diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -976,7 +976,12 @@
                          if (!List)
                            return Reply(List.takeError());
                          CompletionList LSPList;
-                         LSPList.isIncomplete = List->HasMore;
+                         // Set isIncomplete to true regardless of the actual
+                         // results if the corresponding flag was added to
+                         // Clangd invocation.
+                         LSPList.isIncomplete =
+                             ClangdServerOpts.AlwaysIncomplete ? true
+                                                               : List->HasMore;
                          for (const auto &R : List->Completions) {
                            CompletionItem C = R.render(CCOpts);
                            C.kind = adjustKindToCapability(
diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -145,6 +145,9 @@
     /// Enable cross-file rename feature.
     bool CrossFileRename = false;
 
+    /// Always set isIncomplete to true when returning CompletionList.
+    bool AlwaysIncomplete = false;
+
     /// Returns true if the tweak should be enabled.
     std::function<bool(const Tweak &)> TweakFilter = [](const Tweak &T) {
       return !T.hidden(); // only enable non-hidden tweaks.
@@ -295,7 +298,7 @@
 
   /// Get all document links in a file.
   void documentLinks(PathRef File, Callback<std::vector<DocumentLink>> CB);
- 
+
   /// Returns estimated memory usage for each of the currently open files.
   /// The order of results is unspecified.
   /// Overall memory usage of clangd may be significantly more than reported
diff --git a/clang-tools-extra/clangd/test/always-incomplete.test b/clang-tools-extra/clangd/test/always-incomplete.test
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/test/always-incomplete.test
@@ -0,0 +1,39 @@
+# RUN: clangd -lit-test --always-incomplete < %s | FileCheck -strict-whitespace %s
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"struct S { int a; };\nint main() {\nS().\n}"}}}
+---
+{"jsonrpc":"2.0","id":1,"method":"textDocument/completion","params":{"textDocument":{"uri":"test:///main.cpp"},"position":{"line":2,"character":4}}}
+#      CHECK:  "id": 1
+# CHECK-NEXT:  "jsonrpc": "2.0",
+# CHECK-NEXT:  "result": {
+# CHECK-NEXT:    "isIncomplete": true,
+# CHECK-NEXT:    "items": [
+# CHECK-NEXT:    {
+# CHECK-NEXT:      "detail": "int",
+# CHECK-NEXT:      "filterText": "a",
+# CHECK-NEXT:      "insertText": "a",
+# CHECK-NEXT:      "insertTextFormat": 1,
+# CHECK-NEXT:      "kind": 5,
+# CHECK-NEXT:      "label": " a",
+# CHECK-NEXT:      "score": {{[0-9]+.[0-9]+}},
+# CHECK-NEXT:      "sortText": "{{.*}}a"
+# CHECK-NEXT:      "textEdit": {
+# CHECK-NEXT:        "newText": "a",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 4,
+# CHECK-NEXT:            "line": 2
+# CHECK-NEXT:          }
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 4,
+# CHECK-NEXT:            "line": 2
+# CHECK-NEXT:          }
+# CHECK-NEXT:        }
+# CHECK-NEXT:      }
+# CHECK-NEXT:    }
+# CHECK-NEXT:  ]
+---
+{"jsonrpc":"2.0","id":4,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -402,6 +402,14 @@
     init(false),
 };
 
+opt<bool> AlwaysIncomplete{
+    "always-incomplete",
+    cat(Protocol),
+    desc("Always sets code completion list's isIncomplete to true"),
+    init(false),
+    Hidden,
+};
+
 /// Supports a test URI scheme with relaxed constraints for lit tests.
 /// The path in a test URI will be combined with a platform-specific fake
 /// directory to form an absolute path. For example, test:///a.cpp is resolved
@@ -629,6 +637,7 @@
   Opts.StaticIndex = StaticIdx.get();
   Opts.AsyncThreadsCount = WorkerThreadsCount;
   Opts.CrossFileRename = CrossFileRename;
+  Opts.AlwaysIncomplete = AlwaysIncomplete;
 
   clangd::CodeCompleteOptions CCOpts;
   CCOpts.IncludeIneligibleResults = IncludeIneligibleResults;