diff --git a/src/indexer.h b/src/indexer.h
index f7ec4e00..13547e16 100644
--- a/src/indexer.h
+++ b/src/indexer.h
@@ -239,7 +239,7 @@ template <typename TVisitor,
           typename Range>
 void Reflect(TVisitor& visitor,
              TypeDefDefinitionData<TypeId, FuncId, VarId, Range>& value) {
-  REFLECT_MEMBER_START(12);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(short_name);
   REFLECT_MEMBER(detailed_name);
   REFLECT_MEMBER(kind);
@@ -338,7 +338,7 @@ template <typename TVisitor,
 void Reflect(
     TVisitor& visitor,
     FuncDefDefinitionData<TypeId, FuncId, VarId, FuncRef, Range>& value) {
-  REFLECT_MEMBER_START(12);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(short_name);
   REFLECT_MEMBER(detailed_name);
   REFLECT_MEMBER(kind);
@@ -452,7 +452,7 @@ template <typename TVisitor,
           typename Range>
 void Reflect(TVisitor& visitor,
              VarDefDefinitionData<TypeId, FuncId, VarId, Range>& value) {
-  REFLECT_MEMBER_START(9);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(short_name);
   REFLECT_MEMBER(detailed_name);
   REFLECT_MEMBER(kind);
diff --git a/src/language_server_api.cc b/src/language_server_api.cc
index 31940b7e..bd487ac3 100644
--- a/src/language_server_api.cc
+++ b/src/language_server_api.cc
@@ -194,7 +194,7 @@ void lsResponseError::Write(Writer& visitor) {
   auto& value = *this;
   int code2 = static_cast<int>(this->code);
 
-  visitor.StartObject(2 + !!data);
+  visitor.StartObject();
   REFLECT_MEMBER2("code", code2);
   REFLECT_MEMBER(message);
   if (data) {
diff --git a/src/language_server_api.h b/src/language_server_api.h
index 860c942d..4bbbdafd 100644
--- a/src/language_server_api.h
+++ b/src/language_server_api.h
@@ -226,7 +226,7 @@ struct lsCommand {
 };
 template <typename TVisitor, typename T>
 void Reflect(TVisitor& visitor, lsCommand<T>& value) {
-  REFLECT_MEMBER_START(3);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(title);
   REFLECT_MEMBER(command);
   REFLECT_MEMBER(arguments);
@@ -245,7 +245,7 @@ struct lsCodeLens {
 };
 template <typename TVisitor, typename TData, typename TCommandArguments>
 void Reflect(TVisitor& visitor, lsCodeLens<TData, TCommandArguments>& value) {
-  REFLECT_MEMBER_START(3);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(range);
   REFLECT_MEMBER(command);
   REFLECT_MEMBER(data);
@@ -979,7 +979,7 @@ struct Out_TextDocumentPublishDiagnostics
 template <typename TVisitor>
 void Reflect(TVisitor& visitor, Out_TextDocumentPublishDiagnostics& value) {
   std::string method = "textDocument/publishDiagnostics";
-  REFLECT_MEMBER_START(3);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(jsonrpc);
   REFLECT_MEMBER2("method", method);
   REFLECT_MEMBER(params);
@@ -1044,7 +1044,7 @@ struct Out_ShowLogMessage : public lsOutMessage<Out_ShowLogMessage> {
 };
 template <typename TVisitor>
 void Reflect(TVisitor& visitor, Out_ShowLogMessage& value) {
-  REFLECT_MEMBER_START(3);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(jsonrpc);
   std::string method = value.method();
   REFLECT_MEMBER2("method", method);
diff --git a/src/messages/text_document_hover.cc b/src/messages/text_document_hover.cc
index 36ccc3af..61e6dc7e 100644
--- a/src/messages/text_document_hover.cc
+++ b/src/messages/text_document_hover.cc
@@ -58,7 +58,7 @@ struct Out_TextDocumentHover : public lsOutMessage<Out_TextDocumentHover> {
 };
 MAKE_REFLECT_STRUCT(Out_TextDocumentHover::Result, contents, range);
 void Reflect(Writer& visitor, Out_TextDocumentHover& value) {
-  REFLECT_MEMBER_START(3);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(jsonrpc);
   REFLECT_MEMBER(id);
   if (value.result)
diff --git a/src/position.cc b/src/position.cc
index 8f506857..f3514f25 100644
--- a/src/position.cc
+++ b/src/position.cc
@@ -66,9 +66,9 @@ bool Position::operator!=(const Position& that) const {
 }
 
