diff --git a/clang-tools-extra/clangd/Compiler.cpp b/clang-tools-extra/clangd/Compiler.cpp
--- a/clang-tools-extra/clangd/Compiler.cpp
+++ b/clang-tools-extra/clangd/Compiler.cpp
@@ -85,6 +85,11 @@
   // Don't crash on `#pragma clang __debug parser_crash`
   CI->getPreprocessorOpts().DisablePragmaDebugCrash = true;
 
+  // Always default to raw container format as clangd doesn't registry any other
+  // and clang dies when faced with unknown formats.
+  CI->getHeaderSearchOpts().ModuleFormat =
+      PCHContainerOperations().getRawReader().getFormat().str();
+
   return CI;
 }
 
diff --git a/clang-tools-extra/clangd/unittests/ModulesTests.cpp b/clang-tools-extra/clangd/unittests/ModulesTests.cpp
--- a/clang-tools-extra/clangd/unittests/ModulesTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ModulesTests.cpp
@@ -92,6 +92,24 @@
   TU.build();
 }
 
+// Unknown module formats are a fatal failure for clang. Ensure we don't crash.
+TEST(Modules, UnknownFormat) {
+  TestTU TU = TestTU::withCode(R"(#include "modular.h")");
+  TU.OverlayRealFileSystemForModules = true;
+  TU.ExtraArgs.push_back("-Xclang");
+  TU.ExtraArgs.push_back("-fmodule-format=obj");
+  TU.ExtraArgs.push_back("-fmodule-map-file=" + testPath("m.modulemap"));
+  TU.ExtraArgs.push_back("-fmodules");
+  TU.ExtraArgs.push_back("-fimplicit-modules");
+  TU.AdditionalFiles["modular.h"] = "";
+  TU.AdditionalFiles["m.modulemap"] = R"modulemap(
+    module M {
+      header "modular.h"
+    })modulemap";
+
+  // Test that we do not crash.
+  TU.build();
+}
 } // namespace
 } // namespace clangd
 } // namespace clang