diff --git a/dev.oid.toml b/dev.oid.toml
index 9e3ac26..bf70202 100644
--- a/dev.oid.toml
+++ b/dev.oid.toml
@@ -10,6 +10,7 @@ containers = [
     "PWD/types/folly_iobuf_type.toml",
     "PWD/types/folly_iobuf_queue_type.toml",
     "PWD/types/set_type.toml",
+    "PWD/types/multi_set_type.toml",
     "PWD/types/unordered_set_type.toml",
     "PWD/types/seq_type.toml",
     "PWD/types/list_type.toml",
diff --git a/oi/ContainerTypeEnum.h b/oi/ContainerTypeEnum.h
index 664bc7e..a218754 100644
--- a/oi/ContainerTypeEnum.h
+++ b/oi/ContainerTypeEnum.h
@@ -20,6 +20,7 @@
   X(ARRAY_TYPE)                  \
   X(SMALL_VEC_TYPE)              \
   X(SET_TYPE)                    \
+  X(MULTI_SET_TYPE)              \
   X(UNORDERED_SET_TYPE)          \
   X(SEQ_TYPE)                    \
   X(LIST_TYPE)                   \
diff --git a/oi/TreeBuilder.cpp b/oi/TreeBuilder.cpp
index 5fe924e..7aff17d 100644
--- a/oi/TreeBuilder.cpp
+++ b/oi/TreeBuilder.cpp
@@ -843,6 +843,7 @@ void TreeBuilder::processContainer(const Variable& variable, Node& node) {
       containerStats.length = next();
       break;
     case RADIX_TREE_TYPE:
+    case MULTI_SET_TYPE:
     case MULTI_MAP_TYPE:
     case BY_MULTI_QRT_TYPE:
       containerStats.length = containerStats.capacity = next();
diff --git a/test/ci.oid.toml b/test/ci.oid.toml
index 2b39fe1..5ac1a80 100644
--- a/test/ci.oid.toml
+++ b/test/ci.oid.toml
@@ -6,6 +6,7 @@ containers = [
     "../types/folly_iobuf_type.toml",
     "../types/folly_iobuf_queue_type.toml",
     "../types/set_type.toml",
+    "../types/multi_set_type.toml",
     "../types/unordered_set_type.toml",
     "../types/seq_type.toml",
     "../types/list_type.toml",
diff --git a/test/integration/std_multiset_custom_comparator.toml b/test/integration/std_multiset_custom_comparator.toml
new file mode 100644
index 0000000..04d5da8
--- /dev/null
+++ b/test/integration/std_multiset_custom_comparator.toml
@@ -0,0 +1,73 @@
+definitions = '''
+  struct CustomComparator {
+    bool operator()(const int& left, const int& right) const {
+      return left < right;
+    }
+  };
+
+  struct SmallSizedCustomComparator {
+    double a;
+    bool operator()(const int& left, const int& right) const {
+      return left < right;
+    }
+  };
+
+  struct BigSizedCustomComparator {
+    double d[1000];
+    bool operator()(const int& left, const int& right) const {
+      return left < right;
+    }
+  };
+
+  struct Foo {
+    std::multiset<int> m1;
+    std::multiset<int, CustomComparator> m2;
+    std::multiset<int, SmallSizedCustomComparator> m3;
+    std::multiset<int, BigSizedCustomComparator> m4;
+  };
+
+'''
+includes = ["set", "functional"]
+
+[cases]
+  [cases.a]
+    param_types = ["const Foo&"]
+    setup = '''
+      Foo foo;
+
+      for (int i = 0; i < 3; i++) {
+        foo.m1.insert(i);
+      }
+
+      for (int i = 0; i < 5; i++) {
+        foo.m2.insert(i);
+      }
+
+      for (int i = 0; i < 7; i++) {
+        foo.m3.insert(i);
+      }
+
+      for (int i = 0; i < 9; i++) {
+        foo.m4.insert(i);
+      }
+
+      return {foo};
+    '''
+    expect_json = '''[{
+      "staticSize":8184,
+      "dynamicSize":96,
+      "members":[
+        {"name":"m1", "staticSize":48, "dynamicSize":12, "length":3, "capacity":3, "elementStaticSize":4},
+        {"name":"m2", "staticSize":48, "dynamicSize":20, "length":5, "capacity":5, "elementStaticSize":4},
+        {"name":"m3", "staticSize":48, "dynamicSize":28, "length":7, "capacity":7, "elementStaticSize":4},
+        {"name":"m4", "staticSize":8040, "dynamicSize":36, "length":9, "capacity":9, "elementStaticSize":4}
+      ]}]'''
+    expect_json_v2 = '''[{
+      "staticSize":8184,
+      "exclusiveSize": 0,
+      "members":[
+        {"name":"m1", "staticSize":48, "exclusiveSize": 156, "length": 3, "capacity": 3},
+        {"name":"m2", "staticSize":48, "exclusiveSize": 228, "length": 5, "capacity": 5},
+        {"name":"m3", "staticSize":48, "exclusiveSize": 300, "length": 7, "capacity": 7},
+        {"name":"m4", "staticSize":8040, "exclusiveSize": 8364, "length": 9, "capacity": 9}
+      ]}]'''
diff --git a/types/multi_set_type.toml b/types/multi_set_type.toml
new file mode 100644
index 0000000..d693cb4
--- /dev/null
+++ b/types/multi_set_type.toml
@@ -0,0 +1,149 @@
+[info]
+type_name = "std::multiset"
+stub_template_params = [1,2]
+ctype = "MULTI_SET_TYPE"
+header = "set"
+
+# Old:
+typeName = "std::multiset<"
+ns = ["namespace std"]
+numTemplateParams = 1
+replaceTemplateParamIndex = [1]
+allocatorIndex = 2
+
+[codegen]
+decl = """