 bool Position::operator<(const Position& that) const {
-  if (line < that.line)
-    return true;
-  return line == that.line && column < that.column;
+  if (line != that.line)
+    return line < that.line;
+  return column < that.column;
 }
 
 Range::Range() {}
@@ -137,9 +137,9 @@ bool Range::operator!=(const Range& that) const {
 }
 
 bool Range::operator<(const Range& that) const {
-  if (start < that.start)
-    return true;
-  return start == that.start && end < that.end;
+  if (start != that.start)
+    return start < that.start;
+  return end < that.end;
 }
 
 // Position
@@ -148,18 +148,8 @@ void Reflect(Reader& visitor, Position& value) {
     std::string s = visitor.GetString();
     value = Position(s.c_str());
   } else {
-    int i = 0;
-    // FIXME
-    static_cast<MessagePackReader&>(visitor).IterArray([&](Reader& entry) {
-      switch (i++) {
-        case 0:
-          Reflect(entry, value.line);
-          break;
-        case 1:
-          Reflect(entry, value.column);
-          break;
-      }
-    });
+    Reflect(visitor, value.line);
+    Reflect(visitor, value.column);
   }
 }
 void Reflect(Writer& visitor, Position& value) {
@@ -167,10 +157,8 @@ void Reflect(Writer& visitor, Position& value) {
     std::string output = value.ToString();
     visitor.String(output.c_str(), output.size());
   } else {
-    REFLECT_MEMBER_START(2);
     Reflect(visitor, value.line);
     Reflect(visitor, value.column);
-    REFLECT_MEMBER_END();
   }
 }
 
@@ -180,25 +168,10 @@ void Reflect(Reader& visitor, Range& value) {
     std::string s = visitor.GetString();
     value = Range(s.c_str());
   } else {
-    int i = 0;
-    // FIXME
-    static_cast<MessagePackReader&>(visitor).IterArray([&](Reader& entry) {
-      switch (i++) {
-        case 0:
-          Reflect(entry, value.start.line);
-          break;
-        case 1:
-          Reflect(entry, value.start.column);
-          break;
-        case 2:
-          Reflect(entry, value.end.line);
-          break;
-        case 3:
-          Reflect(entry, value.end.column);
-          break;
-      }
-      i++;
-    });
+    Reflect(visitor, value.start.line);
+    Reflect(visitor, value.start.column);
+    Reflect(visitor, value.end.line);
+    Reflect(visitor, value.end.column);
   }
 }
 void Reflect(Writer& visitor, Range& value) {
@@ -206,11 +179,9 @@ void Reflect(Writer& visitor, Range& value) {
     std::string output = value.ToString();
     visitor.String(output.c_str(), output.size());
   } else {
-    REFLECT_MEMBER_START(4);
     Reflect(visitor, value.start.line);
     Reflect(visitor, value.start.column);
     Reflect(visitor, value.end.line);
     Reflect(visitor, value.end.column);
-    REFLECT_MEMBER_END();
   }
 }
diff --git a/src/query.h b/src/query.h
index 6091f990..046c5ac4 100644
--- a/src/query.h
+++ b/src/query.h
@@ -161,7 +161,7 @@ struct MergeableUpdate {
 };
 template <typename TVisitor, typename TId, typename TValue>
 void Reflect(TVisitor& visitor, MergeableUpdate<TId, TValue>& value) {
-  REFLECT_MEMBER_START(3);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(id);
   REFLECT_MEMBER(to_add);
   REFLECT_MEMBER(to_remove);
@@ -177,7 +177,7 @@ struct WithUsr {
 };
 template <typename TVisitor, typename T>
 void Reflect(TVisitor& visitor, WithUsr<T>& value) {
-  REFLECT_MEMBER_START(2);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(usr);
   REFLECT_MEMBER(value);
   REFLECT_MEMBER_END();
diff --git a/src/serializer.cc b/src/serializer.cc
index b6dee5ea..271bce3a 100644
--- a/src/serializer.cc
+++ b/src/serializer.cc
@@ -85,13 +85,13 @@ void ReflectMember(Writer& visitor, const char* name, std::string& value) {
 
