diff --git a/sandboxed_api/BUILD.bazel b/sandboxed_api/BUILD.bazel
index e02db84..187d5e4 100644
--- a/sandboxed_api/BUILD.bazel
+++ b/sandboxed_api/BUILD.bazel
@@ -127,6 +127,7 @@ cc_library(
 cc_library(
     name = "vars",
     srcs = [
+        "proto_helper.cc",
         "rpcchannel.cc",
         "var_abstract.cc",
         "var_int.cc",
diff --git a/sandboxed_api/CMakeLists.txt b/sandboxed_api/CMakeLists.txt
index 7aab157..09624af 100644
--- a/sandboxed_api/CMakeLists.txt
+++ b/sandboxed_api/CMakeLists.txt
@@ -129,6 +129,7 @@ target_link_libraries(sapi_var_type PRIVATE
 
 # sandboxed_api:vars
 add_library(sapi_vars ${SAPI_LIB_TYPE}
+  proto_helper.cc
   proto_helper.h
   rpcchannel.cc
   rpcchannel.h
diff --git a/sandboxed_api/proto_helper.cc b/sandboxed_api/proto_helper.cc
new file mode 100644
index 0000000..a83bdc4
--- /dev/null
+++ b/sandboxed_api/proto_helper.cc
@@ -0,0 +1,55 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "sandboxed_api/proto_helper.h"
+
+#include "absl/status/status.h"
+
+namespace sapi {
+
+namespace internal {
+
+absl::Status DeserializeProto(const char* data, size_t len,
+                              google::protobuf::Message& output) {
+  ProtoArg envelope;
+  if (!envelope.ParseFromArray(data, len)) {
+    return absl::InternalError("Unable to parse proto from array");
+  }
+
+  auto pb_data = envelope.protobuf_data();
+  if (!output.ParseFromArray(pb_data.data(), pb_data.size())) {
+    return absl::InternalError("Unable to parse proto from envelope data");
+  }
+  return absl::OkStatus();
+}
+
+}  // namespace internal
+
+absl::StatusOr<std::vector<uint8_t>> SerializeProto(
+    const google::protobuf::Message& proto) {
+  // Wrap protobuf in a envelope so that we know the name of the protobuf
+  // structure when deserializing in the sandboxee.
+  ProtoArg proto_arg;
+  proto_arg.set_protobuf_data(proto.SerializeAsString());
+  proto_arg.set_full_name(proto.GetDescriptor()->full_name());
+
+  std::vector<uint8_t> serialized_proto(proto_arg.ByteSizeLong());
+  if (!proto_arg.SerializeToArray(serialized_proto.data(),
+                                  serialized_proto.size())) {
+    return absl::InternalError("Unable to serialize proto to array");
+  }
+  return serialized_proto;
+}
+
+}  // namespace sapi
diff --git a/sandboxed_api/proto_helper.h b/sandboxed_api/proto_helper.h
index 63c4294..0c22fab 100644
--- a/sandboxed_api/proto_helper.h
+++ b/sandboxed_api/proto_helper.h
@@ -24,41 +24,27 @@
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "sandboxed_api/proto_arg.pb.h"
+#include "sandboxed_api/util/status_macros.h"
 
 namespace sapi {
 
-template <typename T>
-absl::StatusOr<std::vector<uint8_t>> SerializeProto(const T& proto) {
-  static_assert(std::is_base_of<google::protobuf::Message, T>::value,
-                "Template argument must be a proto message");
-  // Wrap protobuf in a envelope so that we know the name of the protobuf
-  // structure when deserializing in the sandboxee.
-  ProtoArg proto_arg;
-  proto_arg.set_protobuf_data(proto.SerializeAsString());
-  proto_arg.set_full_name(proto.GetDescriptor()->full_name());
+namespace internal {
 
-  std::vector<uint8_t> serialized_proto(proto_arg.ByteSizeLong());
-  if (!proto_arg.SerializeToArray(serialized_proto.data(),
-                                  serialized_proto.size())) {
-    return absl::InternalError("Unable to serialize proto to array");
-  }
-  return serialized_proto;
-}
+absl::Status DeserializeProto(const char* data, size_t len,
+                              google::protobuf::Message& output);
+
+}  // namespace internal
+
+absl::StatusOr<std::vector<uint8_t>> SerializeProto(
+    const google::protobuf::Message& proto);
 
 template <typename T>
 absl::StatusOr<T> DeserializeProto(const char* data, size_t len) {
   static_assert(std::is_base_of<google::protobuf::Message, T>::value,
                 "Template argument must be a proto message");
-  ProtoArg envelope;
-  if (!envelope.ParseFromArray(data, len)) {
-    return absl::InternalError("Unable to parse proto from array");
-  }
-
-  auto pb_data = envelope.protobuf_data();
   T result;
-  if (!result.ParseFromArray(pb_data.data(), pb_data.size())) {
-    return absl::InternalError("Unable to parse proto from envelope data");
-  }
+  SAPI_RETURN_IF_ERROR(
+      internal::DeserializeProto(data, len, /*output=*/result));
   return result;
 }