diff --git a/src/win32/tip/tip_edit_session.cc b/src/win32/tip/tip_edit_session.cc
index cecf844e54..44758b7519 100644
--- a/src/win32/tip/tip_edit_session.cc
+++ b/src/win32/tip/tip_edit_session.cc
@@ -506,6 +506,15 @@ bool OnOutputReceivedImpl(TipTextService *text_service, ITfContext *context,
   auto edit_session = MakeComPtr<SyncEditSessionImpl>(text_service, context,
                                                       std::move(new_output));
 
+  TipThreadContext *thread_context = text_service->GetThreadContext();
+
+  if (mode == kSync && thread_context->use_async_lock_in_key_handler()) {
+    // A workaround for MS Word's failure mode.
+    // See https://github.com/google/mozc/issues/819 for details.
+    // TODO(https://github.com/google/mozc/issues/821): Remove this workaround.
+    mode = kAsync;
+  }
+
   DWORD edit_session_flag = TF_ES_READWRITE;
   switch (mode) {
     case kAsync:
@@ -526,6 +535,14 @@ bool OnOutputReceivedImpl(TipTextService *text_service, ITfContext *context,
   const HRESULT hr = context->RequestEditSession(
       text_service->GetClientID(), edit_session.get(), edit_session_flag,
       &edit_session_result);
+
+  if (mode == kSync && edit_session_result == TF_E_SYNCHRONOUS) {
+    // A workaround for MS Word's failure mode.
+    // See https://github.com/google/mozc/issues/819 for details.
+    // TODO(https://github.com/google/mozc/issues/821): Remove this workaround.
+    thread_context->set_use_async_lock_in_key_handler(true);
+  }
+
   return SUCCEEDED(hr) && SUCCEEDED(edit_session_result);
 }
 
diff --git a/src/win32/tip/tip_thread_context.h b/src/win32/tip/tip_thread_context.h
index 1176acbf52..7f29ed2548 100644
--- a/src/win32/tip/tip_thread_context.h
+++ b/src/win32/tip/tip_thread_context.h
@@ -47,9 +47,21 @@ class TipThreadContext {
   int32_t GetFocusRevision() const { return focus_revision_; }
   void IncrementFocusRevision();
 
+  void set_use_async_lock_in_key_handler(bool value) {
+    use_async_lock_in_key_handler_ = value;
+  }
+  bool use_async_lock_in_key_handler() const {
+    return use_async_lock_in_key_handler_;
+  }
+
  private:
   TipInputModeManager input_mode_manager_;
   int32_t focus_revision_ = 0;
+
+  // A workaround for MS Word's failure mode.
+  // See https://github.com/google/mozc/issues/819 for details.
+  // TODO(https://github.com/google/mozc/issues/821): Remove this workaround.
+  bool use_async_lock_in_key_handler_ = false;
 };
 
 }  // namespace tsf