diff --git a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreImpl.java b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreImpl.java
index ba9a2e9856..38e037b33d 100644
--- a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreImpl.java
+++ b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreImpl.java
@@ -21,8 +21,12 @@
 import com.google.api.core.NanoClock;
 import com.google.api.core.SettableApiFuture;
 import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.BidiStreamObserver;
 import com.google.api.gax.rpc.BidiStreamingCallable;
+import com.google.api.gax.rpc.ClientStream;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
+import com.google.api.gax.rpc.StreamController;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.Timestamp;
 import com.google.cloud.firestore.spi.v1.FirestoreRpc;
@@ -201,7 +205,6 @@ public ApiFuture<List<DocumentSnapshot>> getAll(
     return this.getAll(documentReferences, fieldMask, (ByteString) null);
   }
 
-  @Nonnull
   @Override
   public void getAll(
       final @Nonnull DocumentReference[] documentReferences,
@@ -216,12 +219,15 @@ void getAll(
       @Nullable ByteString transactionId,
       final ApiStreamObserver<DocumentSnapshot> apiStreamObserver) {
 
-    ApiStreamObserver<BatchGetDocumentsResponse> responseObserver =
-        new ApiStreamObserver<BatchGetDocumentsResponse>() {
+    ResponseObserver<BatchGetDocumentsResponse> responseObserver =
+        new ResponseObserver<BatchGetDocumentsResponse>() {
           int numResponses;
 
           @Override
-          public void onNext(BatchGetDocumentsResponse response) {
+          public void onStart(StreamController streamController) {}
+
+          @Override
+          public void onResponse(BatchGetDocumentsResponse response) {
             DocumentReference documentReference;
             DocumentSnapshot documentSnapshot;
 
@@ -270,7 +276,7 @@ public void onError(Throwable throwable) {
           }
 
           @Override
-          public void onCompleted() {
+          public void onComplete() {
             tracer
                 .getCurrentSpan()
                 .addAnnotation(TraceUtil.SPAN_NAME_BATCHGETDOCUMENTS + ": Complete");
@@ -433,19 +439,19 @@ public <RequestT, ResponseT> ApiFuture<ResponseT> sendRequest(
   @Override
   public <RequestT, ResponseT> void streamRequest(
       RequestT requestT,
-      ApiStreamObserver<ResponseT> responseObserverT,
+      ResponseObserver<ResponseT> responseObserverT,
       ServerStreamingCallable<RequestT, ResponseT> callable) {
     Preconditions.checkState(!closed, "Firestore client has already been closed");
-    callable.serverStreamingCall(requestT, responseObserverT);
+    callable.call(requestT, responseObserverT);
   }
 
   /** Request funnel for all bidirectional streaming requests. */
   @Override
-  public <RequestT, ResponseT> ApiStreamObserver<RequestT> streamRequest(
-      ApiStreamObserver<ResponseT> responseObserverT,
+  public <RequestT, ResponseT> ClientStream<RequestT> streamRequest(
+      BidiStreamObserver<RequestT, ResponseT> responseObserverT,
       BidiStreamingCallable<RequestT, ResponseT> callable) {
     Preconditions.checkState(!closed, "Firestore client has already been closed");
-    return callable.bidiStreamingCall(responseObserverT);
+    return callable.splitCall(responseObserverT);
   }
 
   @Override
diff --git a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreRpcContext.java b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreRpcContext.java
index 981d81fdd9..5207bd7701 100644
--- a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreRpcContext.java
+++ b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreRpcContext.java
@@ -20,8 +20,10 @@
 import com.google.api.core.ApiFuture;
 import com.google.api.core.InternalApi;
 import com.google.api.core.InternalExtensionOnly;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.BidiStreamObserver;
 import com.google.api.gax.rpc.BidiStreamingCallable;
+import com.google.api.gax.rpc.ClientStream;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.firestore.spi.v1.FirestoreRpc;
@@ -48,10 +50,10 @@ <RequestT, ResponseT> ApiFuture<ResponseT> sendRequest(
 
   <RequestT, ResponseT> void streamRequest(
       RequestT requestT,
-      ApiStreamObserver<ResponseT> responseObserverT,
+      ResponseObserver<ResponseT> responseObserverT,
       ServerStreamingCallable<RequestT, ResponseT> callable);
 
