diff --git a/src/ros2_medkit_gateway/CMakeLists.txt b/src/ros2_medkit_gateway/CMakeLists.txt
index a394b241..739eeddc 100644
--- a/src/ros2_medkit_gateway/CMakeLists.txt
+++ b/src/ros2_medkit_gateway/CMakeLists.txt
@@ -366,6 +366,10 @@ if(BUILD_TESTING)
   ament_add_gtest(test_update_manager test/test_update_manager.cpp)
   target_link_libraries(test_update_manager gateway_lib)
 
+  # Add data handler tests
+  ament_add_gtest(test_data_handlers test/test_data_handlers.cpp)
+  target_link_libraries(test_data_handlers gateway_lib)
+
   # Add health handler tests
   ament_add_gtest(test_health_handlers test/test_health_handlers.cpp)
   target_link_libraries(test_health_handlers gateway_lib)
@@ -412,6 +416,7 @@ if(BUILD_TESTING)
       test_subscription_manager
       test_cyclic_subscription_handlers
       test_update_manager
+      test_data_handlers
       test_health_handlers
     )
     foreach(_target ${_test_targets})
diff --git a/src/ros2_medkit_gateway/test/test_data_handlers.cpp b/src/ros2_medkit_gateway/test/test_data_handlers.cpp
new file mode 100644
index 00000000..62d708d0
--- /dev/null
+++ b/src/ros2_medkit_gateway/test/test_data_handlers.cpp
@@ -0,0 +1,209 @@
+// Copyright 2026 bburda
+//
+// 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 <gtest/gtest.h>
+
+#include <nlohmann/json.hpp>
+#include <string>
+
+#include "ros2_medkit_gateway/http/error_codes.hpp"
+#include "ros2_medkit_gateway/http/handlers/data_handlers.hpp"
+
+using json = nlohmann::json;
+using ros2_medkit_gateway::AuthConfig;
+using ros2_medkit_gateway::CorsConfig;
+using ros2_medkit_gateway::TlsConfig;
+using ros2_medkit_gateway::handlers::DataHandlers;
+using ros2_medkit_gateway::handlers::HandlerContext;
+
+// DataHandlers uses a null GatewayNode and null AuthManager.
+// This is safe because:
+// - handle_data_categories/handle_data_groups only call HandlerContext::send_error() (static)
+// - handle_list_data/handle_get_data_item/handle_put_data_item check req.matches.size()
+//   before accessing ctx_.node(), so default-constructed requests (size 0) return 400 first
+
+class DataHandlersTest : public ::testing::Test {
+ protected:
+  CorsConfig cors_{};
+  AuthConfig auth_{};
+  TlsConfig tls_{};
+  HandlerContext ctx_{nullptr, cors_, auth_, tls_, nullptr};
+  DataHandlers handlers_{ctx_};
+};
+
+// ============================================================================
+// handle_data_categories — always returns 501 Not Implemented (ISO 17978-3 §7.9)
+// ============================================================================
+
+// @verifies REQ_INTEROP_016
+TEST_F(DataHandlersTest, DataCategoriesReturns501) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_categories(req, res);
+  EXPECT_EQ(res.status, 501);
+}
+
+// @verifies REQ_INTEROP_016
+TEST_F(DataHandlersTest, DataCategoriesResponseBodyIsValidJson) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_categories(req, res);
+  EXPECT_NO_THROW(json::parse(res.body));
+}
+
+// @verifies REQ_INTEROP_016
+TEST_F(DataHandlersTest, DataCategoriesErrorCodeIsNotImplemented) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_categories(req, res);
+  auto body = json::parse(res.body);
+  ASSERT_TRUE(body.contains("error_code"));
+  EXPECT_EQ(body["error_code"], ros2_medkit_gateway::ERR_NOT_IMPLEMENTED);
+}
+
+// @verifies REQ_INTEROP_016
+TEST_F(DataHandlersTest, DataCategoriesErrorBodyContainsMessage) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_categories(req, res);
+  auto body = json::parse(res.body);
+  EXPECT_TRUE(body.contains("message"));
+}
+
+// @verifies REQ_INTEROP_016
+TEST_F(DataHandlersTest, DataCategoriesErrorBodyContainsFeatureParameter) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_categories(req, res);
+  auto body = json::parse(res.body);
+  ASSERT_TRUE(body.contains("parameters"));
+  EXPECT_EQ(body["parameters"]["feature"], "data-categories");
+}
+
+// ============================================================================
