diff --git a/pubsublite/internal/wire/assigner.go b/pubsublite/internal/wire/assigner.go
index e5dcc415158..ea4be1b7e43 100644
--- a/pubsublite/internal/wire/assigner.go
+++ b/pubsublite/internal/wire/assigner.go
@@ -139,30 +139,23 @@ func (a *assigner) onStreamStatusChange(status streamStatus) {
 }
 
 func (a *assigner) onResponse(response interface{}) {
+	assignment, _ := response.(*pb.PartitionAssignment)
+	err := a.receiveAssignment(newPartitionSet(assignment))
+
 	a.mu.Lock()
 	defer a.mu.Unlock()
-
 	if a.status >= serviceTerminating {
 		return
 	}
-
-	assignment, _ := response.(*pb.PartitionAssignment)
-	if err := a.handleAssignment(assignment); err != nil {
+	if err != nil {
 		a.unsafeInitiateShutdown(serviceTerminated, err)
+		return
 	}
-}
-
-func (a *assigner) handleAssignment(assignment *pb.PartitionAssignment) error {
-	if err := a.receiveAssignment(newPartitionSet(assignment)); err != nil {
-		return err
-	}
-
 	a.stream.Send(&pb.PartitionAssignmentRequest{
 		Request: &pb.PartitionAssignmentRequest_Ack{
 			Ack: &pb.PartitionAssignmentAck{},
 		},
 	})
-	return nil
 }
 
 func (a *assigner) unsafeInitiateShutdown(targetStatus serviceStatus, err error) {
diff --git a/pubsublite/internal/wire/subscriber.go b/pubsublite/internal/wire/subscriber.go
index d7b793688df..125eeec06be 100644
--- a/pubsublite/internal/wire/subscriber.go
+++ b/pubsublite/internal/wire/subscriber.go
@@ -465,6 +465,21 @@ func newAssigningSubscriber(allClients apiClients, assignmentClient *vkit.Partit
 }
 
 func (as *assigningSubscriber) handleAssignment(partitions partitionSet) error {
+	removedSubscribers, err := as.doHandleAssignment(partitions)
+	if err != nil {
+		return err
+	}
+
+	// Wait for removed subscribers to completely stop (which waits for commit
+	// acknowledgments from the server) before acking the assignment. This avoids
+	// commits racing with the new assigned client.
+	for _, subscriber := range removedSubscribers {
+		subscriber.WaitStopped()
+	}
+	return nil
+}
+
+func (as *assigningSubscriber) doHandleAssignment(partitions partitionSet) ([]*singlePartitionSubscriber, error) {
 	as.mu.Lock()
 	defer as.mu.Unlock()
 
@@ -474,18 +489,20 @@ func (as *assigningSubscriber) handleAssignment(partitions partitionSet) error {
 			subscriber := as.subFactory.New(partition)
 			if err := as.unsafeAddServices(subscriber); err != nil {
 				// Occurs when the assigningSubscriber is stopping/stopped.
-				return err
+				return nil, err
 			}
 			as.subscribers[partition] = subscriber
 		}
 	}
 
 	// Handle removed partitions.
+	var removedSubscribers []*singlePartitionSubscriber
 	for partition, subscriber := range as.subscribers {
 		if !partitions.Contains(partition) {
 			// Ignore unacked messages from this point on to avoid conflicting with
 			// the commits of the new subscriber that will be assigned this partition.
 			subscriber.Terminate()
+			removedSubscribers = append(removedSubscribers, subscriber)
 
 			as.unsafeRemoveService(subscriber)
 			// Safe to delete map entry during range loop:
@@ -493,7 +510,7 @@ func (as *assigningSubscriber) handleAssignment(partitions partitionSet) error {
 			delete(as.subscribers, partition)
 		}
 	}
-	return nil
+	return removedSubscribers, nil
 }
 
 // Terminate shuts down all singlePartitionSubscribers without waiting for
diff --git a/pubsublite/internal/wire/subscriber_test.go b/pubsublite/internal/wire/subscriber_test.go
index 495e7edcc2a..a45bb281a4c 100644
--- a/pubsublite/internal/wire/subscriber_test.go
+++ b/pubsublite/internal/wire/subscriber_test.go
@@ -887,6 +887,17 @@ func (as *assigningSubscriber) Partitions() []int {
 	return partitions
 }
 
+func (as *assigningSubscriber) Subscribers() []*singlePartitionSubscriber {
+	as.mu.Lock()
+	defer as.mu.Unlock()
+
+	var subscribers []*singlePartitionSubscriber
+	for _, s := range as.subscribers {
+		subscribers = append(subscribers, s)
+	}
+	return subscribers
+}
+
 func (as *assigningSubscriber) FlushCommits() {
 	as.mu.Lock()
 	defer as.mu.Unlock()
@@ -1110,10 +1121,20 @@ func TestAssigningSubscriberIgnoreOutstandingAcks(t *testing.T) {
 	// Partition assignments are initially {1}.
 	receiver.ValidateMsg(msg1).Ack()
 	ack2 := receiver.ValidateMsg(msg2)
+	subscribers := sub.Subscribers()
 
 	// Partition assignments will now be {}.
 	assignmentBarrier1.Release()
-	assignmentBarrier2.Release() // Wait for ack to ensure the test is deterministic
+	assignmentBarrier2.ReleaseAfter(func() {
+		// Verify that the assignment is acked after the subscriber has terminated.
+		if got, want := len(subscribers), 1; got != want {
+			t.Errorf("singlePartitionSubcriber count: got %d, want %d", got, want)
+			return
+		}
+		if got, want := subscribers[0].Status(), serviceTerminated; got != want {
+			t.Errorf("singlePartitionSubcriber status: got %v, want %v", got, want)
+		}
+	})
 
 	// Partition 1 has already been unassigned, so this ack is discarded.
 	ack2.Ack()