-  <RequestT, ResponseT> ApiStreamObserver<RequestT> streamRequest(
-      ApiStreamObserver<ResponseT> responseObserverT,
+  <RequestT, ResponseT> ClientStream<RequestT> streamRequest(
+      BidiStreamObserver<RequestT, ResponseT> responseObserverT,
       BidiStreamingCallable<RequestT, ResponseT> callable);
 }
diff --git a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Query.java b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Query.java
index 8b5e876932..c8dac33964 100644
--- a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Query.java
+++ b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Query.java
@@ -32,7 +32,9 @@
 import com.google.api.core.InternalExtensionOnly;
 import com.google.api.core.SettableApiFuture;
 import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.StatusCode;
+import com.google.api.gax.rpc.StreamController;
 import com.google.auto.value.AutoValue;
 import com.google.cloud.Timestamp;
 import com.google.cloud.firestore.Query.QueryOptions.Builder;
@@ -1502,14 +1504,17 @@ private void internalStream(
 
     final AtomicReference<QueryDocumentSnapshot> lastReceivedDocument = new AtomicReference<>();
 
-    ApiStreamObserver<RunQueryResponse> observer =
-        new ApiStreamObserver<RunQueryResponse>() {
+    ResponseObserver<RunQueryResponse> observer =
+        new ResponseObserver<RunQueryResponse>() {
           Timestamp readTime;
           boolean firstResponse;
           int numDocuments;
 
           @Override
-          public void onNext(RunQueryResponse response) {
+          public void onStart(StreamController streamController) {}
+
+          @Override
+          public void onResponse(RunQueryResponse response) {
             if (!firstResponse) {
               firstResponse = true;
               Tracing.getTracer().getCurrentSpan().addAnnotation("Firestore.Query: First response");
@@ -1557,7 +1562,7 @@ public void onError(Throwable throwable) {
           }
 
           @Override
-          public void onCompleted() {
+          public void onComplete() {
             Tracing.getTracer()
                 .getCurrentSpan()
                 .addAnnotation(
diff --git a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Watch.java b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Watch.java
index f655b58c40..8f07b1332a 100644
--- a/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Watch.java
+++ b/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Watch.java
@@ -21,7 +21,9 @@
 import com.google.api.gax.retrying.ExponentialRetryAlgorithm;
 import com.google.api.gax.retrying.TimedAttemptSettings;
 import com.google.api.gax.rpc.ApiException;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.BidiStreamObserver;
+import com.google.api.gax.rpc.ClientStream;
+import com.google.api.gax.rpc.StreamController;
 import com.google.cloud.Timestamp;
 import com.google.cloud.firestore.DocumentChange.Type;
 import com.google.common.base.Preconditions;
@@ -57,7 +59,7 @@
  * It synchronizes on its own instance so it is advisable not to use this class for external
  * synchronization.
  */
-class Watch implements ApiStreamObserver<ListenResponse> {
+class Watch implements BidiStreamObserver<ListenRequest, ListenResponse> {
   /**
    * Target ID used by watch. Watch uses a fixed target id since we only support one target per
    * stream. The actual target ID we use is arbitrary.
@@ -71,7 +73,7 @@ class Watch implements ApiStreamObserver<ListenResponse> {
   private final ExponentialRetryAlgorithm backoff;
   private final Target target;
   private TimedAttemptSettings nextAttempt;
-  private ApiStreamObserver<ListenRequest> stream;
+  private ClientStream<ListenRequest> stream;
 
   /** The sorted tree of DocumentSnapshots as sent in the last snapshot. */
   private DocumentSet documentSet;
@@ -167,7 +169,13 @@ static Watch forQuery(Query query) {
   }
 
   @Override
-  public synchronized void onNext(ListenResponse listenResponse) {
+  public void onStart(StreamController streamController) {}
+
+  @Override
+  public void onReady(ClientStream<ListenRequest> clientStream) {}
+
+  @Override
+  public synchronized void onResponse(ListenResponse listenResponse) {
     switch (listenResponse.getResponseTypeCase()) {
       case TARGET_CHANGE:
         TargetChange change = listenResponse.getTargetChange();
@@ -258,7 +266,7 @@ public synchronized void onError(Throwable throwable) {
   }
 
