diff --git a/New-LogEntry/Private/Get-NewLogEntryMutexName.ps1 b/New-LogEntry/Private/Get-NewLogEntryMutexName.ps1
index 620049c..79c10e6 100644
--- a/New-LogEntry/Private/Get-NewLogEntryMutexName.ps1
+++ b/New-LogEntry/Private/Get-NewLogEntryMutexName.ps1
@@ -2,7 +2,10 @@ function Get-NewLogEntryMutexName
 {
     param([string]$Path)
 
-    $normalizedPath = if ($IsWindows) { $Path.ToUpperInvariant() } else { $Path }
+    # Conservatively share locks for case aliases on macOS, including its
+    # common case-insensitive volumes. Case-sensitive macOS volumes may serialize
+    # distinct case-only filenames; that is safer than missing a shared file.
+    $normalizedPath = if ($IsWindows -or $IsMacOS) { $Path.ToUpperInvariant() } else { $Path }
     $bytes = [System.Security.Cryptography.SHA256]::HashData([System.Text.Encoding]::UTF8.GetBytes($normalizedPath))
     $hash = [Convert]::ToHexString($bytes).Substring(0, 32)
 
diff --git a/New-LogEntry/Private/Resolve-NewLogEntryPath.ps1 b/New-LogEntry/Private/Resolve-NewLogEntryPath.ps1
index df27c19..7114c59 100644
--- a/New-LogEntry/Private/Resolve-NewLogEntryPath.ps1
+++ b/New-LogEntry/Private/Resolve-NewLogEntryPath.ps1
@@ -4,7 +4,16 @@ function Resolve-NewLogEntryPath
 
     if (-not [string]::IsNullOrWhiteSpace($Path))
     {
-        return $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($Path)
+        $provider = $null
+        $drive = $null
+        $resolved = $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath(
+            $Path, [ref]$provider, [ref]$drive
+        )
+        if ($provider.Name -ne 'FileSystem')
+        {
+            throw [ArgumentException]::new('LogFilePath must use the FileSystem provider.')
+        }
+        return $resolved
     }
 
     $basePath = if (-not [string]::IsNullOrWhiteSpace($CallerScriptPath))
@@ -13,7 +22,12 @@ function Resolve-NewLogEntryPath
     }
     else
     {
-        Join-Path -Path (Get-Location).ProviderPath -ChildPath 'PowerShell'
+        $location = Get-Location
+        if ($location.Provider.Name -ne 'FileSystem')
+        {
+            throw [ArgumentException]::new('A filesystem LogFilePath is required when the current location uses another provider.')
+        }
+        Join-Path -Path $location.ProviderPath -ChildPath 'PowerShell'
     }
 
     $directory = Split-Path -Path $basePath -Parent
diff --git a/New-LogEntry/Private/Write-NewLogEntryLines.ps1 b/New-LogEntry/Private/Write-NewLogEntryLines.ps1
index 21e060b..255b64f 100644
--- a/New-LogEntry/Private/Write-NewLogEntryLines.ps1
+++ b/New-LogEntry/Private/Write-NewLogEntryLines.ps1
@@ -40,7 +40,7 @@ function Write-NewLogEntryLines
         if (-not $hasLock)
         {
             throw [System.TimeoutException]::new(
-                'Timed out waiting {0} seconds for the log file lock: {1}' -f $LockTimeoutSeconds, $resolvedPath
+                ('Timed out waiting {0} seconds for the log file lock: {1}' -f $LockTimeoutSeconds, $resolvedPath)
             )
         }
 
diff --git a/New-LogEntry/README.md b/New-LogEntry/README.md
index f496cf7..12ec52d 100644
--- a/New-LogEntry/README.md
+++ b/New-LogEntry/README.md
@@ -239,3 +239,18 @@ Released under the [MIT License](./LICENSE).
 The New-LogEntry workflow runs the Pester suite on Windows, Linux, and macOS using each GitHub-hosted runner's installed PowerShell. It also checks PowerShell syntax and performs a separate loader smoke test. Pester is pinned to version 5.7.1.
 
