diff --git a/include/oi/exporters/Json.h b/include/oi/exporters/Json.h
index dd88ade..3dd37f4 100644
--- a/include/oi/exporters/Json.h
+++ b/include/oi/exporters/Json.h
@@ -98,8 +98,8 @@ inline void Json::printStringField(std::string_view name,
 inline void Json::printBoolField(std::string_view name,
                                  bool value,
                                  std::string_view indent) {
-  out_ << tab() << '"' << name << "\":" << space() << value << ',' << endl()
-       << indent;
+  out_ << tab() << '"' << name << "\":" << space() << (value ? "true" : "false")
+       << ',' << endl() << indent;
 }
 inline void Json::printUnsignedField(std::string_view name,
                                      uint64_t value,
@@ -159,6 +159,7 @@ inline void Json::printFields(const result::Element& el,
   }
   if (el.is_set_stats.has_value())
     printUnsignedField("is_set", el.is_set_stats->is_set, indent);
+  printBoolField("is_primitive", el.is_primitive, indent);
 }
 
 template <typename It>
diff --git a/include/oi/exporters/inst.h b/include/oi/exporters/inst.h
index 8397cd6..5484376 100644
--- a/include/oi/exporters/inst.h
+++ b/include/oi/exporters/inst.h
@@ -58,19 +58,22 @@ struct Field {
                   std::string_view name_,
                   const std::array<std::string_view, N0>& type_names_,
                   const std::array<Field, N1>& fields_,
-                  const std::array<ProcessorInst, N2>& processors_);
+                  const std::array<ProcessorInst, N2>& processors_,
+                  bool is_primitive_);
   template <size_t N0, size_t N1, size_t N2>
   constexpr Field(size_t static_size_,
                   std::string_view name_,
                   const std::array<std::string_view, N0>& type_names_,
                   const std::array<Field, N1>& fields_,
-                  const std::array<ProcessorInst, N2>& processors_)
+                  const std::array<ProcessorInst, N2>& processors_,
+                  bool is_primitive_)
       : Field(static_size_,
               static_size_,
               name_,
               type_names_,
               fields_,
-              processors_) {
+              processors_,
+              is_primitive_) {
   }
   constexpr Field(const Field&) = default;  // no idea why this is needed
 
@@ -80,6 +83,7 @@ struct Field {
   std::span<const std::string_view> type_names;
   std::span<const Field> fields;
   std::span<const ProcessorInst> processors;
+  bool is_primitive;
 };
 
 template <size_t N0, size_t N1, size_t N2>
@@ -88,13 +92,15 @@ constexpr Field::Field(size_t static_size_,
                        std::string_view name_,
                        const std::array<std::string_view, N0>& type_names_,
                        const std::array<Field, N1>& fields_,
-                       const std::array<ProcessorInst, N2>& processors_)
+                       const std::array<ProcessorInst, N2>& processors_,
+                       bool is_primitive_)
     : static_size(static_size_),
       exclusive_size(exclusive_size_),
       name(name_),
       type_names(type_names_),
       fields(fields_),
-      processors(processors_) {
+      processors(processors_),
+      is_primitive(is_primitive_) {
 }
 
 }  // namespace oi::exporters::inst
diff --git a/include/oi/result/Element.h b/include/oi/result/Element.h
index 1eea86b..c971236 100644
--- a/include/oi/result/Element.h
+++ b/include/oi/result/Element.h
@@ -52,6 +52,7 @@ struct Element {
       std::nullopt};
   std::optional<ContainerStats> container_stats;
   std::optional<IsSetStats> is_set_stats;
+  bool is_primitive;
 };
 