   @Override
-  public synchronized void onCompleted() {
+  public synchronized void onComplete() {
     maybeReopenStream(new StatusException(Status.fromCode(Code.UNKNOWN)));
   }
 
@@ -289,7 +297,7 @@ ListenerRegistration runWatch(
           .execute(
               () -> {
                 synchronized (Watch.this) {
-                  stream.onCompleted();
+                  stream.closeSend();
                   stream = null;
                 }
               });
@@ -321,7 +329,7 @@ private void resetDocs() {
   /** Closes the stream and calls onError() if the stream is still active. */
   private void closeStream(final Throwable throwable) {
     if (stream != null) {
-      stream.onCompleted();
+      stream.closeSend();
       stream = null;
     }
 
@@ -363,7 +371,7 @@ private void maybeReopenStream(Throwable throwable) {
   /** Helper to restart the outgoing stream to the backend. */
   private void resetStream() {
     if (stream != null) {
-      stream.onCompleted();
+      stream.closeSend();
       stream = null;
     }
 
@@ -399,7 +407,7 @@ private void initStream() {
                 request.getAddTargetBuilder().setResumeToken(resumeToken);
               }
 
-              stream.onNext(request.build());
+              stream.send(request.build());
             }
           } catch (Throwable throwable) {
             onError(throwable);
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/BulkWriterTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/BulkWriterTest.java
index 9a6e248ed4..631c543038 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/BulkWriterTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/BulkWriterTest.java
@@ -70,7 +70,6 @@
 import org.mockito.Mockito;
 import org.mockito.Spy;
 import org.mockito.runners.MockitoJUnitRunner;
-import org.mockito.stubbing.Answer;
 
 @RunWith(MockitoJUnitRunner.class)
 public class BulkWriterTest {
@@ -1119,11 +1118,10 @@ public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
         };
 
     doAnswer(
-            (Answer<ApiFuture<GeneratedMessageV3>>)
-                mock -> {
-                  retryAttempts[0]++;
-                  return RETRYABLE_FAILED_FUTURE;
-                })
+            mock -> {
+              retryAttempts[0]++;
+              return RETRYABLE_FAILED_FUTURE;
+            })
         .when(firestoreMock)
         .sendRequest(
             batchWriteCapture.capture(),
@@ -1170,11 +1168,10 @@ public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
         };
 
     doAnswer(
-            (Answer<ApiFuture<GeneratedMessageV3>>)
-                mock -> {
-                  retryAttempts[0]++;
-                  return RESOURCE_EXHAUSTED_FAILED_FUTURE;
-                })
+            mock -> {
+              retryAttempts[0]++;
+              return RESOURCE_EXHAUSTED_FAILED_FUTURE;
+            })
         .when(firestoreMock)
         .sendRequest(
             batchWriteCapture.capture(),
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/ConformanceTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/ConformanceTest.java
index d6d4500859..dd5b215950 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/ConformanceTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/ConformanceTest.java
@@ -31,8 +31,10 @@
 
 import com.google.api.core.ApiFuture;
 import com.google.api.core.SettableApiFuture;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.BidiStreamObserver;
 import com.google.api.gax.rpc.BidiStreamingCallable;
+import com.google.api.gax.rpc.ClientStream;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.Timestamp;
@@ -63,7 +65,6 @@
 import com.google.firestore.v1.ListenRequest;
 import com.google.firestore.v1.ListenResponse;
 import com.google.firestore.v1.RunQueryRequest;
-import com.google.protobuf.AbstractMessage;
 import com.google.protobuf.Message;
 import com.google.protobuf.util.JsonFormat;
 import java.io.IOException;
@@ -90,7 +91,6 @@
 import org.mockito.Mock;
 import org.mockito.Mockito;
 import org.mockito.MockitoAnnotations;
-import org.mockito.stubbing.Answer;
 
 @RunWith(AllTests.class)
 public class ConformanceTest {
@@ -251,7 +251,7 @@ final CollectionReference collection(final String absolutePath) {
   private static final class ConformanceGetTestRunner extends BaseConformanceTestRunner<GetTest> {
 