 // TODO: Move this to indexer.cc
 void Reflect(Reader& visitor, IndexInclude& value) {
-  REFLECT_MEMBER_START(2);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(line);
   REFLECT_MEMBER(resolved_path);
   REFLECT_MEMBER_END();
 }
 void Reflect(Writer& visitor, IndexInclude& value) {
-  REFLECT_MEMBER_START(2);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER(line);
   if (gTestOutputMode) {
     std::string basename = GetBaseName(value.resolved_path);
@@ -106,7 +106,7 @@ void Reflect(Writer& visitor, IndexInclude& value) {
 
 template <typename TVisitor>
 void Reflect(TVisitor& visitor, IndexType& value) {
-  REFLECT_MEMBER_START(17);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER2("id", value.id);
   REFLECT_MEMBER2("usr", value.usr);
   REFLECT_MEMBER2("short_name", value.def.short_name);
@@ -129,7 +129,7 @@ void Reflect(TVisitor& visitor, IndexType& value) {
 
 template <typename TVisitor>
 void Reflect(TVisitor& visitor, IndexFunc& value) {
-  REFLECT_MEMBER_START(17);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER2("id", value.id);
   REFLECT_MEMBER2("is_operator", value.def.is_operator);
   REFLECT_MEMBER2("usr", value.usr);
@@ -152,7 +152,7 @@ void Reflect(TVisitor& visitor, IndexFunc& value) {
 
 template <typename TVisitor>
 void Reflect(TVisitor& visitor, IndexVar& value) {
-  REFLECT_MEMBER_START(13);
+  REFLECT_MEMBER_START();
   REFLECT_MEMBER2("id", value.id);
   REFLECT_MEMBER2("usr", value.usr);
   REFLECT_MEMBER2("short_name", value.def.short_name);
@@ -170,7 +170,7 @@ void Reflect(TVisitor& visitor, IndexVar& value) {
 }
 
 // IndexFile
-bool ReflectMemberStart(Writer& visitor, IndexFile& value, size_t n) {
+bool ReflectMemberStart(Writer& visitor, IndexFile& value) {
   auto it = value.id_cache.usr_to_type_id.find("");
   if (it != value.id_cache.usr_to_type_id.end()) {
     value.Resolve(it->second)->def.short_name = "<fundamental>";
@@ -178,12 +178,12 @@ bool ReflectMemberStart(Writer& visitor, IndexFile& value, size_t n) {
   }
 
