diff --git a/spark/src/main/scala/org/apache/comet/serde/strings.scala b/spark/src/main/scala/org/apache/comet/serde/strings.scala
index 05448ba653..b0f2446354 100644
--- a/spark/src/main/scala/org/apache/comet/serde/strings.scala
+++ b/spark/src/main/scala/org/apache/comet/serde/strings.scala
@@ -152,6 +152,11 @@ object CometStringReplace extends CometScalarFunction[StringReplace]("replace")
 
 object CometSubstring extends CometExpressionSerde[Substring] {
 
+  override def getSupportLevel(expr: Substring): SupportLevel = (expr.pos, expr.len) match {
+    case (_: Literal, _: Literal) => Compatible()
+    case _ => Unsupported(Some("Substring pos and len must be literals"))
+  }
+
   override def convert(
       expr: Substring,
       inputs: Seq[Attribute],
@@ -170,7 +175,7 @@ object CometSubstring extends CometExpressionSerde[Substring] {
             None
         }
       case _ =>
-        withFallbackReason(expr, "Substring pos and len must be literals")
+        // Unreachable: getSupportLevel gates non-literal pos/len.
         None
     }
   }
@@ -213,14 +218,18 @@ object CometLeft extends CometExpressionSerde[Left] {
             None
         }
       case _ =>
-        withFallbackReason(expr, "LEFT len must be a literal")
+        // Unreachable: getSupportLevel gates a non-literal length.
         None
     }
   }
 
   override def getSupportLevel(expr: Left): SupportLevel = {
     expr.str.dataType match {
-      case _: BinaryType | _: StringType => Compatible()
+      case _: BinaryType | _: StringType =>
+        expr.len match {
+          case _: Literal => Compatible()
+          case _ => Unsupported(Some("LEFT len must be a literal"))
+        }
       case _ => Unsupported(Some(s"LEFT does not support ${expr.str.dataType}"))
     }
   }
@@ -256,7 +265,7 @@ object CometRight extends CometExpressionSerde[Right] {
           }
         }
       case _ =>
-        withFallbackReason(expr, "RIGHT len must be a literal")
+        // Unreachable: getSupportLevel gates a non-literal length.
         None
     }
   }
@@ -265,7 +274,11 @@ object CometRight extends CometExpressionSerde[Right] {
 
   override def getSupportLevel(expr: Right): SupportLevel = {
     expr.str.dataType match {
-      case _: StringType => Compatible()
+      case _: StringType =>
+        expr.len match {
+          case _: Literal => Compatible()
+          case _ => Unsupported(Some("RIGHT len must be a literal"))
+        }
       case _ => Unsupported(Some(s"RIGHT does not support ${expr.str.dataType}"))
     }
   }
@@ -303,6 +316,15 @@ object CometConcat extends CometScalarFunction[Concat]("concat") with CometTypeS
 
 object CometConcatWs extends CometExpressionSerde[ConcatWs] {
 
+  override def getSupportLevel(expr: ConcatWs): SupportLevel = expr.children.headOption match {
+    // A NULL separator converts directly to a NULL result, so it stays supported.
+    case Some(Literal(null, _)) => Compatible()
+    // Fall back to Spark for all-literal args so ConstantFolding can handle it.
+    case _ if expr.children.forall(_.foldable) =>
+      Unsupported(Some("all arguments are foldable"))
+    case _ => Compatible()
+  }
+
   override def convert(expr: ConcatWs, inputs: Seq[Attribute], binding: Boolean): Option[Expr] = {
     expr.children.headOption match {
       // Match Spark behavior: when the separator is NULL, the result of concat_ws is NULL.
@@ -310,11 +332,6 @@ object CometConcatWs extends CometExpressionSerde[ConcatWs] {
         val nullLiteral = Literal.create(null, expr.dataType)
         exprToProtoInternal(nullLiteral, inputs, binding)
 
-      case _ if expr.children.forall(_.foldable) =>
-        // Fall back to Spark for all-literal args so ConstantFolding can handle it
-        withFallbackReason(expr, "all arguments are foldable")
-        None
-
       case _ =>
         // For all other cases, use the generic scalar function implementation.
         CometScalarFunction[ConcatWs]("concat_ws").convert(expr, inputs, binding)
@@ -324,22 +341,23 @@ object CometConcatWs extends CometExpressionSerde[ConcatWs] {
 
 object CometLike extends CometExpressionSerde[Like] {
 
-  override def convert(expr: Like, inputs: Seq[Attribute], binding: Boolean): Option[Expr] = {
+  override def getSupportLevel(expr: Like): SupportLevel = {
     if (expr.escapeChar == '\\') {
-      createBinaryExpr(
-        expr,
-        expr.left,
-        expr.right,
-        inputs,
-        binding,
-        (builder, binaryExpr) => builder.setLike(binaryExpr))
+      Compatible()
     } else {
-      withFallbackReason(
-        expr,
-        s"custom escape character ${expr.escapeChar} not supported in LIKE")
-      None
+      Unsupported(Some(s"custom escape character ${expr.escapeChar} not supported in LIKE"))
     }
   }
+
+  override def convert(expr: Like, inputs: Seq[Attribute], binding: Boolean): Option[Expr] = {
+    createBinaryExpr(
+      expr,
+      expr.left,
+      expr.right,
+      inputs,
+      binding,
+      (builder, binaryExpr) => builder.setLike(binaryExpr))
+  }
 }
 
 /**