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
@@ -355,6 +355,9 @@
     InputStyle = JSONStreamStyle::Delimited;
     LogLevel = Logger::Verbose;
     PrettyPrint = true;
+    // Disable background index on lit tests by default to prevent disk writes.
+    if (!EnableBackgroundIndex.getNumOccurrences())
+      EnableBackgroundIndex = false;
     // Ensure background index makes progress.
     BackgroundQueue::preventThreadStarvationInTests();
   }