+// handle_data_groups — always returns 501 Not Implemented (ISO 17978-3 §7.9)
+// ============================================================================
+
+// @verifies REQ_INTEROP_017
+TEST_F(DataHandlersTest, DataGroupsReturns501) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_groups(req, res);
+  EXPECT_EQ(res.status, 501);
+}
+
+// @verifies REQ_INTEROP_017
+TEST_F(DataHandlersTest, DataGroupsResponseBodyIsValidJson) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_groups(req, res);
+  EXPECT_NO_THROW(json::parse(res.body));
+}
+
+// @verifies REQ_INTEROP_017
+TEST_F(DataHandlersTest, DataGroupsErrorCodeIsNotImplemented) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_groups(req, res);
+  auto body = json::parse(res.body);
+  ASSERT_TRUE(body.contains("error_code"));
+  EXPECT_EQ(body["error_code"], ros2_medkit_gateway::ERR_NOT_IMPLEMENTED);
+}
+
+// @verifies REQ_INTEROP_017
+TEST_F(DataHandlersTest, DataGroupsErrorBodyContainsMessage) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_groups(req, res);
+  auto body = json::parse(res.body);
+  EXPECT_TRUE(body.contains("message"));
+}
+
+// @verifies REQ_INTEROP_017
+TEST_F(DataHandlersTest, DataGroupsErrorBodyContainsFeatureParameter) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_data_groups(req, res);
+  auto body = json::parse(res.body);
+  ASSERT_TRUE(body.contains("parameters"));
+  EXPECT_EQ(body["parameters"]["feature"], "data-groups");
+}
+
+// ============================================================================
+// handle_list_data — returns 400 when route matches are missing
+// ============================================================================
+
+// @verifies REQ_INTEROP_018
+TEST_F(DataHandlersTest, ListDataReturnsBadRequestWhenMatchesMissing) {
+  // Default-constructed req has empty matches (size 0 < 2)
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_list_data(req, res);
+  EXPECT_EQ(res.status, 400);
+}
+
+// @verifies REQ_INTEROP_018
+TEST_F(DataHandlersTest, ListDataBadRequestBodyContainsErrorCode) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_list_data(req, res);
+  auto body = json::parse(res.body);
+  EXPECT_TRUE(body.contains("error_code"));
+}
+
+// ============================================================================
+// handle_get_data_item — returns 400 when route matches are missing
+// ============================================================================
+
+// @verifies REQ_INTEROP_019
+TEST_F(DataHandlersTest, GetDataItemReturnsBadRequestWhenMatchesMissing) {
+  // Default-constructed req has empty matches (size 0 < 3)
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_get_data_item(req, res);
+  EXPECT_EQ(res.status, 400);
+}
+
+// @verifies REQ_INTEROP_019
+TEST_F(DataHandlersTest, GetDataItemBadRequestBodyContainsInvalidRequestErrorCode) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_get_data_item(req, res);
+  auto body = json::parse(res.body);
+  ASSERT_TRUE(body.contains("error_code"));
+  EXPECT_EQ(body["error_code"], ros2_medkit_gateway::ERR_INVALID_REQUEST);
+}
+
+// ============================================================================
+// handle_put_data_item — returns 400 when route matches are missing
+// ============================================================================
+
+// @verifies REQ_INTEROP_020
+TEST_F(DataHandlersTest, PutDataItemReturnsBadRequestWhenMatchesMissing) {
+  // Default-constructed req has empty matches (size 0 < 3)
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_put_data_item(req, res);
+  EXPECT_EQ(res.status, 400);
+}
+
+// @verifies REQ_INTEROP_020
+TEST_F(DataHandlersTest, PutDataItemBadRequestBodyContainsInvalidRequestErrorCode) {
+  httplib::Request req;
+  httplib::Response res;
+  handlers_.handle_put_data_item(req, res);
+  auto body = json::parse(res.body);
+  ASSERT_TRUE(body.contains("error_code"));
+  EXPECT_EQ(body["error_code"], ros2_medkit_gateway::ERR_INVALID_REQUEST);
+}