 }  // namespace oi::result
diff --git a/oi/CodeGen.cpp b/oi/CodeGen.cpp
index 84fb9b4..cb5bf54 100644
--- a/oi/CodeGen.cpp
+++ b/oi/CodeGen.cpp
@@ -798,12 +798,15 @@ void CodeGen::genClassTreeBuilderInstructions(const Class& c,
     if (m.name.starts_with(AddPadding::MemberPrefix))
       continue;
     std::string fullName = c.name() + "::" + m.name;
+    bool isPrimitive = dynamic_cast<const Primitive*>(&m.type());
     code += "      inst::Field{sizeof(" + fullName + "), " +
             std::to_string(calculateExclusiveSize(m.type())) + ",\"" +
             m.inputName + "\", member_" + std::to_string(index) +
             "_type_names, TypeHandler<Ctx, decltype(" + fullName +
             ")>::fields, TypeHandler<Ctx, decltype(" + fullName +
-            ")>::processors},\n";
+            ")>::processors, ";
+    code += isPrimitive ? "true" : "false";
+    code += "},\n";
   }
   code += "  };\n";
   code +=
@@ -1074,6 +1077,7 @@ constexpr inst::Field make_field(std::string_view name) {
     NameProvider<T>::names,
     TypeHandler<Ctx, T>::fields,
     TypeHandler<Ctx, T>::processors,
+    std::is_fundamental_v<T>
   };
 }
 )";
diff --git a/oi/FuncGen.cpp b/oi/FuncGen.cpp
index 97d2707..5ac3abf 100644
--- a/oi/FuncGen.cpp
+++ b/oi/FuncGen.cpp
@@ -399,7 +399,8 @@ const std::array<std::string_view, )";
       ", OIInternal::TypeHandler<FakeContext, "
       "OIInternal::__ROOT_TYPE__>::fields, "
       "OIInternal::TypeHandler<FakeContext, "
-      "OIInternal::__ROOT_TYPE__>::processors};\n";
+      "OIInternal::__ROOT_TYPE__>::processors, "
+      "std::is_fundamental_v<OIInternal::__ROOT_TYPE__>};\n";
   code += "} // namespace\n";
   code +=
       "extern const exporters::inst::Inst __attribute__((used, retain)) "
@@ -640,13 +641,7 @@ void FuncGen::DefineBasicTypeHandlers(std::string& code, FeatureSet features) {
         }
         static void process_pointer_content(result::Element& el, std::function<void(inst::Inst)> stack_ins, ParsedData d) {
           static constexpr std::array<std::string_view, 1> names{"TODO"};
-          static constexpr auto childField = inst::Field{
-            sizeof(T),
-            "*",
-            names,
-            TypeHandler<Ctx, T>::fields,
-            TypeHandler<Ctx, T>::processors,
-          };
+          static constexpr auto childField = make_field<Ctx, T>("*");
 
           const ParsedData::Sum& sum = std::get<ParsedData::Sum>(d.val);
 
diff --git a/oi/IntrospectionResult.cpp b/oi/IntrospectionResult.cpp
index bc59f5e..5310001 100644
--- a/oi/IntrospectionResult.cpp
+++ b/oi/IntrospectionResult.cpp
@@ -78,6 +78,7 @@ IntrospectionResult::const_iterator::operator++() {
                 .exclusive_size = ty.exclusive_size,
                 .container_stats = std::nullopt,
                 .is_set_stats = std::nullopt,
+                .is_primitive = ty.is_primitive,
             };
 
