diff --git a/src/IdLE.Provider.AD/Private/New-IdleADAdapter.ps1 b/src/IdLE.Provider.AD/Private/New-IdleADAdapter.ps1
index c938f803..3c3a3ec1 100644
--- a/src/IdLE.Provider.AD/Private/New-IdleADAdapter.ps1
+++ b/src/IdLE.Provider.AD/Private/New-IdleADAdapter.ps1
@@ -667,7 +667,9 @@ function New-IdleADAdapter {
         }
     } -Force
 
-    $adapter | Add-Member -MemberType ScriptMethod -Name AddGroupMember -Value {
+    # Helper method to check if a user is a member of a group
+    # Returns $true if member, $false if not a member, $null if check fails
+    $adapter | Add-Member -MemberType ScriptMethod -Name TestGroupMembership -Value {
         param(
             [Parameter(Mandatory)]
             [ValidateNotNullOrEmpty()]
@@ -678,29 +680,29 @@ function New-IdleADAdapter {
             [string] $MemberIdentity
         )
 
-        $params = @{
+        $getMembersParams = @{
             Identity    = $GroupIdentity
-            Members     = $MemberIdentity
             ErrorAction = 'Stop'
         }
         if ($null -ne $this.Credential) {
-            $params['Credential'] = $this.Credential
+            $getMembersParams['Credential'] = $this.Credential
         }
 
         try {
-            Add-ADGroupMember @params
-            return $true
+            # For large groups, short-circuit after finding the first match
+            $existingMember = Get-ADGroupMember @getMembersParams |
+                Where-Object { $_.DistinguishedName -eq $MemberIdentity } |
+                Select-Object -First 1
+            return ($null -ne $existingMember)
         }
         catch {
-            # Idempotency: already a member is a no-op
-            if ($_.Exception.Message -match 'already a member') {
-                return $false
-            }
-            throw
+            # Return null to signal check failure (let calling method proceed to authoritative operation)
+            Write-Verbose "TestGroupMembership: Get-ADGroupMember failed for group '$GroupIdentity' and member '$MemberIdentity': $($_.Exception.Message)"
+            return $null
         }
     } -Force
 
-    $adapter | Add-Member -MemberType ScriptMethod -Name RemoveGroupMember -Value {
+    $adapter | Add-Member -MemberType ScriptMethod -Name AddGroupMember -Value {
         param(
             [Parameter(Mandatory)]
             [ValidateNotNullOrEmpty()]
@@ -711,27 +713,60 @@ function New-IdleADAdapter {
             [string] $MemberIdentity
         )
 
+        # Check if already a member (idempotency + reliable change detection)
+        $isMember = $this.TestGroupMembership($GroupIdentity, $MemberIdentity)
+        
+        if ($isMember -eq $true) {
+            # Already a member - no change needed
+            return $false
+        }
+        # If $isMember is $null (check failed), proceed to Add-ADGroupMember for authoritative error
+
         $params = @{
             Identity    = $GroupIdentity
             Members     = $MemberIdentity
-            Confirm     = $false
             ErrorAction = 'Stop'
         }
         if ($null -ne $this.Credential) {
             $params['Credential'] = $this.Credential
         }
 
-        try {
-            Remove-ADGroupMember @params
-            return $true
+        Add-ADGroupMember @params
+        return $true
+    } -Force
+
+    $adapter | Add-Member -MemberType ScriptMethod -Name RemoveGroupMember -Value {
+        param(
+            [Parameter(Mandatory)]
+            [ValidateNotNullOrEmpty()]
+            [string] $GroupIdentity,
+
+            [Parameter(Mandatory)]
+            [ValidateNotNullOrEmpty()]
+            [string] $MemberIdentity
+        )
+
+        # Check if actually a member (idempotency + reliable change detection)
+        $isMember = $this.TestGroupMembership($GroupIdentity, $MemberIdentity)
+        
+        if ($isMember -eq $false) {
+            # Not a member - no change needed
+            return $false
         }
-        catch {
-            # Idempotency: not a member is a no-op
-            if ($_.Exception.Message -match 'not a member|Member does not exist') {
-                return $false
-            }
-            throw
+        # If $isMember is $null (check failed), proceed to Remove-ADGroupMember for authoritative error
+
+        $params = @{
+            Identity    = $GroupIdentity
+            Members     = $MemberIdentity
+            Confirm     = $false
+            ErrorAction = 'Stop'
         }
+        if ($null -ne $this.Credential) {
+            $params['Credential'] = $this.Credential
+        }
+
+        Remove-ADGroupMember @params
+        return $true
     } -Force
 
