diff --git a/src/reader/wgsl/parser_impl.cc b/src/reader/wgsl/parser_impl.cc
index 649589b..dcf8794 100644
--- a/src/reader/wgsl/parser_impl.cc
+++ b/src/reader/wgsl/parser_impl.cc
@@ -103,10 +103,10 @@
   if (str == "position") {
     return ast::Builtin::kPosition;
   }
-  if (str == "vertex_idx" || str == "vertex_index") {
+  if (str == "vertex_index") {
     return ast::Builtin::kVertexIndex;
   }
-  if (str == "instance_idx" || str == "instance_index") {
+  if (str == "instance_index") {
     return ast::Builtin::kInstanceIndex;
   }
   if (str == "front_facing") {
@@ -121,7 +121,7 @@
   if (str == "local_invocation_id") {
     return ast::Builtin::kLocalInvocationId;
   }
-  if (str == "local_invocation_idx" || str == "local_invocation_index") {
+  if (str == "local_invocation_index") {
     return ast::Builtin::kLocalInvocationIndex;
   }
   if (str == "global_invocation_id") {
diff --git a/src/reader/wgsl/parser_impl_variable_decoration_test.cc b/src/reader/wgsl/parser_impl_variable_decoration_test.cc
index 128ec8a..db50003 100644
--- a/src/reader/wgsl/parser_impl_variable_decoration_test.cc
+++ b/src/reader/wgsl/parser_impl_variable_decoration_test.cc
@@ -113,17 +113,13 @@
     ParserImplTest,
     BuiltinTest,
     testing::Values(BuiltinData{"position", ast::Builtin::kPosition},
-                    BuiltinData{"vertex_idx", ast::Builtin::kVertexIndex},
                     BuiltinData{"vertex_index", ast::Builtin::kVertexIndex},
-                    BuiltinData{"instance_idx", ast::Builtin::kInstanceIndex},
                     BuiltinData{"instance_index", ast::Builtin::kInstanceIndex},
                     BuiltinData{"front_facing", ast::Builtin::kFrontFacing},
                     BuiltinData{"frag_coord", ast::Builtin::kFragCoord},
                     BuiltinData{"frag_depth", ast::Builtin::kFragDepth},
                     BuiltinData{"local_invocation_id",
                                 ast::Builtin::kLocalInvocationId},
-                    BuiltinData{"local_invocation_idx",
-                                ast::Builtin::kLocalInvocationIndex},
                     BuiltinData{"local_invocation_index",
                                 ast::Builtin::kLocalInvocationIndex},
                     BuiltinData{"global_invocation_id",