             for (const auto& [dy, handler] : ty.processors) {
diff --git a/test/integration/simple.toml b/test/integration/simple.toml
index 0945f70..4557319 100644
--- a/test/integration/simple.toml
+++ b/test/integration/simple.toml
@@ -24,12 +24,21 @@ definitions = '''
       "staticSize":16,
       "dynamicSize":0,
       "exclusiveSize": 3,
-      "size": 16,
       "members":[
         {"name":"a", "staticSize":4, "dynamicSize":0, "exclusiveSize": 4, "size": 4},
         {"name":"b", "staticSize":1, "dynamicSize":0, "exclusiveSize": 1, "size": 1},
         {"name":"c", "staticSize":8, "dynamicSize":0, "exclusiveSize": 8, "size": 8}
       ]}]'''
+    expect_json_v2 = '''[{
+      "staticSize":16,
+      "exclusiveSize": 3,
+      "size": 16,
+      "is_primitive": false,
+      "members":[
+        {"name":"a", "staticSize":4, "exclusiveSize": 4, "size": 4, "is_primitive": true},
+        {"name":"b", "staticSize":1, "exclusiveSize": 1, "size": 1, "is_primitive": true},
+        {"name":"c", "staticSize":8, "exclusiveSize": 8, "size": 8, "is_primitive": true}
+      ]}]'''
   [cases.class]
     param_types = ["const SimpleClass&"]
     setup = "return {};"
@@ -37,18 +46,32 @@ definitions = '''
       "staticSize":16,
       "dynamicSize":0,
       "exclusiveSize": 3,
-      "size": 16,
       "members":[
         {"name":"a", "staticSize":4, "dynamicSize":0, "exclusiveSize": 4, "size": 4},
         {"name":"b", "staticSize":1, "dynamicSize":0, "exclusiveSize": 1, "size": 1},
         {"name":"c", "staticSize":8, "dynamicSize":0, "exclusiveSize": 8, "size": 8}
       ]}]'''
+    expect_json_v2 = '''[{
+      "staticSize":16,
+      "exclusiveSize": 3,
+      "size": 16,
+      "is_primitive": false,
+      "members":[
+        {"name":"a", "staticSize":4, "exclusiveSize": 4, "size": 4, "is_primitive": true},
+        {"name":"b", "staticSize":1, "exclusiveSize": 1, "size": 1, "is_primitive": true},
+        {"name":"c", "staticSize":8, "exclusiveSize": 8, "size": 8, "is_primitive": true}
+      ]}]'''
   [cases.union]
     param_types = ["const SimpleUnion&"]
     setup = "return {};"
     expect_json = '''[{
       "staticSize":8,
       "dynamicSize":0,
-      "exclusiveSize":8,
-      "size":8
+      "exclusiveSize":8
+    }]'''
+    expect_json_v2 = '''[{
+      "staticSize":8,
+      "exclusiveSize":8,
+      "size":8,
+      "is_primitive":false
     }]'''
diff --git a/types/f14_fast_map.toml b/types/f14_fast_map.toml
index 06c6f72..69a5173 100644
--- a/types/f14_fast_map.toml
+++ b/types/f14_fast_map.toml
@@ -95,6 +95,7 @@ static constexpr inst::Field element{
   std::array<std::string_view, 0>{},
   element_fields,
   std::array<inst::ProcessorInst, 0>{},
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 for (size_t i = 0; i < list.length; i++)
diff --git a/types/f14_node_map.toml b/types/f14_node_map.toml
index de1dcda..cc89f06 100644
--- a/types/f14_node_map.toml
+++ b/types/f14_node_map.toml
@@ -95,6 +95,7 @@ static constexpr inst::Field element{
   std::array<std::string_view, 0>{},
   element_fields,
   std::array<inst::ProcessorInst, 0>{},
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 for (size_t i = 0; i < list.length; i++)
diff --git a/types/f14_value_map.toml b/types/f14_value_map.toml
index c09bbf0..aa6aa59 100644
--- a/types/f14_value_map.toml
+++ b/types/f14_value_map.toml
@@ -95,6 +95,7 @@ static constexpr inst::Field element{
   std::array<std::string_view, 0>{},
   element_fields,
   std::array<inst::ProcessorInst, 0>{},
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 for (size_t i = 0; i < list.length; i++)
diff --git a/types/f14_vector_map.toml b/types/f14_vector_map.toml
index a673bbd..f640dcc 100644
--- a/types/f14_vector_map.toml
+++ b/types/f14_vector_map.toml
@@ -95,6 +95,7 @@ static constexpr inst::Field element{
   std::array<std::string_view, 0>{},
   element_fields,
   std::array<inst::ProcessorInst, 0>{},
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 for (size_t i = 0; i < list.length; i++)
diff --git a/types/map_seq_type.toml b/types/map_seq_type.toml
index a09efff..f4281eb 100644
--- a/types/map_seq_type.toml
+++ b/types/map_seq_type.toml
@@ -91,6 +91,7 @@ static constexpr auto entry = inst::Field {
   std::array<std::string_view, 0>{},
   entryFields,
   processors,
+  entryFields[0].is_primitive && entryFields[1].is_primitive,
 };
 
 auto list = std::get<ParsedData::List>(d.val);
diff --git a/types/multi_map_type.toml b/types/multi_map_type.toml
index a5d628f..e313417 100644
--- a/types/multi_map_type.toml
+++ b/types/multi_map_type.toml
@@ -118,6 +118,7 @@ static constexpr auto element = inst::Field {
   std::array<std::string_view, 0>{},
   elementFields,
   std::array<inst::ProcessorInst, 0>{},
+  elementFields[0].is_primitive && elementFields[1].is_primitive,
 };
 
 auto list = std::get<ParsedData::List>(d.val);
diff --git a/types/std_map_type.toml b/types/std_map_type.toml
index ee652ec..b972b70 100644
--- a/types/std_map_type.toml
+++ b/types/std_map_type.toml
@@ -135,6 +135,7 @@ static constexpr inst::Field element{
   std::array<std::string_view, 0>{},
   element_fields,
   processors,
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 auto list = std::get<ParsedData::List>(d.val);
diff --git a/types/std_unordered_map_type.toml b/types/std_unordered_map_type.toml
index 5b2402f..108731e 100644
--- a/types/std_unordered_map_type.toml
+++ b/types/std_unordered_map_type.toml
@@ -135,6 +135,7 @@ static constexpr auto element = inst::Field{
   std::array<std::string_view, 0>{},
   element_fields,
   processors,
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 for (size_t i = 0; i < list.length; i++)
diff --git a/types/std_unordered_multimap_type.toml b/types/std_unordered_multimap_type.toml
index a6f7671..8452154 100644
--- a/types/std_unordered_multimap_type.toml
+++ b/types/std_unordered_multimap_type.toml
@@ -121,6 +121,7 @@ static constexpr auto element = inst::Field{
   std::array<std::string_view, 0>{},
   element_fields,
   std::array<inst::ProcessorInst, 0>{},
+  element_fields[0].is_primitive && element_fields[1].is_primitive,
 };
 
 for (size_t i = 0; i < list.length; i++)