diff --git a/src/convert_json.cpp b/src/convert_json.cpp
index c774aa002..e9ff1e56c 100644
--- a/src/convert_json.cpp
+++ b/src/convert_json.cpp
@@ -68,12 +68,17 @@ Json::Value fetchJsonValue(const std::string &url,
 	return root;
 }
 
-std::string fastWriteJson(const Json::Value &value)
+void fastWriteJson(const Json::Value &value, std::ostream &to)
 {
-	std::ostringstream oss;
 	Json::StreamWriterBuilder builder;
 	builder["indentation"] = "";
 	std::unique_ptr<Json::StreamWriter> writer(builder.newStreamWriter());
-	writer->write(value, &oss);
+	writer->write(value, &to);
+}
+
+std::string fastWriteJson(const Json::Value &value)
+{
+	std::ostringstream oss;
+	fastWriteJson(value, oss);
 	return oss.str();
 }
diff --git a/src/convert_json.h b/src/convert_json.h
index d8825acdc..2c094a946 100644
--- a/src/convert_json.h
+++ b/src/convert_json.h
@@ -20,8 +20,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include <json/json.h>
+#include <ostream>
 
 Json::Value fetchJsonValue(const std::string &url,
 		std::vector<std::string> *extra_headers);
 
+void fastWriteJson(const Json::Value &value, std::ostream &to);
+
 std::string fastWriteJson(const Json::Value &value);
diff --git a/src/tool.cpp b/src/tool.cpp
index 90f4f9c12..3f639a69e 100644
--- a/src/tool.cpp
+++ b/src/tool.cpp
@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "log.h"
 #include "inventory.h"
 #include "exceptions.h"
+#include "convert_json.h"
 #include "util/serialize.h"
 #include "util/numeric.h"
 
@@ -142,7 +143,7 @@ void ToolCapabilities::serializeJson(std::ostream &os) const
 	}
 	root["damage_groups"] = damage_groups_object;
 
-	os << root;
+	fastWriteJson(root, os);
 }
 
 void ToolCapabilities::deserializeJson(std::istream &is)