     @Captor private ArgumentCaptor<BatchGetDocumentsRequest> getAllCapture;
-    @Captor private ArgumentCaptor<ApiStreamObserver<AbstractMessage>> streamObserverCapture;
+    @Captor private ArgumentCaptor<ResponseObserver<Message>> streamObserverCapture;
 
     private ConformanceGetTestRunner(String description, GetTest testParameters) {
       super(description, testParameters);
@@ -480,7 +480,7 @@ private static final class ConformanceQueryTestRunner
       extends BaseConformanceTestRunner<TestDefinition.QueryTest> {
 
     @Captor private ArgumentCaptor<RunQueryRequest> runQueryCapture;
-    @Captor private ArgumentCaptor<ApiStreamObserver<AbstractMessage>> streamObserverCapture;
+    @Captor private ArgumentCaptor<ResponseObserver<Message>> streamObserverCapture;
 
     private ConformanceQueryTestRunner(
         String description, TestDefinition.QueryTest testParameters) {
@@ -624,8 +624,8 @@ private DocumentSnapshot convertDocument(DocSnapshot snapshot) {
   private static final class ConformanceListenTestRunner
       extends BaseConformanceTestRunner<ListenTest> {
 
-    @Captor private ArgumentCaptor<ApiStreamObserver<AbstractMessage>> streamObserverCapture;
-    @Mock private ApiStreamObserver<ListenRequest> noOpRequestObserver;
+    @Captor private ArgumentCaptor<BidiStreamObserver<Message, Message>> streamObserverCapture;
+    @Mock private ClientStream<ListenRequest> noOpRequestObserver;
 
     private final Query watchQuery;
 
@@ -641,11 +641,10 @@ public void runTest() throws Throwable {
       final SettableApiFuture<Void> testCaseFinished = SettableApiFuture.create();
 
       doAnswer(
-              (Answer<ApiStreamObserver<ListenRequest>>)
-                  invocationOnMock -> {
-                    testCaseStarted.set(null);
-                    return noOpRequestObserver;
-                  })
+              invocationOnMock -> {
+                testCaseStarted.set(null);
+                return noOpRequestObserver;
+              })
           .when(firestore)
           .streamRequest(
               streamObserverCapture.capture(), Matchers.any(BidiStreamingCallable.class));
@@ -680,7 +679,7 @@ public void runTest() throws Throwable {
       testCaseStarted.get();
 
       for (ListenResponse response : testParameters.getResponsesList()) {
-        streamObserverCapture.getValue().onNext(response);
+        streamObserverCapture.getValue().onResponse(response);
       }
 
       testCaseFinished.get();
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/DocumentReferenceTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/DocumentReferenceTest.java
index 7cc0832ac8..aecc8b6bea 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/DocumentReferenceTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/DocumentReferenceTest.java
@@ -64,7 +64,7 @@
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
 
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.Timestamp;
@@ -111,7 +111,7 @@ public class DocumentReferenceTest {
 
   @Captor private ArgumentCaptor<BatchGetDocumentsRequest> getAllCapture;
 
-  @Captor private ArgumentCaptor<ApiStreamObserver> streamObserverCapture;
+  @Captor private ArgumentCaptor<ResponseObserver<CommitResponse>> streamObserverCapture;
 
   private DocumentReference documentReference;
 
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/FirestoreTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/FirestoreTest.java
index c90692b4ba..9d685022d9 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/FirestoreTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/FirestoreTest.java
@@ -32,14 +32,14 @@
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
 
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.firestore.spi.v1.FirestoreRpc;
 import com.google.firestore.v1.BatchGetDocumentsRequest;
 import com.google.firestore.v1.CommitRequest;
 import com.google.firestore.v1.CommitResponse;
-import com.google.firestore.v1.ListCollectionIdsRequest;
+import com.google.protobuf.Message;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -64,9 +64,7 @@ public class FirestoreTest {
 
   @Captor private ArgumentCaptor<BatchGetDocumentsRequest> getAllCapture;
 
-  @Captor private ArgumentCaptor<ListCollectionIdsRequest> listCollectionIdsCapture;
-
-  @Captor private ArgumentCaptor<ApiStreamObserver> streamObserverCapture;
+  @Captor private ArgumentCaptor<ResponseObserver<Message>> streamObserverCapture;
 
   @Captor private ArgumentCaptor<CommitRequest> commitCapture;
 
