diff --git a/src/proto/grpc/testing/control.proto b/src/proto/grpc/testing/control.proto
index 2ff2e4e8a28..57592662c44 100644
--- a/src/proto/grpc/testing/control.proto
+++ b/src/proto/grpc/testing/control.proto
@@ -108,6 +108,9 @@ message ClientConfig {
 
   // Number of messages on a stream before it gets finished/restarted
   int32 messages_per_stream = 18;
+
+  // Use coalescing API when possible.
+  bool use_coalesce_api = 19;
 }
 
 message ClientStatus { ClientStats stats = 1; }
diff --git a/test/cpp/qps/client_async.cc b/test/cpp/qps/client_async.cc
index 7cf9d3ea7e1..e3fba36a7a7 100644
--- a/test/cpp/qps/client_async.cc
+++ b/test/cpp/qps/client_async.cc
@@ -82,6 +82,7 @@ class ClientRpcContextUnaryImpl : public ClientRpcContext {
         prepare_req_(prepare_req) {}
   ~ClientRpcContextUnaryImpl() override {}
   void Start(CompletionQueue* cq, const ClientConfig& config) override {
+    GPR_ASSERT(!config.use_coalesce_api());  // not supported.
     StartInternal(cq);
   }
   bool RunNextState(bool ok, HistogramEntry* entry) override {
@@ -349,10 +350,11 @@ class ClientRpcContextStreamingPingPongImpl : public ClientRpcContext {
         next_state_(State::INVALID),
         callback_(on_done),
         next_issue_(next_issue),
-        prepare_req_(prepare_req) {}
+        prepare_req_(prepare_req),
+        coalesce_(false) {}
   ~ClientRpcContextStreamingPingPongImpl() override {}
   void Start(CompletionQueue* cq, const ClientConfig& config) override {
-    StartInternal(cq, config.messages_per_stream());
+    StartInternal(cq, config.messages_per_stream(), config.use_coalesce_api());
   }
   bool RunNextState(bool ok, HistogramEntry* entry) override {
     while (true) {
@@ -375,7 +377,12 @@ class ClientRpcContextStreamingPingPongImpl : public ClientRpcContext {
           }
           start_ = UsageTimer::Now();
           next_state_ = State::WRITE_DONE;
-          stream_->Write(req_, ClientRpcContext::tag(this));
+          if (coalesce_ && messages_issued_ == messages_per_stream_ - 1) {
+            stream_->WriteLast(req_, WriteOptions(),
+                               ClientRpcContext::tag(this));
+          } else {
+            stream_->Write(req_, ClientRpcContext::tag(this));
+          }
           return true;
         case State::WRITE_DONE:
           if (!ok) {
@@ -391,6 +398,11 @@ class ClientRpcContextStreamingPingPongImpl : public ClientRpcContext {
           if ((messages_per_stream_ != 0) &&
               (++messages_issued_ >= messages_per_stream_)) {
             next_state_ = State::WRITES_DONE_DONE;
+            if (coalesce_) {
+              // WritesDone should have been called on the last Write.
+              // loop around to call Finish.
+              break;
+            }
             stream_->WritesDone(ClientRpcContext::tag(this));
             return true;
           }
@@ -413,7 +425,7 @@ class ClientRpcContextStreamingPingPongImpl : public ClientRpcContext {
   void StartNewClone(CompletionQueue* cq) override {
     auto* clone = new ClientRpcContextStreamingPingPongImpl(
         stub_, req_, next_issue_, prepare_req_, callback_);
-    clone->StartInternal(cq, messages_per_stream_);
+    clone->StartInternal(cq, messages_per_stream_, coalesce_);
   }
   void TryCancel() override { context_.TryCancel(); }
 
@@ -449,14 +461,27 @@ class ClientRpcContextStreamingPingPongImpl : public ClientRpcContext {
   // Allow a limit on number of messages in a stream
   int messages_per_stream_;
   int messages_issued_;
+  // Whether to use coalescing API.
+  bool coalesce_;
 
-  void StartInternal(CompletionQueue* cq, int messages_per_stream) {
+  void StartInternal(CompletionQueue* cq, int messages_per_stream,
+                     bool coalesce) {
     cq_ = cq;
     messages_per_stream_ = messages_per_stream;
     messages_issued_ = 0;
+    coalesce_ = coalesce;
+    if (coalesce_) {
+      GPR_ASSERT(messages_per_stream_ != 0);
+      context_.set_initial_metadata_corked(true);
+    }
     stream_ = prepare_req_(stub_, &context_, cq);
     next_state_ = State::STREAM_IDLE;
     stream_->StartCall(ClientRpcContext::tag(this));
+    if (coalesce_) {
+      // When the intial metadata is corked, the tag will not come back and we
+      // need to manually drive the state machine.
+      RunNextState(true, nullptr);
+    }
   }
 };
 
@@ -512,6 +537,7 @@ class ClientRpcContextStreamingFromClientImpl : public ClientRpcContext {
         prepare_req_(prepare_req) {}
   ~ClientRpcContextStreamingFromClientImpl() override {}
   void Start(CompletionQueue* cq, const ClientConfig& config) override {
+    GPR_ASSERT(!config.use_coalesce_api());  // not supported yet.
     StartInternal(cq);
   }
   bool RunNextState(bool ok, HistogramEntry* entry) override {
@@ -641,6 +667,7 @@ class ClientRpcContextStreamingFromServerImpl : public ClientRpcContext {
         prepare_req_(prepare_req) {}
   ~ClientRpcContextStreamingFromServerImpl() override {}
   void Start(CompletionQueue* cq, const ClientConfig& config) override {
+    GPR_ASSERT(!config.use_coalesce_api());  // not supported
     StartInternal(cq);
   }
   bool RunNextState(bool ok, HistogramEntry* entry) override {
@@ -753,6 +780,7 @@ class ClientRpcContextGenericStreamingImpl : public ClientRpcContext {
         prepare_req_(prepare_req) {}
   ~ClientRpcContextGenericStreamingImpl() override {}
   void Start(CompletionQueue* cq, const ClientConfig& config) override {
+    GPR_ASSERT(!config.use_coalesce_api());  // not supported yet.
     StartInternal(cq, config.messages_per_stream());
   }
   bool RunNextState(bool ok, HistogramEntry* entry) override {
diff --git a/test/cpp/qps/client_sync.cc b/test/cpp/qps/client_sync.cc
index 82a3f0042d1..a2ddbeb508a 100644
--- a/test/cpp/qps/client_sync.cc
+++ b/test/cpp/qps/client_sync.cc
@@ -402,6 +402,7 @@ class SynchronousStreamingBothWaysClient final
 };
 
 std::unique_ptr<Client> CreateSynchronousClient(const ClientConfig& config) {
+  GPR_ASSERT(!config.use_coalesce_api());  // not supported yet.
   switch (config.rpc_type()) {
     case UNARY:
       return std::unique_ptr<Client>(new SynchronousUnaryClient(config));