diff --git a/scripts/Build-Tweaks.ps1 b/scripts/Build-Tweaks.ps1
index 763b1a8..e1833a9 100644
--- a/scripts/Build-Tweaks.ps1
+++ b/scripts/Build-Tweaks.ps1
@@ -122,8 +122,44 @@ foreach ($dir in $tweakDirs) {
         }
     } finally { $zip.Dispose() }
 
+    # ── Post-build validation ────────────────────────────────────────────────
+    # Catch structural problems BEFORE the jar gets bundled into the manifest
+    # and shipped to clients. NeoForge silently rejects invalid jars with a
+    # bland "not a valid mod file" warning that's easy to miss in a 500-line
+    # game log; this fails the build loudly with a specific reason instead.
+    $verify = [System.IO.Compression.ZipFile]::OpenRead($jarPath)
+    try {
+        $names = @($verify.Entries | ForEach-Object { $_.FullName })
+
+        # 1. No backslashes in entry names. PowerShell 5.1's CreateFromDirectory
+        #    leaks Windows path separators into ZIP entries, which makes
+        #    NeoForge fail to find META-INF/neoforge.mods.toml. We build with
+        #    explicit CreateEntry above, but if anyone "simplifies" the zip
+        #    path back to CreateFromDirectory, this guard fires.
+        $badSep = $names | Where-Object { $_ -match '\\' }
+        if ($badSep) {
+            throw ("$jarName has entries with backslash separators (NeoForge will reject it):`n  - " + ($badSep -join "`n  - "))
+        }
+
+        # 2. META-INF/neoforge.mods.toml must be at the canonical path.
+        if (-not ($names -contains 'META-INF/neoforge.mods.toml')) {
+            throw "$jarName is missing META-INF/neoforge.mods.toml at the canonical forward-slash path. Found: $($names -join ', ')"
+        }
+
+        # 3. modId from the embedded toml must match the jar's filename, so a
+        #    misnamed source folder doesn't ship a jar whose manifest declares
+        #    a different mod (which loads silently under the wrong identity).
+        $tomlEntry = $verify.GetEntry('META-INF/neoforge.mods.toml')
+        $reader = New-Object System.IO.StreamReader($tomlEntry.Open())
+        try { $tomlContent = $reader.ReadToEnd() } finally { $reader.Dispose() }
+        $tomlModId = [regex]::Match($tomlContent, '(?m)^\s*modId\s*=\s*"([^"]+)"').Groups[1].Value
+        if ($tomlModId -ne $meta.ModId) {
+            throw "$jarName modId mismatch: source folder advertises '$($meta.ModId)' but built jar's neoforge.mods.toml says '$tomlModId'."
+        }
+    } finally { $verify.Dispose() }
+
     $size = (Get-Item $jarPath).Length
-    Write-Host ("  built {0,-40} {1,8:N0} bytes" -f $jarName, $size)
+    Write-Host ("  built {0,-40} {1,8:N0} bytes  [validated]" -f $jarName, $size)
 }
 
 Write-Host ""