src/Tools/VSCode/src/protocol.scala

       (source match { case Some(x) => Map("source" -> x) case None => Map.empty })
   }
 
   object DiagnosticSeverity
   {
-    val Error = 1
-    val Warning = 2
-    val Information = 3
-    val Hint = 4
+    val Error = 0
+    val Warning = 1
+    val Information = 2
+    val Hint = 3
   }
 
   object PublishDiagnostics
   {
     def apply(uri: String, diagnostics: List[Diagnostic]): JSON.T =