diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 64e2d38..11a08fe 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -83,6 +83,15 @@ jobs:
             exit 1
           }
 
+      - name: Run custom feature apply tests
+        shell: pwsh
+        run: |
+          $result = Invoke-Pester -Path "Tests/Unit/Test-CustomFeatureApply.ps1" -Output Detailed -PassThru
+          if ($result.FailedCount -gt 0) {
+            Write-Error "$($result.FailedCount) test(s) failed in Test-CustomFeatureApply.ps1"
+            exit 1
+          }
+
       - name: Run preset tests
         shell: pwsh
         run: |
diff --git a/Tests/Unit/Test-CustomFeatureApply.ps1 b/Tests/Unit/Test-CustomFeatureApply.ps1
new file mode 100644
index 0000000..d60933b
--- /dev/null
+++ b/Tests/Unit/Test-CustomFeatureApply.ps1
@@ -0,0 +1,324 @@
+#Requires -Modules Pester
+<#
+.SYNOPSIS
+    Unit tests for the custom-feature apply and undo bodies.
+.DESCRIPTION
+    Enable-GamingMode, Enable-PerformanceTweaks, Enable-SecurityHardening and
+    Disable-ExtendedAIPurge (plus their undo counterparts) do real registry,
+    service, power, firewall and optional-feature changes. They were only
+    exercised end to end by the Windows Sandbox mutating suite; the static
+    drift guard in Test-ModuleRollback checks the declared target lists but
+    never runs a body. These tests run each body with every OS-touching command
+    mocked and assert what it writes, that it creates missing keys, that -WhatIf
+    gates every change, and that its undo reverses the right values.
+
+    Two safety measures keep this off the real machine: powercfg and netsh are
+    shadowed by no-op functions before anything runs, so the native executables
+    can never be reached even if a mock were missing, and every mutating cmdlet
+    is mocked. Nothing here is tagged Mutating; it is pure unit coverage.
+
+    The runtime drift check is stronger than the static one: it confirms every
+    Path+Name a body actually writes is declared by that module's
+    registry-target provider, so a wrong key path (not just a wrong value name)
+    is caught. The reverse direction is deliberately not asserted, since a body
+    skips conditional writes when a key is absent.
+#>
+
+BeforeAll {
+    $script:repoRoot = Resolve-Path (Join-Path $PSScriptRoot '..\..') | Select-Object -ExpandProperty Path
+    $script:featuresPath = Join-Path $script:repoRoot 'Scripts\Features'
+    . (Join-Path $script:featuresPath 'GamingMode.ps1')
+    . (Join-Path $script:featuresPath 'PerformanceTweaks.ps1')
+    . (Join-Path $script:featuresPath 'SecurityHardening.ps1')
+    . (Join-Path $script:featuresPath 'ExtendedAIPurge.ps1')
+
+    # Shadow the native executables so a missed mock can never touch the real
+    # power plan or firewall. A function outranks an application in command
+    # resolution, so these win even before Mock replaces them.
+    function powercfg { }
+    function netsh { }
+
+    # Two fake network interfaces so the Nagle writes and the provider that
+    # enumerates them both see the same set of interface keys.
+    $script:fakeInterfaces = @(
+        [pscustomobject]@{ PSPath = 'Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{iface-1}' }
+        [pscustomobject]@{ PSPath = 'Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{iface-2}' }
+    )
+
+    # Every write a body issues, so the drift check can compare against the
+    # module's declared targets.
+    $script:regWrites = [System.Collections.Generic.List[object]]::new()
+    $script:regRemoves = [System.Collections.Generic.List[object]]::new()
+    $script:keysCreated = [System.Collections.Generic.List[string]]::new()
+
+    function Get-UndeclaredWrites {
+        param([object[]]$Targets)
+        $undeclared = New-Object System.Collections.Generic.List[string]
+        foreach ($write in $script:regWrites) {
+            $match = @($Targets | Where-Object { $_.Path -ieq $write.Path -and $_.Name -ieq $write.Name })
+            if ($match.Count -eq 0) {
+                $undeclared.Add(('{0}\{1}' -f $write.Path, $write.Name))
+            }
+        }
+        return $undeclared
+    }
+
+    # Mocks common to every context. Recording mocks push into the script-scoped
+    # lists that BeforeEach clears before each test. Invoked from each Describe's
+    # BeforeAll so the mocks bind to that Describe.
