fix(grpc): transport validation and error message

client/transport/grpc/src/main/java/org/a2aproject/sdk/client/transport/grpc/GrpcTransport.java

@@ -252,9 +252,7 @@ public TaskPushNotificationConfig getTaskPushNotificationConfiguration(GetTaskPu
             @Nullable ClientCallContext context) throws A2AClientException {
         checkNotNullParam("request", request);
         checkNotNullParam("taskId", request.taskId());
-        if(request.id() == null) {
-             throw new IllegalArgumentException("Id must not be null");
-        }
+        checkNotNullParam("id", request.id());
 
         org.a2aproject.sdk.grpc.GetTaskPushNotificationConfigRequest grpcRequest = org.a2aproject.sdk.grpc.GetTaskPushNotificationConfigRequest.newBuilder()
                 .setTaskId(request.taskId())
@@ -333,7 +331,7 @@ public void subscribeToTask(TaskIdParams request, Consumer<StreamingEventKind> e
             A2AServiceStub stubWithMetadata = createAsyncStubWithMetadata(context, payloadAndHeaders);
             stubWithMetadata.subscribeToTask(grpcRequest, streamObserver);
         } catch (StatusRuntimeException e) {
-            throw GrpcErrorMapper.mapGrpcError(e, "Failed to subscribe task push notification config: ");
+            throw GrpcErrorMapper.mapGrpcError(e, "Failed to subscribe to task: ");
         }
     }
 

spec/src/main/java/org/a2aproject/sdk/spec/DataPart.java

@@ -7,6 +7,9 @@
 import com.google.gson.ToNumberPolicy;
 import org.a2aproject.sdk.util.Assert;
 import org.a2aproject.sdk.spec.util.CollectionCopies;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 import java.util.Map;
 import org.jspecify.annotations.Nullable;
 
@@ -23,7 +26,6 @@
  *   <li>JSON objects: {@code Map<String, Object>}</li>
  *   <li>JSON arrays: {@code List<Object>}</li>
  *   <li>Primitives: {@code String}, {@code Number}, {@code Boolean}</li>
- *   <li>Null values: {@code null}</li>
  * </ul>
  * <p>
  * Example usage:
@@ -42,7 +44,7 @@
  * DataPart primitive = new DataPart(42);
  * }</pre>
  *
- * @param data the structured data (required, supports JSON objects, arrays, primitives, and null)
+ * @param data the structured data (required, supports JSON objects, arrays, and primitives)
  * @param metadata additional metadata for the part
  * @see Part
  * @see Message
@@ -59,24 +61,25 @@ public record DataPart(Object data, @Nullable Map<String, Object> metadata) impl
     public static final String DATA = "data";
 
     /**
-     * Compact constructor with validation.
+     * Compact constructor with validation and defensive copying.
      * <p>
-     * Note: For mutable data types (Map, List), callers should ensure immutability
-     * by using {@code Map.copyOf()} or {@code List.copyOf()} before passing to this constructor.
+     * For mutable data types ({@code Map} and {@code List}), an unmodifiable defensive
+     * copy is created. Primitives and other immutable values are stored as-is.
      *
-     * @param data the structured data (supports JSON objects, arrays, primitives, and null)
+     * @param data the structured data (required, supports JSON objects, arrays, and primitives)
+     * @param metadata additional metadata for the part
      * @throws IllegalArgumentException if data is null
      */
     public DataPart (Object data, @Nullable Map<String, Object> metadata) {
         Assert.checkNotNullParam("data", data);
         this.metadata = CollectionCopies.unmodifiableNullableShallowMap(metadata);
-        this.data = data;
+        this.data = defensivelyCopy(data);
     }
 
     /**
      * Constructor.
      *
-     * @param data the structured data (supports JSON objects, arrays, primitives, and not null)
+     * @param data the structured data (required, supports JSON objects, arrays, and primitives)
      * @throws IllegalArgumentException if data is null
      */
     public DataPart(Object data) {
@@ -137,4 +140,24 @@ public static DataPart fromJson(String json, @Nullable Map<String, Object> metad
     private static final Gson JSON_PARSER = new GsonBuilder()
             .setObjectToNumberStrategy(ToNumberPolicy.LONG_OR_DOUBLE)
             .create();
+
+    /**
+     * Creates a defensive copy of mutable collection types to ensure immutability.
+     * <p>
+     * For {@code Map} and {@code List} instances, returns an unmodifiable copy preserving
+     * null elements. For all other types (primitives, Strings, immutable objects), returns
+     * the value as-is.
+     *
+     * @param data the data value to potentially copy
+     * @return an unmodifiable copy for collections, or the original value for immutable types
+     */
+    private static Object defensivelyCopy(Object data) {
+        if (data instanceof Map<?, ?> map) {
+            return CollectionCopies.unmodifiableShallowMap(map);
+        }
+        if (data instanceof List<?> list) {
+            return Collections.unmodifiableList(new ArrayList<>(list));
+        }
+        return data;
+    }
 }

spec/src/test/java/org/a2aproject/sdk/spec/DataPartTest.java

@@ -3,9 +3,11 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -108,4 +110,62 @@ void testFromJson_nullLiteralThrows() {
     void testFromJson_invalidJsonThrows() {
         assertThrows(IllegalArgumentException.class, () -> DataPart.fromJson("{invalid}"));
     }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    void testDataMapIsDefensivelyCopiedAndImmutable() {
+        Map<String, Object> data = new HashMap<>();
+        data.put("temperature", 22.5);
+
+        DataPart part = new DataPart(data);
+
+        assertThrows(UnsupportedOperationException.class, () -> ((Map<String, Object>) part.data()).put("humidity", 65));
+        data.put("humidity", 65);
+        assertEquals(Map.of("temperature", 22.5), part.data());
+    }
+
+    @Test
+    void testDataListIsDefensivelyCopiedAndImmutable() {
+        List<Object> data = new ArrayList<>();
+        data.add("a");
+
+        DataPart part = new DataPart(data);
+
+        assertThrows(UnsupportedOperationException.class, () -> ((List<Object>) part.data()).add("b"));
+        data.add("b");
+        assertEquals(List.of("a"), part.data());
+    }
+
+    @Test
+    void testDataMapPreservesNullValues() {
+        Map<String, Object> data = new HashMap<>();
+        data.put("source", null);
+
+        DataPart part = new DataPart(data);
+
+        assertTrue(part.data() instanceof Map);
+        assertTrue(((Map<?, ?>) part.data()).containsKey("source"));
+        assertNull(((Map<?, ?>) part.data()).get("source"));
+    }
+
+    @Test
+    void testDataListPreservesNullElements() {
+        List<Object> data = new ArrayList<>();
+        data.add(null);
+
+        DataPart part = new DataPart(data);
+
+        assertTrue(part.data() instanceof List);
+        assertEquals(1, ((List<?>) part.data()).size());
+        assertNull(((List<?>) part.data()).get(0));
+    }
+
+    @Test
+    void testDataPrimitiveStoredAsIs() {
+        Integer data = 42;
+
+        DataPart part = new DataPart(data);
+
+        assertSame(data, part.data());
+    }
 }