diff --git a/pack/pack.lock.json b/pack/pack.lock.json
index c8b03bf..5330146 100644
--- a/pack/pack.lock.json
+++ b/pack/pack.lock.json
@@ -1,7 +1,7 @@
 {
     "$schema":  "Brass-and-Sigil pack.lock.json - generated, do not edit by hand unless you know what you are doing",
     "name":  "Brass and Sigil",
-    "version":  "0.9.2",
+    "version":  "0.9.3",
     "minecraft":  "1.21.1",
     "loader":  {
                    "type":  "neoforge",
diff --git a/scripts/Build-Tweaks.ps1 b/scripts/Build-Tweaks.ps1
index 4e8d819..763b1a8 100644
--- a/scripts/Build-Tweaks.ps1
+++ b/scripts/Build-Tweaks.ps1
@@ -100,12 +100,27 @@ foreach ($dir in $tweakDirs) {
 
     if (Test-Path $jarPath) { Remove-Item $jarPath -Force }
 
-    [System.IO.Compression.ZipFile]::CreateFromDirectory(
-        $dir.FullName,
-        $jarPath,
-        [System.IO.Compression.CompressionLevel]::Optimal,
-        $false
-    )
+    # NB: PowerShell 5.1's [ZipFile]::CreateFromDirectory() writes Windows-
+    # native path separators (\) into ZIP entry names on Windows, producing
+    # entries like "META-INF\neoforge.mods.toml" instead of the spec-required
+    # "META-INF/neoforge.mods.toml". NeoForge's mod scanner then can't find
+    # the manifest and silently rejects the jar with "not a valid mod file".
+    # We build the archive manually so we control the entry name format.
+    # 'Create' string maps to ZipArchiveMode.Create -- PowerShell 5.1 doesn't
+    # auto-load the enum type, so the literal string form is more portable.
+    $zip = [System.IO.Compression.ZipFile]::Open($jarPath, 'Create')
+    try {
+        $sourceLen = $dir.FullName.Length
+        Get-ChildItem -Path $dir.FullName -Recurse -File | ForEach-Object {
+            $relPath = $_.FullName.Substring($sourceLen).TrimStart('\','/').Replace('\','/')
+            $entry = $zip.CreateEntry($relPath, [System.IO.Compression.CompressionLevel]::Optimal)
+            $entryStream = $entry.Open()
+            try {
+                $bytes = [System.IO.File]::ReadAllBytes($_.FullName)
+                $entryStream.Write($bytes, 0, $bytes.Length)
+            } finally { $entryStream.Dispose() }
+        }
+    } finally { $zip.Dispose() }
 
     $size = (Get-Item $jarPath).Length
     Write-Host ("  built {0,-40} {1,8:N0} bytes" -f $jarName, $size)