fix: handle empty Struct in valueToObject to prevent NullPointerException

spec-grpc/src/main/java/io/a2a/grpc/mapper/A2ACommonFieldMapper.java

@@ -207,6 +207,12 @@ private Value objectToValue(Object value) {
     private Object valueToObject(Value value) {
         switch (value.getKindCase()) {
             case STRUCT_VALUE:
+                if (value.getStructValue() == null) {
+                    return null;
+                }
+                if (value.getStructValue().getFieldsCount() == 0) {
+                    return java.util.Collections.emptyMap();
+                }
                 return structToMap(value.getStructValue());
             case LIST_VALUE:
                 return value.getListValue().getValuesList().stream()

spec-grpc/src/test/java/io/a2a/grpc/mapper/A2ACommonFieldMapperTest.java

@@ -0,0 +1,108 @@
+package io.a2a.grpc.mapper;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import java.util.Map;
+
+import com.google.protobuf.InvalidProtocolBufferException;
+import com.google.protobuf.util.JsonFormat;
+
+import io.a2a.grpc.utils.ProtoUtils;
+import io.a2a.spec.MessageSendParams;
+import org.junit.jupiter.api.Test;
+
+public class A2ACommonFieldMapperTest {
+
+    /**
+     * Test that valueToObject handles empty struct correctly without throwing NullPointerException.
+     *
+     * This test verifies the fix for the bug where an empty struct in the JSON
+     * (e.g., "response": {}) would cause a NullPointerException because structToMap
+     * returns null for empty structs.
+     */
+    @Test
+    void testValueToObject_WithEmptyStruct_ReturnsEmptyMap() throws InvalidProtocolBufferException {
+        // JSON containing an empty struct in "response" field
+        String json = "{\n" +
+                "  \"message\": {\n" +
+                "    \"messageId\": \"b3b1ab58-c3d0-4e6d-9e47-9d8a12fe0809\",\n" +
+                "    \"role\": \"ROLE_USER\",\n" +
+                "    \"parts\": [{\n" +
+                "      \"text\": \"Hello\"\n" +
+                "    }, {\n" +
+                "      \"data\": {\n" +
+                "        \"data\": {\n" +
+                "          \"id\": \"call_94yo5ymj3qi5glbpkw5eicfd\",\n" +
+                "          \"args\": {\n" +
+                "            \"agent_name\": \"Default Agent\"\n" +
+                "          },\n" +
+                "          \"name\": \"transfer_to_agent\"\n" +
+                "        }\n" +
+                "      }\n" +
+                "    }, {\n" +
+                "      \"data\": {\n" +
+                "        \"data\": {\n" +
+                "          \"response\": {\n" +
+                "          },\n" +
+                "          \"id\": \"call_94yo5ymj3qi5glbpkw5eicfd\",\n" +
+                "          \"name\": \"transfer_to_agent\"\n" +
+                "        }\n" +
+                "      }\n" +
+                "    }, {\n" +
+                "      \"text\": \"World\"\n" +
+                "    }],\n" +
+                "    \"metadata\": {\n" +
+                "    }\n" +
+                "  },\n" +
+                "  \"configuration\": {\n" +
+                "    \"blocking\": true\n" +
+                "  },\n" +
+                "  \"metadata\": {\n" +
+                "  }\n" +
+                "}";
+
+        io.a2a.grpc.SendMessageRequest.Builder builder = io.a2a.grpc.SendMessageRequest.newBuilder();
+        JsonFormat.parser().merge(json, builder);
+
+        // This should not throw NullPointerException
+        MessageSendParams messageSendParams = ProtoUtils.FromProto.messageSendParams(builder);
+
+        assertNotNull(messageSendParams);
+        assertNotNull(messageSendParams.message());
+        assertEquals(4, messageSendParams.message().parts().size());
+    }
+
+    /**
+     * Test that valueToObject handles nested empty struct correctly.
+     */
+    @Test
+    void testValueToObject_WithNestedEmptyStruct_ReturnsEmptyMap() throws InvalidProtocolBufferException {
+        String json = "{\n" +
+                "  \"message\": {\n" +
+                "    \"messageId\": \"test-id\",\n" +
+                "    \"role\": \"ROLE_USER\",\n" +
+                "    \"parts\": [{\n" +
+                "      \"data\": {\n" +
+                "        \"data\": {\n" +
+                "          \"nested\": {\n" +
+                "            \"empty\": {\n" +
+                "            }\n" +
+                "          }\n" +
+                "        }\n" +
+                "      }\n" +
+                "    }]\n" +
+                "  }\n" +
+                "}";
+
+        io.a2a.grpc.SendMessageRequest.Builder builder = io.a2a.grpc.SendMessageRequest.newBuilder();
+        JsonFormat.parser().merge(json, builder);
+
+        // This should not throw NullPointerException
+        MessageSendParams messageSendParams = ProtoUtils.FromProto.messageSendParams(builder);
+
+        assertNotNull(messageSendParams);
+        assertNotNull(messageSendParams.message());
+    }
+}