   value.version = IndexFile::kCurrentVersion;
-  DefaultReflectMemberStart(visitor, n);
+  DefaultReflectMemberStart(visitor);
   return true;
 }
 template <typename TVisitor>
 void Reflect(TVisitor& visitor, IndexFile& value) {
-  REFLECT_MEMBER_START(5 + (gTestOutputMode ? 0 : 6));
+  REFLECT_MEMBER_START();
   if (!gTestOutputMode) {
     REFLECT_MEMBER(version);
     REFLECT_MEMBER(last_modification_time);
@@ -257,10 +257,12 @@ std::unique_ptr<IndexFile> Deserialize(SerializeFormat format,
       if (serialized.empty())
         return nullptr;
       try {
-        msgpack::object_handle oh =
-            msgpack::unpack(serialized.data(), serialized.size());
+        msgpack::unpacker upk;
+        upk.reserve_buffer(serialized.size());
+        memcpy(upk.buffer(), serialized.data(), serialized.size());
+        upk.buffer_consumed(serialized.size());
         file = MakeUnique<IndexFile>(path);
-        MessagePackReader reader(oh.get());
+        MessagePackReader reader(&upk);
         Reflect(reader, *file);
         if (file->version != expected_version)
           return nullptr;
diff --git a/src/serializer.h b/src/serializer.h
index 0143cef6..801236af 100644
--- a/src/serializer.h
+++ b/src/serializer.h
@@ -23,6 +23,7 @@ class Reader {
   //virtual bool IsUint64() = 0;
   virtual bool IsString() = 0;
 
+  virtual void GetNull() = 0;
   virtual bool GetBool() = 0;
   virtual int GetInt() = 0;
   virtual int64_t GetInt64() = 0;
@@ -50,19 +51,19 @@ class Writer {
   virtual void String(const char* x, size_t len) = 0;
   virtual void StartArray(size_t) = 0;
   virtual void EndArray() = 0;
-  virtual void StartObject(size_t) = 0;
+  virtual void StartObject() = 0;
   virtual void EndObject() = 0;
   virtual void Key(const char* name) = 0;
 };
 
 struct IndexFile;
 
-#define REFLECT_MEMBER_START(n)             \
-  if (!ReflectMemberStart(visitor, value, n)) \
-  return
-#define REFLECT_MEMBER_START1(value)       \
-  if (!ReflectMemberStart(visitor, value)) \
-  return
+#define REFLECT_MEMBER_START()                  \
+  if (!ReflectMemberStart(visitor, value))      \
+    return
+#define REFLECT_MEMBER_START1(value)            \
+  if (!ReflectMemberStart(visitor, value))      \
+    return
 #define REFLECT_MEMBER_END() ReflectMemberEnd(visitor, value);
 #define REFLECT_MEMBER_END1(value) ReflectMemberEnd(visitor, value);
 #define REFLECT_MEMBER(name) ReflectMember(visitor, #name, value.name)
@@ -76,38 +77,38 @@ struct IndexFile;
     value = static_cast<type>(value0);           \
   }
 
-// clang-format off
-// Config has many fields, we need to support at least its number of fields.
-#define NUM_VA_ARGS_IMPL(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,_11,_12,_13,_14,_15,_16,_17,_18,_19,_20,_21,_22,_23,_24,_25,_26,_27,_28,_29,_30,N,...) N
-#define NUM_VA_ARGS(...) NUM_VA_ARGS_IMPL(__VA_ARGS__,30,29,28,27,26,25,24,23,22,21,20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1)
-// clang-format on
-
 #define _MAPPABLE_REFLECT_MEMBER(name) REFLECT_MEMBER(name);
 
 #define MAKE_REFLECT_EMPTY_STRUCT(type, ...)     \
   template <typename TVisitor>                   \
   void Reflect(TVisitor& visitor, type& value) { \
-    REFLECT_MEMBER_START(0);                     \
+    REFLECT_MEMBER_START();                      \
     REFLECT_MEMBER_END();                        \
   }
 
 #define MAKE_REFLECT_STRUCT(type, ...)               \
   template <typename TVisitor>                       \
   void Reflect(TVisitor& visitor, type& value) {     \
-    REFLECT_MEMBER_START(NUM_VA_ARGS(__VA_ARGS__));  \
+    REFLECT_MEMBER_START();                          \
     MACRO_MAP(_MAPPABLE_REFLECT_MEMBER, __VA_ARGS__) \
     REFLECT_MEMBER_END();                            \
   }
 