     $adapter | Add-Member -MemberType ScriptMethod -Name GetUserGroups -Value {
diff --git a/tests/Providers/New-IdleADAdapter.MembershipChecks.Tests.ps1 b/tests/Providers/New-IdleADAdapter.MembershipChecks.Tests.ps1
new file mode 100644
index 00000000..c1cd7711
--- /dev/null
+++ b/tests/Providers/New-IdleADAdapter.MembershipChecks.Tests.ps1
@@ -0,0 +1,200 @@
+Set-StrictMode -Version Latest
+
+BeforeAll {
+    . (Join-Path -Path $PSScriptRoot -ChildPath '..\_testHelpers.ps1')
+    Import-IdleTestModule
+
+    $repoRoot = Split-Path -Path (Split-Path -Path $PSScriptRoot -Parent) -Parent
+    $adapterPath = Join-Path -Path $repoRoot -ChildPath 'src\IdLE.Provider.AD\Private\New-IdleADAdapter.ps1'
+    
+    if (-not (Test-Path -LiteralPath $adapterPath -PathType Leaf)) {
+        throw "New-IdleADAdapter.ps1 not found at: $adapterPath"
+    }
+
+    # Mock the AD cmdlets globally for this test
+    function Get-ADGroupMember {
+        param($Identity, $Credential, $ErrorAction)
+        
+        # Simulate group membership data
+        if ($script:MockGroupMembers.ContainsKey($Identity)) {
+            return $script:MockGroupMembers[$Identity]
+        }
+        
+        throw "Group '$Identity' not found"
+    }
+
+    function Add-ADGroupMember {
+        param($Identity, $Members, $Credential, $ErrorAction)
+        return
+    }
+
+    function Remove-ADGroupMember {
+        param($Identity, $Members, $Confirm, $Credential, $ErrorAction)
+        return
+    }
+
+    # Now source the adapter (which will use our mocked functions)
+    . $adapterPath
+}
+
+Describe 'New-IdleADAdapter membership checks' {
+
+    BeforeEach {
+        $script:MockGroupMembers = @{}
+    }
+
+    Context 'TestGroupMembership method' {
+        It 'returns $true when user is a member' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User1,DC=contoso,DC=com' }
+                [pscustomobject]@{ DistinguishedName = 'CN=User2,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.TestGroupMembership('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $true
+        }
+
+        It 'returns $false when user is not a member' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User1,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.TestGroupMembership('CN=TestGroup,DC=contoso,DC=com', 'CN=User2,DC=contoso,DC=com')
+            
+            $result | Should -Be $false
+        }
+
+        It 'returns $null when Get-ADGroupMember fails' {
+            # Don't add the group to MockGroupMembers, so Get-ADGroupMember will throw
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.TestGroupMembership('CN=NonExistentGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -BeNullOrEmpty
+        }
+
+        It 'short-circuits after finding first match (performance)' {
+            # Create a large group
+            $members = @()
+            for ($i = 1; $i -le 1000; $i++) {
+                $members += [pscustomobject]@{ DistinguishedName = "CN=User$i,DC=contoso,DC=com" }
+            }
+            $script:MockGroupMembers['CN=LargeGroup,DC=contoso,DC=com'] = $members
+
+            $adapter = New-IdleADAdapter
+            # User1 is at the beginning - should find quickly
+            $result = $adapter.TestGroupMembership('CN=LargeGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $true
+        }
+    }
+
+    Context 'AddGroupMember change detection' {
+        It 'returns $false when user is already a member (no-op)' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User1,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.AddGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $false
+        }
+
+        It 'returns $true when user is added (change occurred)' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User2,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.AddGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $true
+        }
+
+        It 'proceeds to Add-ADGroupMember when membership check fails (fail-forward)' {
+            # Don't add the group to MockGroupMembers, so membership check will fail
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.AddGroupMember('CN=NonExistentGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $true
+        }
+    }
+
+    Context 'RemoveGroupMember change detection' {
+        It 'returns $false when user is not a member (no-op)' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User2,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.RemoveGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $false
+        }
+
+        It 'returns $true when user is removed (change occurred)' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User1,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.RemoveGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $true
+        }
+
+        It 'proceeds to Remove-ADGroupMember when membership check fails (fail-forward)' {
+            # Don't add the group to MockGroupMembers, so membership check will fail
+
+            $adapter = New-IdleADAdapter
+            $result = $adapter.RemoveGroupMember('CN=NonExistentGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            
+            $result | Should -Be $true
+        }
+    }
+
+    Context 'Idempotency across multiple calls' {
+        It 'AddGroupMember is idempotent - second call returns $false' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @()
+
+            $adapter = New-IdleADAdapter
+            
+            # First call - should add
+            $result1 = $adapter.AddGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            $result1 | Should -Be $true
+
+            # Simulate the user now being a member
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User1,DC=contoso,DC=com' }
+            )
+
+            # Second call - should be no-op
+            $result2 = $adapter.AddGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            $result2 | Should -Be $false
+        }
+
+        It 'RemoveGroupMember is idempotent - second call returns $false' {
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @(
+                [pscustomobject]@{ DistinguishedName = 'CN=User1,DC=contoso,DC=com' }
+            )
+
+            $adapter = New-IdleADAdapter
+            
+            # First call - should remove
+            $result1 = $adapter.RemoveGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            $result1 | Should -Be $true
+
+            # Simulate the user no longer being a member
+            $script:MockGroupMembers['CN=TestGroup,DC=contoso,DC=com'] = @()
+
+            # Second call - should be no-op
+            $result2 = $adapter.RemoveGroupMember('CN=TestGroup,DC=contoso,DC=com', 'CN=User1,DC=contoso,DC=com')
+            $result2 | Should -Be $false
+        }
+    }
+}