+    function Set-CommonFeatureMocks {
+        Mock Write-Host { }
+        Mock Set-ItemProperty { $script:regWrites.Add([pscustomobject]@{ Path = $Path; Name = $Name; Value = $Value }) }
+        Mock Remove-ItemProperty { $script:regRemoves.Add([pscustomobject]@{ Path = $Path; Name = $Name }) }
+        Mock New-Item { $script:keysCreated.Add([string]$Path) }
+        Mock Test-Path { $false }
+        Mock Get-ChildItem { $script:fakeInterfaces }
+        Mock Set-Service { }
+        Mock Stop-Service { }
+        Mock Start-Service { }
+        Mock Set-SmbServerConfiguration { }
+        Mock Disable-WindowsOptionalFeature { }
+        Mock Get-WindowsOptionalFeature { [pscustomobject]@{ State = 'Enabled' } }
+        Mock Get-ScheduledTask { [pscustomobject]@{ State = 'Ready' } }
+        Mock Disable-ScheduledTask { }
+        Mock powercfg { 'Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e  (High performance)' }
+        Mock netsh { }
+    }
+}
+
+Describe 'Enable-GamingMode' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 'writes the curated tweak values and creates the keys it needs' {
+        Enable-GamingMode
+
+        ($script:regWrites | Where-Object { $_.Path -eq 'HKCU:\Control Panel\Mouse' -and $_.Name -eq 'MouseSpeed' -and $_.Value -eq '0' }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'HwSchMode' -and $_.Value -eq 2 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'AllowGameDVR' -and $_.Value -eq 0 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'MaintenanceDisabled' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+        $script:keysCreated | Should -Contain 'HKCU:\Software\Microsoft\Windows\CurrentVersion\GameDVR'
+    }
+
+    It 'disables Nagle on every network interface' {
+        Enable-GamingMode
+
+        $nagle = @($script:regWrites | Where-Object { $_.Name -eq 'TcpAckFrequency' })
+        $nagle.Count | Should -Be $script:fakeInterfaces.Count
+    }
+
+    It 'sets the High Performance power plan by GUID' {
+        Enable-GamingMode
+        Should -Invoke powercfg -Times 1 -Exactly -ParameterFilter { $args -contains '-setactive' -and $args -contains '381b4222-f694-41f0-9685-ff5bb260df2e' }
+    }
+
+    It 'writes nothing under -WhatIf' {
+        Enable-GamingMode -WhatIf
+        Should -Invoke Set-ItemProperty -Times 0
+        Should -Invoke New-Item -Times 0
+    }
+
+    It 'writes only values its registry-target provider declares' {
+        Enable-GamingMode
+        Get-UndeclaredWrites -Targets @(Get-GamingModeRegistryTargets) | Should -BeNullOrEmpty
+    }
+}
+
+Describe 'Disable-GamingMode' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 'removes the Nagle values and restores automatic maintenance' {
+        Disable-GamingMode
+
+        @($script:regRemoves | Where-Object { $_.Name -eq 'TcpAckFrequency' }) | Should -Not -BeNullOrEmpty
+        @($script:regRemoves | Where-Object { $_.Name -eq 'MaintenanceDisabled' }) | Should -Not -BeNullOrEmpty
+    }
+
+    It 'restores the Balanced power plan' {
+        Disable-GamingMode
+        Should -Invoke powercfg -Times 1 -Exactly -ParameterFilter { $args -contains 'SCHEME_BALANCED' }
+    }
+}
+
+Describe 'Enable-PerformanceTweaks' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 'disables the heavy background services' {
+        Enable-PerformanceTweaks
+
+        Should -Invoke Set-Service -ParameterFilter { $Name -eq 'SysMain' -and $StartupType -eq 'Disabled' }