+// clang-format off
+// Config has many fields, we need to support at least its number of fields.
+#define NUM_VA_ARGS_IMPL(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,_11,_12,_13,_14,_15,_16,_17,_18,_19,_20,_21,_22,_23,_24,_25,_26,_27,_28,_29,_30,N,...) N
+#define NUM_VA_ARGS(...) NUM_VA_ARGS_IMPL(__VA_ARGS__,30,29,28,27,26,25,24,23,22,21,20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1)
+// clang-format on
+
 #define _MAPPABLE_REFLECT_ARRAY(name) Reflect(visitor, value.name);
 
 // Reflects the struct so it is serialized as an array instead of an object.
 // This currently only supports writers.
-#define MAKE_REFLECT_STRUCT_WRITER_AS_ARRAY(type, ...) \
-  inline void Reflect(Writer& visitor, type& value) {  \
-    visitor.StartArray(NUM_VA_ARGS(__VA_ARGS__));      \
-    MACRO_MAP(_MAPPABLE_REFLECT_ARRAY, __VA_ARGS__)    \
-    visitor.EndArray();                                \
+#define MAKE_REFLECT_STRUCT_WRITER_AS_ARRAY(type, ...)  \
+  inline void Reflect(Writer& visitor, type& value) {   \
+    visitor.StartArray(NUM_VA_ARGS(__VA_ARGS__));       \
+    MACRO_MAP(_MAPPABLE_REFLECT_ARRAY, __VA_ARGS__)     \
+        visitor.EndArray();                             \
   }
 
 // API:
@@ -157,8 +158,10 @@ void Reflect(Writer& visitor, std::string& value);
 // std::optional
 template <typename T>
 void Reflect(Reader& visitor, optional<T>& value) {
-  if (visitor.IsNull())
+  if (visitor.IsNull()) {
+    visitor.GetNull();
     return;
+  }
   T real_value{};
   Reflect(visitor, real_value);
   value = real_value;
@@ -201,12 +204,12 @@ void Reflect(Writer& visitor, std::vector<T>& values) {
 
 // Writer:
 
-inline void DefaultReflectMemberStart(Writer& visitor, size_t n) {
-  visitor.StartObject(n);
+inline void DefaultReflectMemberStart(Writer& visitor) {
+  visitor.StartObject();
 }
 template <typename T>
-bool ReflectMemberStart(Writer& visitor, T& value, size_t n) {
-  visitor.StartObject(n);
+bool ReflectMemberStart(Writer& visitor, T& value) {
+  visitor.StartObject();
   return true;
 }
 template <typename T>
@@ -238,9 +241,9 @@ void ReflectMember(Writer& visitor, const char* name, std::string& value);
 // Reader:
 
 
-inline void DefaultReflectMemberStart(Reader& visitor, size_t n) {}
+inline void DefaultReflectMemberStart(Reader& visitor) {}
 template <typename T>
-bool ReflectMemberStart(Reader& visitor, T& value, size_t n) {
+bool ReflectMemberStart(Reader& visitor, T& value) {
   return true;
 }
 template <typename T>
diff --git a/src/serializers/json.h b/src/serializers/json.h
index 8cb92bc8..38b2cf65 100644
--- a/src/serializers/json.h
+++ b/src/serializers/json.h
@@ -20,6 +20,7 @@ class JsonReader : public Reader {
   //bool IsUint64() override { return m_->IsUint64(); }
   bool IsString() override { return m_->IsString(); }
 
+  void GetNull() override {}
   bool GetBool() override { return m_->GetBool(); }
   int GetInt() override { return m_->GetInt(); }
   int64_t GetInt64() override { return m_->GetInt64(); }
@@ -67,7 +68,7 @@ class JsonWriter : public Writer {
   void String(const char* x, size_t len) override { m_->String(x, len); }
   void StartArray(size_t) override { m_->StartArray(); }
   void EndArray() override { m_->EndArray(); }
-  void StartObject(size_t) override { m_->StartObject(); }
+  void StartObject() override { m_->StartObject(); }
   void EndObject() override { m_->EndObject(); }
   void Key(const char* name) override { m_->Key(name); }
 };
diff --git a/src/serializers/msgpack.h b/src/serializers/msgpack.h
index c2468e8a..9d5a57e2 100644
--- a/src/serializers/msgpack.h
+++ b/src/serializers/msgpack.h
@@ -5,27 +5,34 @@
 #include <msgpack.hpp>
 
