Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -303,12 +303,15 @@
   if (Server)
     return Reply(make_error<LSPError>("server already initialized",
                                       ErrorCode::InvalidRequest));
-  if (Params.initializationOptions)
+  std::vector<std::string> FallbackFlags;
+  if (Params.initializationOptions) {
     CompileCommandsDir = Params.initializationOptions->compilationDatabasePath;
+    FallbackFlags = Params.initializationOptions->fallbackFlags;
+  }
   if (UseDirBasedCDB)
     BaseCDB = llvm::make_unique<DirectoryBasedGlobalCompilationDatabase>(
         CompileCommandsDir);
-  CDB.emplace(BaseCDB.get());
+  CDB.emplace(BaseCDB.get(), std::move(FallbackFlags));
   Server.emplace(*CDB, FSProvider, static_cast<DiagnosticsConsumer &>(*this),
                  ClangdServerOpts);
   if (Params.initializationOptions)
Index: clangd/Protocol.h
===================================================================
--- clangd/Protocol.h
+++ clangd/Protocol.h
@@ -367,6 +367,10 @@
   ClangdConfigurationParamsChange ParamsChange;
 
   llvm::Optional<std::string> compilationDatabasePath;
+  // Additional flags to be included in the "fallback command" used when
+  // the compilation database doesn't describe an opened file.
+  // The command used will be approximately `clang $FILE $fallbackFlags`.
+  std::vector<std::string> fallbackFlags;
 };
 bool fromJSON(const llvm::json::Value &, ClangdInitializationOptions &);
 
Index: clangd/Protocol.cpp
===================================================================
--- clangd/Protocol.cpp
+++ clangd/Protocol.cpp
@@ -667,7 +667,10 @@
   }
 
   json::ObjectMapper O(Params);
-  return O && O.map("compilationDatabasePath", Opts.compilationDatabasePath);
+  if (!O && !O.map("compilationDatabasePath", Opts.compilationDatabasePath))
+    return false;
+  O.map("fallbackFlags", Opts.fallbackFlags);
+  return true;
 }
 
 bool fromJSON(const json::Value &Params, ReferenceParams &R) {