 PowerShell 7.1 is the API minimum because the mutex helper uses .NET 5 APIs. CI tests the runner versions, not every historical PowerShell release.
+
+### Logger path and lock boundaries
+
+Explicit log destinations must use the FileSystem provider. Non-filesystem paths
+such as `Env:NAME` are rejected before file operations. Interactive calls from a
+non-filesystem location require an explicit filesystem log destination; a calling
+script can still supply the default directory through its own filesystem path.
+A rejected flush preserves the buffer.
+
+On Windows and macOS, case-only path aliases share a mutex. On case-sensitive Mac
+volumes, distinct case-only filenames may therefore serialize unnecessarily.
+Locks remain path-based: symlink/hard-link aliases are not resolved to file identity.
+Upgrade all writers together: older macOS implementations can use different lock
+names for lowercase paths. Keep one consistent path spelling during mixed-version
+operation.
diff --git a/New-LogEntry/Tests/LoggerPath.Tests.ps1 b/New-LogEntry/Tests/LoggerPath.Tests.ps1
new file mode 100644
index 0000000..b1db985
--- /dev/null
+++ b/New-LogEntry/Tests/LoggerPath.Tests.ps1
@@ -0,0 +1,100 @@
+BeforeAll {
+    $loggerLoader = (Resolve-Path (Join-Path $PSScriptRoot '../New-LogEntry.ps1')).Path
+    $loggerModule = New-Module -Name LoggerPathStandalone -ArgumentList $loggerLoader -ScriptBlock {
+        param($Loader)
+        . $Loader
+    } | Import-Module -Force -PassThru
+    $loggerIsModule = $false
+}
+AfterAll { Remove-Module LoggerPathStandalone }
+Describe 'Logger destination boundaries' {
+    BeforeEach { New-LogEntry -ClearBuffer }
+    It 'rejects non-filesystem destinations before creating a file: <Provider>' -ForEach @(
+        @{Provider='Env'}; @{Provider='Variable'}; @{Provider='Function'}
+    ) {
+        $name = 'LoggerProbe' + [guid]::NewGuid().ToString('N')
+        $destination = '{0}:{1}' -f $Provider,$name
+        $originalDirectory = [Environment]::CurrentDirectory
+        try {
+            [Environment]::CurrentDirectory = $TestDrive
+            { New-LogEntry -LogMessage 'must not be written' -LogFilePath $destination -NoConsole } | Should -Throw '*FileSystem*'
+            @(Get-ChildItem -LiteralPath $TestDrive -Filter $name -Force).Count | Should -Be 0
+            New-LogEntry -LogMessage 'keep buffered' -BufferOnly
+            { New-LogEntry -FlushBuffer -LogFilePath $destination -NoConsole } | Should -Throw '*FileSystem*'
+            @(New-LogEntry -GetBuffer).Count | Should -Be 1
+            (New-LogEntry -GetBuffer) | Should -Match 'keep buffered$'
+        }
+        finally { [Environment]::CurrentDirectory = $originalDirectory }
+    }
+    It 'rejects an interactive default path from a non-filesystem location' {
+        Push-Location Env:
+        try {
+            { & $loggerModule { Resolve-NewLogEntryPath } } | Should -Throw '*filesystem LogFilePath*'
+        }
+        finally { Pop-Location }
+    }
+    It 'still resolves an explicit filesystem destination from another provider' {
+        $path = Join-Path $TestDrive 'explicit.log'
+        Push-Location Env:
+        try { New-LogEntry -LogMessage 'explicit destination' -LogFilePath $path -NoConsole }
+        finally { Pop-Location }
+        Get-Content -LiteralPath $path | Should -Match 'explicit destination$'
+    }
+}
+
+Describe 'Logger case-alias locking' {
+    It 'blocks a different process using the same file or an available case alias' {
+        $path = Join-Path $TestDrive 'case-probe.log'
+        $aliasPath = Join-Path $TestDrive 'CASE-PROBE.log'
+        Set-Content -LiteralPath $path -Value 'seed'
+        if (-not (Test-Path -LiteralPath $aliasPath)) {
+            $aliasPath = $path
+
+        }
+        $name = & $loggerModule { param($p) Get-NewLogEntryMutexName -Path $p } $path
+        $aliasName = & $loggerModule { param($p) Get-NewLogEntryMutexName -Path $p } $aliasPath
+        $aliasName | Should -BeExactly $name
+        $mutex = [Threading.Mutex]::new($false,$name)
+        $locked = $false
+        $process = $null
+        try {
+            $locked = $mutex.WaitOne(1000)
+            $locked | Should -BeTrue
+            $escapedLoader = $loggerLoader.Replace("'","''")
+            $escapedPath = $aliasPath.Replace("'","''")
+            $load = if ($loggerIsModule) { "Import-Module '$escapedLoader' -Force" } else { ". '$escapedLoader'" }
+            $code = @"
+`$ErrorActionPreference='Stop'
+$load
+try {
+    New-LogEntry -LogMessage 'must time out' -LogFilePath '$escapedPath' -LockTimeoutSeconds 1 -NoConsole
+    Write-Output 'UNEXPECTED_WRITE'
+    exit 2
+} catch {
+    if (`$_.Exception.Message -like '*Timed out waiting*') { Write-Output 'EXPECTED_TIMEOUT'; exit 0 }
+    Write-Output `$_.Exception.Message
+    exit 3
+}
+"@
+            $start = [Diagnostics.ProcessStartInfo]::new((Get-Process -Id $PID).Path)
+            $start.UseShellExecute = $false
+            $start.RedirectStandardOutput = $true
+            $start.RedirectStandardError = $true
+            foreach ($argument in @('-NoProfile','-EncodedCommand',[Convert]::ToBase64String([Text.Encoding]::Unicode.GetBytes($code)))) {
+                $start.ArgumentList.Add($argument)
+            }
+            $process = [Diagnostics.Process]::Start($start)
+            if (-not $process.WaitForExit(15000)) { $process.Kill($true); throw 'Logger child process exceeded test timeout.' }
+            $output = $process.StandardOutput.ReadToEnd()
+            $errorOutput = $process.StandardError.ReadToEnd()
+            $process.ExitCode | Should -Be 0 -Because ($output + $errorOutput)
+            $output | Should -Match 'EXPECTED_TIMEOUT'
+            Get-Content -LiteralPath $path | Should -Be 'seed'
+        }
+        finally {
+            if ($null -ne $process) { $process.Dispose() }
+            if ($locked) { $mutex.ReleaseMutex() }
+            $mutex.Dispose()
+        }
+    }
+}