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/LocalFirestoreHelper.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/LocalFirestoreHelper.java
index 4cc8eda767..e73241a002 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/LocalFirestoreHelper.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/LocalFirestoreHelper.java
@@ -22,7 +22,7 @@
 import com.google.api.core.ApiFuture;
 import com.google.api.core.ApiFutures;
 import com.google.api.gax.retrying.RetrySettings;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.Timestamp;
 import com.google.common.base.Preconditions;
@@ -316,14 +316,14 @@ public static <T> Answer<T> streamingResponse(
       final T[] response, @Nullable final Throwable throwable) {
     return invocation -> {
       Object[] args = invocation.getArguments();
-      ApiStreamObserver<T> observer = (ApiStreamObserver<T>) args[1];
+      ResponseObserver<T> observer = (ResponseObserver<T>) args[1];
       for (T resp : response) {
-        observer.onNext(resp);
+        observer.onResponse(resp);
       }
       if (throwable != null) {
         observer.onError(throwable);
       }
-      observer.onCompleted();
+      observer.onComplete();
       return null;
     };
   }
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/PartitionQuery.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/PartitionQuery.java
index 26d7c4cde6..04203313be 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/PartitionQuery.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/PartitionQuery.java
@@ -26,7 +26,7 @@
 import static org.mockito.Mockito.when;
 
 import com.google.api.core.ApiFutures;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.firestore.spi.v1.FirestoreRpc;
@@ -75,7 +75,7 @@ public class PartitionQuery {
   @Mock private PartitionQueryPage queryPage;
 
   @Captor private ArgumentCaptor<RunQueryRequest> runQuery;
-  @Captor private ArgumentCaptor<ApiStreamObserver> streamObserverCapture;
+  @Captor private ArgumentCaptor<ResponseObserver<PartitionQueryResponse>> streamObserverCapture;
   @Captor private ArgumentCaptor<PartitionQueryRequest> requestCaptor;
 
   @Test
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/QueryTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/QueryTest.java
index 23673507f8..90520e3c5c 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/QueryTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/QueryTest.java
@@ -42,6 +42,7 @@
 
 import com.google.api.core.ApiClock;
 import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.cloud.Timestamp;
 import com.google.cloud.firestore.Query.ComparisonFilter;
@@ -74,7 +75,6 @@
 import org.mockito.Mockito;
 import org.mockito.Spy;
 import org.mockito.runners.MockitoJUnitRunner;
-import org.mockito.stubbing.Answer;
 import org.threeten.bp.Duration;
 
 @RunWith(MockitoJUnitRunner.class)
@@ -106,7 +106,7 @@ public long millisTime() {
 
   @Captor private ArgumentCaptor<RunQueryRequest> runQuery;
 
-  @Captor private ArgumentCaptor<ApiStreamObserver> streamObserverCapture;
+  @Captor private ArgumentCaptor<ResponseObserver<RunQueryResponse>> streamObserverCapture;
 
   private Query query;
 
@@ -957,20 +957,19 @@ public void retriesAfterRetryableError() throws Exception {
     final boolean[] returnError = new boolean[] {true};
 
     doAnswer(
-            (Answer<RunQueryResponse>)
-                invocation -> {
-                  if (returnError[0]) {
-                    returnError[0] = false;
-                    return queryResponse(
-                            FirestoreException.forServerRejection(
-                                Status.DEADLINE_EXCEEDED, "Simulated test failure"),
-                            DOCUMENT_NAME + "1",
-                            DOCUMENT_NAME + "2")
-                        .answer(invocation);
-                  } else {
-                    return queryResponse(DOCUMENT_NAME + "3").answer(invocation);
-                  }
-                })
+            invocation -> {
+              if (returnError[0]) {
+                returnError[0] = false;
+                return queryResponse(
+                        FirestoreException.forServerRejection(
+                            Status.DEADLINE_EXCEEDED, "Simulated test failure"),
+                        DOCUMENT_NAME + "1",
+                        DOCUMENT_NAME + "2")
+                    .answer(invocation);
+              } else {
+                return queryResponse(DOCUMENT_NAME + "3").answer(invocation);
+              }
+            })
         .when(firestoreMock)
         .streamRequest(
             runQuery.capture(),
@@ -1080,22 +1079,21 @@ public void retriesWithoutTimeout() throws Exception {
     final boolean[] returnError = new boolean[] {true};
 