 class MessagePackReader : public Reader {
-  msgpack::object o_;
-  size_t idx_ = 0;
-  std::vector<msgpack::object> children_;
+  msgpack::unpacker* pk_;
+  msgpack::object_handle oh_;
+
+  template <typename T>
+  T Get() {
+    T ret = oh_.get().as<T>();
+    pk_->next(oh_);
+    return ret;
+  }
 
  public:
-  MessagePackReader(msgpack::object o) : o_(o) {}
+  MessagePackReader(msgpack::unpacker* pk) : pk_(pk) { pk->next(oh_); }
   SerializeFormat Format() const override { return SerializeFormat::MessagePack; }
 
-  bool IsNull() override { return o_.is_nil(); }
-  bool IsArray() override { return o_.type == msgpack::type::ARRAY; }
+  bool IsNull() override { return oh_.get().is_nil(); }
+  bool IsArray() override { return oh_.get().type == msgpack::type::ARRAY; }
   bool IsInt() override {
-    return o_.type == msgpack::type::POSITIVE_INTEGER ||
-           o_.type == msgpack::type::NEGATIVE_INTEGER;
+    return oh_.get().type == msgpack::type::POSITIVE_INTEGER ||
+           oh_.get().type == msgpack::type::NEGATIVE_INTEGER;
   }
-  bool IsString() override { return o_.type == msgpack::type::STR; }
+  bool IsString() override { return oh_.get().type == msgpack::type::STR; }
 
-  bool GetBool() override { return o_.as<bool>(); }
-  int GetInt() override { return o_.as<int>(); }
-  int64_t GetInt64() override { return o_.as<int64_t>(); }
-  uint64_t GetUint64() override { return o_.as<uint64_t>(); }
-  std::string GetString() override { return o_.as<std::string>(); }
+  void GetNull() override { pk_->next(oh_); }
+  bool GetBool() override { return Get<bool>(); }
+  int GetInt() override { return Get<int>(); }
+  int64_t GetInt64() override { return Get<int64_t>(); }
+  uint64_t GetUint64() override { return Get<uint64_t>(); }
+  std::string GetString() override { return Get<std::string>(); }
 
   bool HasMember(const char* x) override { return true; }
   std::unique_ptr<Reader> operator[](const char* x) override {
@@ -33,17 +40,13 @@ class MessagePackReader : public Reader {
   }
 
   void IterArray(std::function<void(Reader&)> fn) override {
-    for (auto& entry : o_.as<std::vector<msgpack::object>>()) {
-      MessagePackReader sub(entry);
-      fn(sub);
-    }
+    size_t n = Get<size_t>();
+    for (size_t i = 0; i < n; i++)
+      fn(*this);
   }
 
-  void DoMember(const char* name, std::function<void(Reader&)> fn) override {
-    if (idx_ == 0)
-      children_ = o_.as<std::vector<msgpack::object>>();
-    MessagePackReader sub(children_[idx_++]);
-    fn(sub);
+  void DoMember(const char*, std::function<void(Reader&)> fn) override {
+    fn(*this);
   }
 };
 
@@ -62,9 +65,9 @@ class MessagePackWriter : public Writer {
   void String(const char* x) override { m_->pack(x); }
   // TODO Remove std::string
   void String(const char* x, size_t len) override { m_->pack(std::string(x, len)); }
-  void StartArray(size_t n) override { m_->pack_array(uint32_t(n)); }
+  void StartArray(size_t n) override { m_->pack(n); }
   void EndArray() override {}
-  void StartObject(size_t n) override { m_->pack_array(uint32_t(n)); }
+  void StartObject() override {}
   void EndObject() override {}
   void Key(const char* name) override {}
 };