+        Should -Invoke Set-Service -ParameterFilter { $Name -eq 'WSearch' -and $StartupType -eq 'Disabled' }
+        Should -Invoke Set-Service -ParameterFilter { $Name -eq 'Spooler' -and $StartupType -eq 'Disabled' }
+        Should -Invoke Stop-Service -ParameterFilter { $Name -eq 'SysMain' }
+    }
+
+    It 'turns off hibernate and writes the responsiveness tweaks' {
+        Enable-PerformanceTweaks
+
+        Should -Invoke powercfg -ParameterFilter { $args -contains '-h' -and $args -contains 'off' }
+        ($script:regWrites | Where-Object { $_.Name -eq 'StartupDelayInMSec' -and $_.Value -eq 0 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'ShowSecondsInSystemClock' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+    }
+
+    It 'touches no service and writes nothing under -WhatIf' {
+        Enable-PerformanceTweaks -WhatIf
+        Should -Invoke Set-Service -Times 0
+        Should -Invoke Stop-Service -Times 0
+        Should -Invoke Set-ItemProperty -Times 0
+    }
+}
+
+Describe 'Disable-PerformanceTweaks' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 're-enables and starts the services it disabled' {
+        Disable-PerformanceTweaks
+
+        Should -Invoke Set-Service -ParameterFilter { $Name -eq 'SysMain' -and $StartupType -eq 'Automatic' }
+        Should -Invoke Set-Service -ParameterFilter { $Name -eq 'WSearch' -and $StartupType -eq 'Automatic' }
+        Should -Invoke Start-Service -ParameterFilter { $Name -eq 'Spooler' }
+    }
+
+    It 're-enables hibernate' {
+        Disable-PerformanceTweaks
+        Should -Invoke powercfg -ParameterFilter { $args -contains '-h' -and $args -contains 'on' }
+    }
+}
+
+Describe 'Enable-SecurityHardening' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 'disables SMBv1' {
+        Enable-SecurityHardening
+        Should -Invoke Set-SmbServerConfiguration -ParameterFilter { $EnableSMB1Protocol -eq $false }
+    }
+
+    It 'denies RDP, hardens AutoRun, disables the legacy TLS versions and blocks the Windows Script Host' {
+        Enable-SecurityHardening
+
+        ($script:regWrites | Where-Object { $_.Name -eq 'fDenyTSConnections' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'NoDriveTypeAutoRun' -and $_.Value -eq 255 }) | Should -Not -BeNullOrEmpty
+        @($script:regWrites | Where-Object { $_.Name -eq 'DisabledByDefault' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Path -like '*Windows Script Host*' -and $_.Name -eq 'Enabled' -and $_.Value -eq 0 }) | Should -Not -BeNullOrEmpty
+    }
+
+    It 'blocks the SMB and NetBIOS ports through the firewall' {
+        Enable-SecurityHardening
+        Should -Invoke netsh -ParameterFilter { $args -contains 'localport=445' }
+        Should -Invoke netsh -ParameterFilter { $args -contains 'localport=139' }
+    }
+
+    It 'writes nothing under -WhatIf' {
+        Enable-SecurityHardening -WhatIf
+        Should -Invoke Set-ItemProperty -Times 0
+        Should -Invoke Set-SmbServerConfiguration -Times 0
+        Should -Invoke netsh -Times 0
+    }
+
+    It 'writes only values its registry-target provider declares' {
+        Enable-SecurityHardening
+        Get-UndeclaredWrites -Targets @(Get-SecurityHardeningRegistryTargets) | Should -BeNullOrEmpty
+    }
+}
+
+Describe 'Disable-SecurityHardening' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 're-enables RDP, restores the AutoRun default and re-enables the Windows Script Host' {
+        Disable-SecurityHardening
+