     doAnswer(
-            (Answer<RunQueryResponse>)
-                invocation -> {
-                  // Advance clock by an hour
-                  clock.advance(Duration.ofHours(1).toNanos());
-
-                  if (returnError[0]) {
-                    returnError[0] = false;
-                    return queryResponse(
-                            FirestoreException.forServerRejection(
-                                Status.DEADLINE_EXCEEDED, "Simulated test failure"),
-                            DOCUMENT_NAME + "1")
-                        .answer(invocation);
-                  } else {
-                    return queryResponse(DOCUMENT_NAME + "2").answer(invocation);
-                  }
-                })
+            invocation -> {
+              // Advance clock by an hour
+              clock.advance(Duration.ofHours(1).toNanos());
+
+              if (returnError[0]) {
+                returnError[0] = false;
+                return queryResponse(
+                        FirestoreException.forServerRejection(
+                            Status.DEADLINE_EXCEEDED, "Simulated test failure"),
+                        DOCUMENT_NAME + "1")
+                    .answer(invocation);
+              } else {
+                return queryResponse(DOCUMENT_NAME + "2").answer(invocation);
+              }
+            })
         .when(firestoreMock)
         .streamRequest(
             runQuery.capture(),
@@ -1113,18 +1111,17 @@ public void doesNotRetryWithTimeout() {
     doReturn(Duration.ofMinutes(1)).when(firestoreMock).getTotalRequestTimeout();
 
     doAnswer(
-            (Answer<RunQueryResponse>)
-                invocation -> {
-                  // Advance clock by an hour
-                  clock.advance(Duration.ofHours(1).toNanos());
-
-                  return queryResponse(
-                          FirestoreException.forServerRejection(
-                              Status.DEADLINE_EXCEEDED, "Simulated test failure"),
-                          DOCUMENT_NAME + "1",
-                          DOCUMENT_NAME + "2")
-                      .answer(invocation);
-                })
+            invocation -> {
+              // Advance clock by an hour
+              clock.advance(Duration.ofHours(1).toNanos());
+
+              return queryResponse(
+                      FirestoreException.forServerRejection(
+                          Status.DEADLINE_EXCEEDED, "Simulated test failure"),
+                      DOCUMENT_NAME + "1",
+                      DOCUMENT_NAME + "2")
+                  .answer(invocation);
+            })
         .when(firestoreMock)
         .streamRequest(
             runQuery.capture(),
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/RecursiveDeleteTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/RecursiveDeleteTest.java
index 5d8392a548..84713a679c 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/RecursiveDeleteTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/RecursiveDeleteTest.java
@@ -44,7 +44,7 @@
 import com.google.api.core.ApiFuture;
 import com.google.api.core.ApiFutures;
 import com.google.api.core.SettableApiFuture;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.firestore.LocalFirestoreHelper.ResponseStubber;
@@ -58,6 +58,7 @@
 import com.google.firestore.v1.StructuredQuery.FieldFilter.Operator;
 import com.google.firestore.v1.Value;
 import com.google.firestore.v1.Write;
+import com.google.protobuf.Message;
 import com.google.rpc.Code;
 import io.grpc.Status;
 import java.util.ArrayList;
@@ -105,7 +106,7 @@ public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit)
 
   @Captor private ArgumentCaptor<BatchWriteRequest> batchWriteCapture;
   @Captor private ArgumentCaptor<RunQueryRequest> runQueryCapture;
-  @Captor private ArgumentCaptor<ApiStreamObserver> streamObserverCapture;
+  @Captor private ArgumentCaptor<ResponseObserver<Message>> streamObserverCapture;
 
   private BulkWriter bulkWriter;
   private ResponseStubber responseStubber;
@@ -295,7 +296,7 @@ public void createsRetryQueryAfterStreamExceptionWithLastReceivedDoc() throws Ex
             streamObserverCapture.capture(),
             Matchers.<ServerStreamingCallable>any());
 
