conflicts in the memory management stack

Sometimes the problem isn't a mod—it's a corrupt Windows component that handles memory mapping. The ReadProcessMemory API relies on the Windows memory manager being healthy. If a driver (especially an old GPU driver or a third-party antivirus filter) has a bug, it can cause partial transfers.

Real-world scenario: A user updated their NVIDIA driver to version 531.18 (a known buggy release) and started getting 0x0000012B in GTA V with FiveM. Rolling back the driver fixed it.

Fix it:

1. Run the System File Checker. Open Command Prompt as Administrator (press Win + X, choose "Windows Terminal (Admin)" or "Command Prompt (Admin)"). Type:
   sfc /scannow
   Wait for it to finish. It may take 10–15 minutes. If it finds corrupted files, reboot.
2. If SFC finds nothing, run DISM to fix the system image:
   DISM /Online /Cleanup-Image /RestoreHealth
   This downloads fresh copies of system files. Let it complete—it can take 20 minutes.
3. Update your GPU driver to the latest stable version. Don't use beta drivers. Go to the manufacturer's site (NVIDIA, AMD, Intel) and get the Game Ready or recommended driver.
4. If you're using a third-party antivirus (like Norton or McAfee), temporarily disable real-time protection. Some AV drivers hook into ReadProcessMemory and can cause partial copies. Test with it off—if the error stops, consider switching to Windows Defender.

After completing these steps, test again. If the error persists, move to Cause 3.

Cause 3: Process is running with different integrity levels or is 64-bit vs 32-bit mismatch

Here's a tricky one that catches advanced users. The ReadProcessMemory call can fail with ERROR_PARTIAL_COPY if the target process is 64-bit and your calling process is 32-bit, or vice versa. Windows can't map the memory layout correctly across architecture boundaries. Also, if the target process runs at a higher integrity level (like "System" or "Admin") and your process runs at "Medium" (standard user), you'll get partial access.

What to check: Look at the process that's throwing the error. If it's a memory editor or debugger, it must match the target's bitness. For example, a 32-bit Cheat Engine can't fully read a 64-bit game—you'll get partial copies.

Fix it:

1. Check the bitness of the application that's failing. Right-click its shortcut, choose "Properties," and look under "Compatibility" or "Details." If it says "32-bit" and the target process is "64-bit," you need a 64-bit version of the tool.
2. If you're writing a custom program, ensure you compile it with the same architecture as the target. For a 64-bit target, compile as x64. For 32-bit, compile as x86.
3. Run the tool as Administrator. Right-click its .exe and choose "Run as administrator." This elevates your process's integrity level to match high-integrity targets.
4. If the target is a Windows system process (like svchost.exe), you can't read its memory without special kernel-level access—that's by design. You'll need to write a kernel driver, which is advanced territory beyond this guide.

If none of these fixes work, the error might indicate a failing RAM stick—but that's rare. Run Windows Memory Diagnostic (search "Windows Memory Diagnostic" in Start, click "Restart now and check for problems") to rule out hardware.

Quick-reference summary