@@ -56,7 +56,7 @@ type: docs
   * Controllers can now be unregistered from the cluster metadata. A new `kafka-cluster.sh unregister-controller` command and a `--unregister` flag on `kafka-metadata-quorum.sh remove-controller` are provided, backed by the new `Admin#unregisterController` API and the new `UnregisterController` RPC. This introduces the error code `CONTROLLER_ID_NOT_REGISTERED` (136) and requires metadata version `4.4-IV2` (`IBP_4_4_IV2`). For further details, please refer to [KIP-1312](https://cwiki.apache.org/confluence/spaces/KAFKA/pages/406623954/KIP-1312+Support+unregistering+controllers).
   * A new client configuration, `bootstrap.resolve.timeout.ms`, controls how DNS resolution of `bootstrap.servers` is performed. The default value of `0` preserves the historical synchronous behavior. Setting a positive value opts into asynchronous bootstrap resolution: the client constructor returns immediately, `NetworkClient` retries DNS resolution up to the configured timeout, and failures are surfaced later as `BootstrapResolutionExceptions` from subsequent API calls. This failure is unrecoverable: once raised, the exception is re-thrown by every API call until the client is closed and re-created after fixing the underlying DNS or `bootstrap.servers` configuration. Both asynchronous bootstrap resolution (enabled by setting `bootstrap.resolve.timeout.ms` to a positive value) and `BootstrapResolutionException` are evolving features and may undergo compatibility-breaking changes in a minor release. For further details, please refer to [KIP-909](https://cwiki.apache.org/confluence/x/MJtbDg). Note that Kafka Streams and Kafka Connect do not support asynchronous bootstrap resolution yet: for every client they create internally, `bootstrap.resolve.timeout.ms` is forced to `0` and a user-supplied positive value is ignored with a warning.
   * For tiered-storage topics, time-based local retention (`local.retention.ms`) now uses a segment's last modified time as the age anchor when the segment's largest record timestamp is in the future. Previously such a segment was never removed by time-based retention (its future timestamp kept it perpetually within the retention window) and was only reclaimed once `local.retention.bytes` was breached, even though it had already been uploaded to remote storage. For further details, please refer to [KAFKA-20609](https://issues.apache.org/jira/browse/KAFKA-20609).
-  * `group.consumer.assignors` no longer allows a custom assignor to reuse the name of a built-in assignor (e.g. `uniform`, `range`), even when it is configured on its own to intentionally replace the built-in's behavior. Such a configuration used to be accepted and now fails broker startup with a `ConfigException`. For further details, please refer to [KAFKA-20843](https://issues.apache.org/jira/browse/KAFKA-20843).
+  * `group.consumer.assignors` now fails broker startup with a `ConfigException` when two configured assignors resolve to the same name, for example a built-in listed both by its name and by its class name, or a custom assignor reusing the name of another configured assignor. Such a configuration used to pass startup validation and fail later during group coordinator loading. For further details, please refer to [KAFKA-20843](https://issues.apache.org/jira/browse/KAFKA-20843).
 
 ## Upgrading to 4.3.1
 
 
@@ -883,15 +883,9 @@ public static GroupCoordinatorConfig fromProps(
     protected List<ConsumerGroupPartitionAssignor> consumerGroupAssignors(
         AbstractConfig config
     ) {
-        Map<String, ConsumerGroupPartitionAssignor> builtInAssignors = CONSUMER_GROUP_BUILTIN_ASSIGNORS
+        Map<String, ConsumerGroupPartitionAssignor> defaultAssignors = CONSUMER_GROUP_BUILTIN_ASSIGNORS
             .stream()
             .collect(Collectors.toMap(ConsumerGroupPartitionAssignor::name, Function.identity()));
-        // A built-in may be configured either by its name or by its class name, so it is recognised
-        // by class rather than by how it was resolved below.
-        Set<Class<? extends ConsumerGroupPartitionAssignor>> builtInAssignorClasses = CONSUMER_GROUP_BUILTIN_ASSIGNORS
-            .stream()
-            .map(ConsumerGroupPartitionAssignor::getClass)
-            .collect(Collectors.toSet());
 
