diff --git a/scripts/install_source.ps1 b/scripts/install_source.ps1
index 40a332bc0f..ad46b116a6 100644
--- a/scripts/install_source.ps1
+++ b/scripts/install_source.ps1
@@ -300,6 +300,34 @@ function Install-WindowsVCRedistIfNeeded {
     Write-Warning 'After installing, reopen PowerShell and restart OpenSquilla.'
 }
 
+function Assert-OpenSquillaNotRunning {
+    if (-not $script:isWindowsHost) {
+        return
+    }
+
+    try {
+        $running = @(Get-CimInstance Win32_Process -ErrorAction Stop | Where-Object {
+            $_.ProcessId -ne $PID -and
+            $_.CommandLine -and
+            $_.CommandLine -match '(?i)opensquilla' -and
+            $_.CommandLine -notmatch '(?i)install_source\.ps1'
+        })
+    } catch {
+        Write-Warning 'install_source.ps1: could not inspect running processes; continuing with the installer.'
+        return
+    }
+
+    if ($running.Count -eq 0) {
+        return
+    }
+
+    $details = @($running | ForEach-Object {
+        $name = if ($_.Name) { $_.Name } else { 'unknown process' }
+        "$name (PID $($_.ProcessId))"
+    }) -join ', '
+    Write-Error "install_source.ps1: OpenSquilla is running ($details). Stop it before upgrading so the existing CLI remains recoverable if installation cannot replace locked files."
+}
+
 # --- installer selection ----------------------------------------------------
 
 $installer = $null
@@ -454,6 +482,7 @@ if ($dryRun) {
 
 # --- execute ---------------------------------------------------------------
 
+Assert-OpenSquillaNotRunning
 Test-SquillaRouterAssets
 Build-WebUI
 Install-WindowsVCRedistIfNeeded
diff --git a/tests/test_install_scripts.py b/tests/test_install_scripts.py
index d657a2b830..89929e370f 100644
--- a/tests/test_install_scripts.py
+++ b/tests/test_install_scripts.py
@@ -89,6 +89,16 @@ def test_windows_installer_stops_when_native_install_command_fails() -> None:
     assert "exit $installExitCode" in ps1
 
 
+def test_windows_source_installer_preflights_running_opensquilla() -> None:
+    ps1 = SOURCE_PS1.read_text(encoding="utf-8")
+
+    assert "function Assert-OpenSquillaNotRunning" in ps1
+    assert "Get-CimInstance Win32_Process" in ps1
+    assert "install_source.ps1: OpenSquilla is running" in ps1
+    assert "Stop it before upgrading" in ps1
+    assert "Assert-OpenSquillaNotRunning" in ps1
+
+
 def test_install_script_banners_are_ascii_for_windows_terminals() -> None:
     scripts = [
         RELEASE_PS1.read_text(encoding="utf-8"),