+        ($script:regWrites | Where-Object { $_.Name -eq 'fDenyTSConnections' -and $_.Value -eq 0 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'NoDriveTypeAutoRun' -and $_.Value -eq 91 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Path -like '*Windows Script Host*' -and $_.Name -eq 'Enabled' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+    }
+}
+
+Describe 'Disable-ExtendedAIPurge' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 'writes the user-and-machine AI policies to both hives' {
+        Disable-ExtendedAIPurge
+
+        @($script:regWrites | Where-Object { $_.Path -eq 'HKLM:\SOFTWARE\Policies\Microsoft\Windows\WindowsAI' -and $_.Name -eq 'DisableClickToDo' }) | Should -Not -BeNullOrEmpty
+        @($script:regWrites | Where-Object { $_.Path -eq 'HKCU:\SOFTWARE\Policies\Microsoft\Windows\WindowsAI' -and $_.Name -eq 'DisableClickToDo' }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'AllowRecallEnablement' -and $_.Value -eq 0 }) | Should -Not -BeNullOrEmpty
+        ($script:regWrites | Where-Object { $_.Name -eq 'TurnOffWindowsCopilot' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+    }
+
+    It 'writes the Paint AI policy values at the Paint policy key' {
+        Disable-ExtendedAIPurge
+
+        foreach ($name in 'DisableCocreator', 'DisableGenerativeFill', 'DisableImageCreator') {
+            ($script:regWrites | Where-Object { $_.Path -like '*\Policies\Paint' -and $_.Name -eq $name }) | Should -Not -BeNullOrEmpty
+        }
+    }
+
+    It 'disables each sluggishness telemetry task that is present and not already disabled' {
+        Disable-ExtendedAIPurge
+        Should -Invoke Disable-ScheduledTask -Times 4 -Exactly
+    }
+
+    It 'leaves the tasks alone when they are already disabled' {
+        Mock Get-ScheduledTask { [pscustomobject]@{ State = 'Disabled' } }
+        Disable-ExtendedAIPurge
+        Should -Invoke Disable-ScheduledTask -Times 0
+    }
+
+    It 'removes the Recall optional component only when it is present and enabled' {
+        Disable-ExtendedAIPurge
+        Should -Invoke Disable-WindowsOptionalFeature -ParameterFilter { $FeatureName -eq 'Recall' }
+    }
+
+    It 'does not remove the Recall component when the feature is absent' {
+        Mock Get-WindowsOptionalFeature { [pscustomobject]@{ State = 'Disabled' } }
+        Disable-ExtendedAIPurge
+        Should -Invoke Disable-WindowsOptionalFeature -ParameterFilter { $FeatureName -eq 'Recall' } -Times 0
+    }
+
+    It 'writes nothing under -WhatIf' {
+        Disable-ExtendedAIPurge -WhatIf
+        Should -Invoke Set-ItemProperty -Times 0
+        Should -Invoke Disable-ScheduledTask -Times 0
+        Should -Invoke Disable-WindowsOptionalFeature -Times 0
+    }
+
+    It 'writes only values its registry-target provider declares' {
+        Disable-ExtendedAIPurge
+        Get-UndeclaredWrites -Targets @(Get-ExtendedAIPurgeRegistryTargets) | Should -BeNullOrEmpty
+    }
+}
+
+Describe 'Enable-ExtendedAIPurgeRevert' {
+    BeforeAll { Set-CommonFeatureMocks }
+    BeforeEach {
+        $script:regWrites.Clear(); $script:regRemoves.Clear(); $script:keysCreated.Clear()
+    }
+
+    It 'restores Phone Link when its key exists and clears the cloud clipboard opt-in' {
+        Mock Test-Path { $true }
+        Enable-ExtendedAIPurgeRevert
+
+        ($script:regWrites | Where-Object { $_.Name -eq 'PhoneLinkEnabled' -and $_.Value -eq 1 }) | Should -Not -BeNullOrEmpty
+        @($script:regRemoves | Where-Object { $_.Name -eq 'EnableCloudClipboard' }) | Should -Not -BeNullOrEmpty
+    }
+}