+template<typename Key, typename Compare, typename Alloc>
+void getSizeType(const %1%<Key, Compare, Alloc> &container, size_t& returnArg);
+"""
+
+func = """
+template<typename Key, typename Compare, typename Alloc>
+void getSizeType(const %1%<Key, Compare, Alloc> &container, size_t& returnArg)
+{
+    constexpr size_t nodeSize = sizeof(typename %1%<Key, Compare, Alloc>::node_type);
+    size_t numElems = container.size();
+
+    SAVE_SIZE(sizeof(%1%<Key, Compare, Alloc>) + (nodeSize * numElems));
+
+    SAVE_DATA((uintptr_t)numElems);
+
+    // The double ampersand is needed otherwise this loop doesn't work with vector<bool>
+    for (auto&& it: container) {
+    getSizeType(it, returnArg);
+    }
+}
+"""
+
+handler = """
+template <typename DB, typename T0, typename T1, typename T2>
+struct TypeHandler<DB, %1% <T0, T1, T2>> {
+  using type = types::st::List<DB, typename TypeHandler<DB, T0>::type>;
+
+  static types::st::Unit<DB> getSizeType(
+      const %1%<T0, T1, T2>& container,
+      typename TypeHandler<DB, %1%<T0, T1, T2>>::type returnArg) {
+    constexpr size_t nodeSize = sizeof(typename %1%<T0, T1, T2>::node_type);
+
+    auto tail = returnArg.write(container.size());
+
+    // The double ampersand is needed otherwise this loop doesn't work with
+    // vector<bool>
+    for (auto&& it : container) {
+      tail = tail.delegate([&it](auto ret) {
+        return OIInternal::getSizeType<DB>(it, ret);
+      });
+    }
+
+    return tail.finish();
+  }
+};
+"""
+
+traversal_func = """
+auto tail = returnArg.write((uintptr_t)&container)
+                .write(container.size());
+
+// The double ampersand is needed otherwise this loop doesn't work with
+// vector<bool>
+for (auto&& it : container) {
+  tail = tail.delegate([&it](auto ret) {
+    return OIInternal::getSizeType<DB>(it, ret);
+  });
+}
+
+return tail.finish();
+"""
+
+[[codegen.processor]]
+type = "types::st::VarInt<DB>"
+func = """
+el.pointer = std::get<ParsedData::VarInt>(d.val).value;
+"""
+
+[[codegen.processor]]
+type = "types::st::List<DB, typename TypeHandler<DB, T0>::type>"
+func = """
+#ifdef __GLIBCXX__
+/* We don't have access to the _Rb_tree_node struct, so we manually re-create it
+ * to get the effective size of a multiset entry. Is there a better way to do this?
+ *
+ * https://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a00716_source.html#l00216
+ * From the source of <bits/stl_tree.h>, an _Rb_tree_node has the following members:
+ */
+struct OI_Rb_tree_node {
+  using _Rb_tree_color = int;       // enum
+  using _Base_ptr = std::nullptr_t; // pointer
+
+  _Rb_tree_color      _M_color;  // from parent _Rb_tree_node_base
+  _Base_ptr           _M_parent; // from parent _Rb_tree_node_base
+  _Base_ptr           _M_left;   // from parent _Rb_tree_node_base
+  _Base_ptr           _M_right;  // from parent _Rb_tree_node_base
+  T0                  _M_value;
+};
+
+static constexpr size_t element_size = sizeof(OI_Rb_tree_node);
+#elif _LIBCPP_VERSION
+static_assert(false && "libc++ is currently not supported");
+
+/* We don't have access to the __tree_node struct, so we manually re-create it
+ * to get the effective size of a multiset entry. Is there a better way to do this?
+ *
+ * https://github.com/llvm/llvm-project/blob/1b10920164695a487669405223f8bbe93799430c/libcxx/include/__tree#L729-L781
+ * From the source of <__tree>, a __tree_node has the following members:
+ */
+struct OI__tree_node {
+  using pointer = std::nullptr_t;          // pointer
+  using __parent_pointer = std::nullptr_t; // pointer
+
+  pointer           __left_;     // from parent __tree_end_node
+  pointer           __right_;    // from parent __tree_node_base
+  __parent_pointer  __parent_;   // from parent __tree_node_base
+  bool              __is_black_; // from parent __tree_node_base
+  T0                __value_;
+};
+
+static constexpr size_t element_size = sizeof(OI__tree_node);
+#else
+static_assert(false && "No known element_size for multisets. See types/multi_set_type.toml");
+#endif
+
+static constexpr std::array<std::string_view, 1> names{"TODO"};
+static constexpr auto childField = inst::Field{
+  sizeof(T0),
+  "[]",
+  names,
+  TypeHandler<DB, T0>::fields,
+  TypeHandler<DB, T0>::processors,
+};
+
+auto list = std::get<ParsedData::List>(d.val);
+el.container_stats.emplace(result::Element::ContainerStats {
+  .capacity = list.length,
+  .length = list.length,
+});
+el.exclusive_size += el.container_stats->length * (element_size - sizeof(T0));
+
+for (size_t i = 0; i < list.length; i++)
+  ins.emplace(childField);
+"""