         List<ConsumerGroupPartitionAssignor> assignors = new ArrayList<>();
         Set<String> assignorNames = new HashSet<>();
@@ -900,7 +894,7 @@ protected List<ConsumerGroupPartitionAssignor> consumerGroupAssignors(
             // `configuredAssignor` is either the name of a built-in assignor,
             // or a fully qualified class name of a custom assignor
             for (String configuredAssignor : config.getList(GroupCoordinatorConfig.CONSUMER_GROUP_ASSIGNORS_CONFIG)) {
-                ConsumerGroupPartitionAssignor assignor = builtInAssignors.get(configuredAssignor);
+                ConsumerGroupPartitionAssignor assignor = defaultAssignors.get(configuredAssignor);
                 if (assignor == null) {
                     try {
                         assignor = Utils.newInstance(configuredAssignor, ConsumerGroupPartitionAssignor.class);
@@ -919,12 +913,6 @@ protected List<ConsumerGroupPartitionAssignor> consumerGroupAssignors(
 
                 assignors.add(assignor);
 
-                if (!builtInAssignorClasses.contains(assignor.getClass()) && builtInAssignors.containsKey(assignor.name())) {
-                    throw new ConfigException(CONSUMER_GROUP_ASSIGNORS_CONFIG, configuredAssignor,
-                        "Assignor name '" + assignor.name() + "' is reserved by a built-in assignor. " +
-                            "A custom assignor must not reuse the name of a built-in assignor");
-                }
-
                 if (!assignorNames.add(assignor.name())) {
                     throw new ConfigException(CONSUMER_GROUP_ASSIGNORS_CONFIG, configuredAssignor,
                         "Assignor name '" + assignor.name() + "' is already registered by another configured assignor. " +
 
@@ -102,7 +102,7 @@ public GroupAssignment assign(
     public static class UniformNamedAssignor implements ConsumerGroupPartitionAssignor {
         @Override
         public String name() {
-            // Collides with the built-in "uniform" assignor.
+            // Overrides the built-in "uniform" assignor.
             return "uniform";
         }
 
@@ -207,28 +207,28 @@ public void testConsumerGroupAssignorsWithDuplicateNamesFails() {
     }
 
     @Test
-    public void testConsumerGroupAssignorsWithReservedBuiltinNameFails() {
-        // A custom assignor must not take the name of a built-in, whether or not the built-in is
-        // itself configured: a member selecting that name would otherwise silently get the custom one.
+    public void testConsumerGroupAssignorsOverridingBuiltinName() {
+        // A custom assignor may reuse the name of a built-in; members selecting that name get the custom one.
         Map<String, Object> configs = new HashMap<>();
         configs.put(GroupCoordinatorConfig.CONSUMER_GROUP_ASSIGNORS_CONFIG, UniformNamedAssignor.class.getName());
-        assertEquals("Invalid value " + UniformNamedAssignor.class.getName() +
-                " for configuration group.consumer.assignors: Assignor name 'uniform' is reserved by a " +
-                "built-in assignor. A custom assignor must not reuse the name of a built-in assignor",
-            assertThrows(ConfigException.class, () -> createConfig(configs)).getMessage());
+        GroupCoordinatorConfig config = createConfig(configs);
+        List<ConsumerGroupPartitionAssignor> assignors = config.consumerGroupAssignors();
+        assertEquals(1, assignors.size());
+        assertInstanceOf(UniformNamedAssignor.class, assignors.get(0));
+        assertEquals("uniform", assignors.get(0).name());
 
+        // Configuring the built-in alongside the custom assignor overriding its name still fails the duplicate check.
         configs.put(GroupCoordinatorConfig.CONSUMER_GROUP_ASSIGNORS_CONFIG,
             List.of("uniform", UniformNamedAssignor.class.getName()));
         assertEquals("Invalid value " + UniformNamedAssignor.class.getName() +
-                " for configuration group.consumer.assignors: Assignor name 'uniform' is reserved by a " +
-                "built-in assignor. A custom assignor must not reuse the name of a built-in assignor",
+                " for configuration group.consumer.assignors: Assignor name 'uniform' is already " +
+                "registered by another configured assignor. Assignor names, whether built-in or custom, must be unique",
             assertThrows(ConfigException.class, () -> createConfig(configs)).getMessage());
     }
 
     @Test
     public void testConsumerGroupAssignorsBuiltinByClassName() {
-        // A built-in may also be configured by its class name, so the reserved-name check must
-        // recognise it by class rather than by name.
+        // A built-in may also be configured by its class name and resolves to the built-in itself.
         Map<String, Object> configs = new HashMap<>();
         configs.put(GroupCoordinatorConfig.CONSUMER_GROUP_ASSIGNORS_CONFIG,
             List.of(UniformAssignor.class.getName(), RangeAssignor.class.getName()));