-    doAnswer((Answer<ApiFuture<BatchWriteResponse>>) mock -> successResponse(1))
+    doAnswer(mock -> successResponse(1))
         .when(firestoreMock)
         .sendRequest(
             batchWriteCapture.capture(),
@@ -355,15 +356,14 @@ public void createsSecondQueryWithCorrectStartAfter() throws Exception {
 
     doAnswer(queryResponse(firstStream.toArray(new String[0])))
         .doAnswer(
-            (Answer<RunQueryResponse>)
-                invocation -> {
-                  secondQueryFuture.set(null);
-                  Object[] args = invocation.getArguments();
-                  ApiStreamObserver<RunQueryResponse> observer =
-                      (ApiStreamObserver<RunQueryResponse>) args[1];
-                  observer.onCompleted();
-                  return null;
-                })
+            invocation -> {
+              secondQueryFuture.set(null);
+              Object[] args = invocation.getArguments();
+              ResponseObserver<RunQueryResponse> observer =
+                  (ResponseObserver<RunQueryResponse>) args[1];
+              observer.onComplete();
+              return null;
+            })
         .when(firestoreMock)
         .streamRequest(
             runQueryCapture.capture(),
@@ -371,32 +371,31 @@ public void createsSecondQueryWithCorrectStartAfter() throws Exception {
             Matchers.<ServerStreamingCallable>any());
 
     doAnswer(
-            (Answer<ApiFuture<BatchWriteResponse>>)
-                mock -> {
-                  if (numDeletesBuffered[0] < cutoff) {
-                    numDeletesBuffered[0] += batchWriteResponse.size();
-                    // By waiting for `bufferFuture` to complete, we can guarantee that the writes
-                    // complete after all documents are streamed. Without this future, the test can
-                    // race and complete the writes before the stream is finished, which is a
-                    // different scenario this test is not for.
-                    return ApiFutures.transformAsync(
-                        bufferFuture,
-                        (ApiAsyncFunction<Void, BatchWriteResponse>)
-                            unused -> mergeResponses(batchWriteResponse.toArray(new ApiFuture[0])),
-                        MoreExecutors.directExecutor());
-                  } else {
-                    // Once there are `cutoff` pending deletes, completing the future allows enough
-                    // responses to be returned such that the number of pending deletes should be
-                    // less than `minPendingOps`. This allows us to test that the second query is
-                    // made.
-                    bufferFuture.set(null);
-                    return ApiFutures.transformAsync(
-                        secondQueryFuture,
-                        (ApiAsyncFunction<Void, BatchWriteResponse>)
-                            unused -> mergeResponses(batchWriteResponse.toArray(new ApiFuture[0])),
-                        MoreExecutors.directExecutor());
-                  }
-                })
+            mock -> {
+              if (numDeletesBuffered[0] < cutoff) {
+                numDeletesBuffered[0] += batchWriteResponse.size();
+                // By waiting for `bufferFuture` to complete, we can guarantee that the writes
+                // complete after all documents are streamed. Without this future, the test can
+                // race and complete the writes before the stream is finished, which is a
+                // different scenario this test is not for.
+                return ApiFutures.transformAsync(
+                    bufferFuture,
+                    (ApiAsyncFunction<Void, BatchWriteResponse>)
+                        unused -> mergeResponses(batchWriteResponse.toArray(new ApiFuture[0])),
+                    MoreExecutors.directExecutor());
+              } else {
+                // Once there are `cutoff` pending deletes, completing the future allows enough
+                // responses to be returned such that the number of pending deletes should be
+                // less than `minPendingOps`. This allows us to test that the second query is
+                // made.
+                bufferFuture.set(null);
+                return ApiFutures.transformAsync(
+                    secondQueryFuture,
+                    (ApiAsyncFunction<Void, BatchWriteResponse>)
+                        unused -> mergeResponses(batchWriteResponse.toArray(new ApiFuture[0])),
+                    MoreExecutors.directExecutor());
+              }
+            })
         .when(firestoreMock)
         .sendRequest(
             batchWriteCapture.capture(),
@@ -677,11 +676,10 @@ public void usesSameBulkWriterInstanceAcrossCalls() throws Exception {
     final int[] callCount = {0};
     final BulkWriter bulkWriter = firestoreMock.bulkWriter();
     doAnswer(
-            (Answer<BulkWriter>)
-                mock -> {
-                  callCount[0]++;
-                  return bulkWriter;
-                })
+            mock -> {
+              callCount[0]++;
+              return bulkWriter;
+            })
         .when(firestoreMock)
         .bulkWriter();
 
