diff --git a/.github/scripts/backwards_compatibility_check.sh b/.github/scripts/backwards_compatibility_check.sh
index 070e9c17..03eee014 100755
--- a/.github/scripts/backwards_compatibility_check.sh
+++ b/.github/scripts/backwards_compatibility_check.sh
@@ -58,14 +58,18 @@ is_non_public_class() {
   return $?
 }
 
-# Ignore methods that change from abstract to non-abstract (and vice versa) if the class is an interface.
-ignore_abstract_changes_in_interfaces() {
+# Ignore methods that change from abstract to non-abstract (and vice-versa) if the class is an interface.\
+# Ignore methods that change from synchronized to non-synchronized (and vice-versa)
+ignore_non_breaking_changes() {
   local current_class="$1"
   local class_definition=$(javap -classpath "$LATEST_JAR" "$current_class" | head -2 | tail -1)
   if [[ $class_definition == *"interface"* ]]
   then
-    LATEST_METHODS=${LATEST_METHODS// abstract / }
-    CURRENT_METHODS=${CURRENT_METHODS// abstract / }
+    LATEST_METHODS=${LATEST_METHODS//abstract /}
+    CURRENT_METHODS=${CURRENT_METHODS//abstract /}
+  else
+    LATEST_METHODS=${LATEST_METHODS//synchronized /}
+    CURRENT_METHODS=${CURRENT_METHODS//synchronized /}
   fi
 }
 
@@ -103,7 +107,7 @@ find_removed_methods() {
 
     LATEST_METHODS=$(javap -classpath "$LATEST_JAR" "$class")
 
-    ignore_abstract_changes_in_interfaces "$class"
+    ignore_non_breaking_changes "$class"
 
     local removed_methods=$(diff <(echo "$LATEST_METHODS") <(echo "$CURRENT_METHODS") | grep '^<')
 
diff --git a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/assignment/LeaseAssignmentDecider.java b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/assignment/LeaseAssignmentDecider.java
index a39866ae..66042651 100644
--- a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/assignment/LeaseAssignmentDecider.java
+++ b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/assignment/LeaseAssignmentDecider.java
@@ -17,8 +17,10 @@ package software.amazon.kinesis.coordinator.assignment;
 
 import java.util.List;
 
+import software.amazon.kinesis.annotations.KinesisClientInternalApi;
 import software.amazon.kinesis.leases.Lease;
 
+@KinesisClientInternalApi
 public interface LeaseAssignmentDecider {
 
     /**
diff --git a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationClientVersionState.java b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationClientVersionState.java
index c1d8507e..c2e3feac 100644
--- a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationClientVersionState.java
+++ b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationClientVersionState.java
@@ -14,11 +14,13 @@
  */
 package software.amazon.kinesis.coordinator.migration;
 
+import software.amazon.kinesis.annotations.KinesisClientInternalApi;
 import software.amazon.kinesis.leases.exceptions.DependencyException;
 
 /**
  * Interface of a state implementation for the MigrationStateMachine
  */
+@KinesisClientInternalApi
 public interface MigrationClientVersionState {
 
     /**
diff --git a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationStateMachine.java b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationStateMachine.java
index 4698feb0..6dff4e0c 100644
--- a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationStateMachine.java
+++ b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/coordinator/migration/MigrationStateMachine.java
@@ -14,6 +14,7 @@
  */
 package software.amazon.kinesis.coordinator.migration;
 
+import software.amazon.kinesis.annotations.KinesisClientInternalApi;
 import software.amazon.kinesis.leases.exceptions.DependencyException;
 import software.amazon.kinesis.leases.exceptions.InvalidStateException;
 
@@ -28,6 +29,7 @@ import software.amazon.kinesis.leases.exceptions.InvalidStateException;
  * 3. Instant roll-forwards - Once any issue has been mitigated, rollfowards are supported instantly
  *    with KCL Migration tool.
  */
+@KinesisClientInternalApi
 public interface MigrationStateMachine {
     /**
      * Initialize the state machine by identifying the initial state when the KCL worker comes up for the first time.
diff --git a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/LeaseDiscoverer.java b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/LeaseDiscoverer.java
index 9f6ce776..6056d1fd 100644
--- a/amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/LeaseDiscoverer.java
+++ b/amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/LeaseDiscoverer.java
@@ -17,10 +17,12 @@ package software.amazon.kinesis.leases;
 
 import java.util.List;
 
+import software.amazon.kinesis.annotations.KinesisClientInternalApi;
 import software.amazon.kinesis.leases.exceptions.DependencyException;
 import software.amazon.kinesis.leases.exceptions.InvalidStateException;
 import software.amazon.kinesis.leases.exceptions.ProvisionedThroughputException;
 
+@KinesisClientInternalApi
 public interface LeaseDiscoverer {
     /**
      * Identifies the leases that are assigned to the current worker but are not being tracked and processed by the