diff --git a/ParallelKit.Harness.ps1 b/ParallelKit.Harness.ps1
index 39bfebe..9bc7a65 100644
--- a/ParallelKit.Harness.ps1
+++ b/ParallelKit.Harness.ps1
@@ -43,6 +43,9 @@ function New-JobList {
 }
 
 $cmdExe  = Join-Path $env:SystemRoot "System32\cmd.exe"
+# Whichever PowerShell is running the harness (powershell.exe or pwsh.exe),
+# for the tests that need a fresh process of the same version.
+$selfExe = [System.Diagnostics.Process]::GetCurrentProcess().MainModule.FileName
 $pingExe = Join-Path $env:SystemRoot "System32\ping.exe"
 
 # A user function the workers share.
@@ -688,6 +691,17 @@ $probeThreads = 0
 if ($probe.Stdout -match 'threads=(\d+)') { $probeThreads = [int]$Matches[1] }
 Test-Assert ($probeThreads -ge 128) ("and raises the thread pool by itself (minimum " + $probeThreads + ")")
 
+# Loading the kit in Constrained Language Mode fails with a message that
+# says so. Needs a fresh process: the mode can't be set back.
+$clmScript = Join-Path $env:TEMP ("pk-clm-" + [guid]::NewGuid().ToString("N") + ".ps1")
+[System.IO.File]::WriteAllText($clmScript, @"
+`$ExecutionContext.SessionState.LanguageMode = 'ConstrainedLanguage'
+try { . '$kitPath'; Write-Output 'loaded' } catch { Write-Output ('error: ' + `$_) }
+"@)
+$clm = Invoke-PkProcessWithTimeout -FilePath $selfExe -Arguments ('-NoProfile -ExecutionPolicy Bypass -File "' + $clmScript + '"') -TimeoutMs 60000
+[System.IO.File]::Delete($clmScript)
+Test-Assert ($clm.Stdout -like '*ConstrainedLanguage mode, but the kit needs FullLanguage*') ("loading in Constrained Language Mode fails with a message naming the problem (" + ([string]$clm.Stdout).Trim() + ")")
+
 # The dispatcher sizes the pool for its own concurrency.
 $feed = @{ Given = $false }
 $null = Invoke-PkProcessDispatch -MaxConcurrent 100 -GroupProvider { return $null } -JobBuilder { param($g) } -JobFinalize { param($j, $p) }
diff --git a/ParallelKit.ps1 b/ParallelKit.ps1
index b4aa176..2c16ae9 100644
--- a/ParallelKit.ps1
+++ b/ParallelKit.ps1
@@ -13,6 +13,14 @@
 # function called from inside a .GetNewClosure()'d block can see an empty
 # $script: scope, and a static has no scope to lose.
 
+# Everything below needs Add-Type, which Constrained Language Mode blocks with
+# an error that doesn't say why. Say why.
+if ($ExecutionContext.SessionState.LanguageMode -ne 'FullLanguage') {
+    throw ("ParallelKit: this PowerShell session runs in " + $ExecutionContext.SessionState.LanguageMode + " mode, but the kit needs FullLanguage " +
+        "(it compiles C# with Add-Type). This is usually set by AppLocker or Windows Defender Application Control policy. " +
+        "Ask whoever manages the machine to allow this script, or run it where that policy doesn't apply.")
+}
+
 # ==============================================================================
 #  Native types (guarded so the file can be dot-sourced more than once)
 # ==============================================================================
diff --git a/README.md b/README.md
index 8452e2d..0398a16 100644
--- a/README.md
+++ b/README.md
@@ -282,6 +282,7 @@ Other pieces: `Request-PkAbort` triggers the same abort from code. `Test-PkAbort
 ## Limits
 
 - Windows PowerShell 5.1, Windows only (console and Job Object APIs).
+- It needs FullLanguage mode, since it compiles a little C# with `Add-Type` when loaded. On a machine locked down to Constrained Language Mode (usually by AppLocker or Windows Defender Application Control), loading it fails with a message saying so. Some endpoint security products also flag scripts that compile code calling Windows APIs at runtime; if yours blocks the kit, that's the likely reason.
 - After changing `ParallelKit.ps1` itself, start a new PowerShell window. A session keeps the kit's compiled parts until it closes; the kit tells you if it finds an older copy loaded.
 - One run at a time per process. The abort state is shared. A nested `Invoke-PkRun` just runs inside the outer one.
 - A worker stuck inside a long .NET call can't be interrupted. Force-stop stops waiting for it and returns. The worker is NOT killed: once that call returns it can carry on to the end of its script. Before any step that can't be undone (deleting a source file, say), have the worker check `Test-PkCancelRequested` and skip the step if it's true.