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/TransactionTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/TransactionTest.java
index 9e35ee1b77..87c0e3866a 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/TransactionTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/TransactionTest.java
@@ -47,7 +47,7 @@
 import com.google.api.core.ApiFutures;
 import com.google.api.gax.grpc.GrpcStatusCode;
 import com.google.api.gax.rpc.ApiException;
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.ResponseObserver;
 import com.google.api.gax.rpc.ServerStreamingCallable;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.cloud.Timestamp;
@@ -100,7 +100,7 @@ public class TransactionTest {
           firestoreRpc);
 
   @Captor private ArgumentCaptor<Message> requestCapture;
-  @Captor private ArgumentCaptor<ApiStreamObserver<Message>> streamObserverCapture;
+  @Captor private ArgumentCaptor<ResponseObserver<Message>> streamObserverCapture;
 
   private DocumentReference documentReference;
   private Query queryReference;
diff --git a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WatchTest.java b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WatchTest.java
index 4d6f5d44a7..3ec83f1a1b 100644
--- a/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WatchTest.java
+++ b/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WatchTest.java
@@ -29,12 +29,12 @@
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
-import static org.junit.Assert.fail;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
 
-import com.google.api.gax.rpc.ApiStreamObserver;
+import com.google.api.gax.rpc.BidiStreamObserver;
 import com.google.api.gax.rpc.BidiStreamingCallable;
+import com.google.api.gax.rpc.ClientStream;
 import com.google.cloud.firestore.Query.Direction;
 import com.google.cloud.firestore.WatchTest.SnapshotDocument.ChangeType;
 import com.google.cloud.firestore.spi.v1.FirestoreRpc;
@@ -103,7 +103,8 @@ public class WatchTest {
               .build(),
           firestoreRpc);
 
-  @Captor private ArgumentCaptor<ApiStreamObserver<ListenResponse>> streamObserverCapture;
+  @Captor
+  private ArgumentCaptor<BidiStreamObserver<ListenRequest, ListenResponse>> streamObserverCapture;
 
   /** Executor that executes delayed tasks without delay. */
   private final ScheduledExecutorService immediateExecutor =
@@ -272,6 +273,17 @@ public void queryWatchRemoveTargetWithStatus() throws InterruptedException {
     awaitException(Code.ABORTED);
   }
 
+  @Test
+  public void queryWatchShutsDownStreamOnPermissionDenied() throws InterruptedException {
+    addQueryListener();
+
+    awaitAddTarget();
+    send(removeTarget(Code.PERMISSION_DENIED));
+    awaitClose();
+
+    awaitException(Code.PERMISSION_DENIED);
+  }
+
   @Test
   public void queryWatchReopensOnUnexceptedStreamEnd() throws InterruptedException {
     addQueryListener();
@@ -975,7 +987,7 @@ private ListenResponse docRemove(String docPath) {
   }
 
   private void send(ListenResponse response) {
-    streamObserverCapture.getValue().onNext(response);
+    streamObserverCapture.getValue().onResponse(response);
   }
 
   private void destroy(Code code) {
@@ -983,27 +995,30 @@ private void destroy(Code code) {
   }
 
   private void close() {
-    streamObserverCapture.getValue().onCompleted();
+    streamObserverCapture.getValue().onComplete();
   }
 
   /** Returns a new request observer that persists its input. */
-  private Answer newRequestObserver() {
+  private Answer<ClientStream<ListenRequest>> newRequestObserver() {
     return invocationOnMock ->
-        new ApiStreamObserver<ListenRequest>() {
+        new ClientStream<ListenRequest>() {
           @Override
-          public void onNext(ListenRequest listenRequest) {
+          public void send(ListenRequest listenRequest) {
             requests.add(listenRequest);
           }
 
           @Override
-          public void onError(Throwable throwable) {
-            fail("Received unexpected error");
-          }
+          public void closeSendWithError(Throwable throwable) {}
 
           @Override
-          public void onCompleted() {
+          public void closeSend() {
             closes.release();
           }
+
+          @Override
+          public boolean isSendReady() {
+            return true;
+          }
         };
   }
 }