diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -197,8 +197,6 @@
   }
   if (DynamicIdx)
     AddIndex(DynamicIdx.get());
-  // Suppress warnings of this being unused, will be used later.
-  (void)this->DirtyFS;
 }
 
 void ClangdServer::addDocument(PathRef File, llvm::StringRef Contents,
@@ -429,7 +427,8 @@
 // May generate several candidate selections, due to SelectionTree ambiguity.
 // vector of pointers because GCC doesn't like non-copyable Selection.
 static llvm::Expected<std::vector<std::unique_ptr<Tweak::Selection>>>
-tweakSelection(const Range &Sel, const InputsAndAST &AST) {
+tweakSelection(const Range &Sel, const InputsAndAST &AST,
+               llvm::vfs::FileSystem *FS) {
   auto Begin = positionToOffset(AST.Inputs.Contents, Sel.start);
   if (!Begin)
     return Begin.takeError();
@@ -441,7 +440,7 @@
       AST.AST.getASTContext(), AST.AST.getTokens(), *Begin, *End,
       [&](SelectionTree T) {
         Result.push_back(std::make_unique<Tweak::Selection>(
-            AST.Inputs.Index, AST.AST, *Begin, *End, std::move(T)));
+            AST.Inputs.Index, AST.AST, *Begin, *End, std::move(T), FS));
         return false;
       });
   assert(!Result.empty() && "Expected at least one SelectionTree");
@@ -454,12 +453,14 @@
   // Tracks number of times a tweak has been offered.
   static constexpr trace::Metric TweakAvailable(
       "tweak_available", trace::Metric::Counter, "tweak_id");
-  auto Action = [File = File.str(), Sel, CB = std::move(CB),
+  auto Action = [File = File.str(), Sel, CB = std::move(CB), &TFS = this->TFS,
                  Filter =
                      std::move(Filter)](Expected<InputsAndAST> InpAST) mutable {
     if (!InpAST)
       return CB(InpAST.takeError());
-    auto Selections = tweakSelection(Sel, *InpAST);
+    // FIXME: Should we use the dirty fs here?
+    auto FS = TFS.view(llvm::None);
+    auto Selections = tweakSelection(Sel, *InpAST, FS.get());
     if (!Selections)
       return CB(Selections.takeError());
     std::vector<TweakRef> Res;
@@ -497,7 +498,8 @@
                  this](Expected<InputsAndAST> InpAST) mutable {
     if (!InpAST)
       return CB(InpAST.takeError());
-    auto Selections = tweakSelection(Sel, *InpAST);
+    auto FS = DirtyFS.view(llvm::None);
+    auto Selections = tweakSelection(Sel, *InpAST, FS.get());
     if (!Selections)
       return CB(Selections.takeError());
     llvm::Optional<llvm::Expected<Tweak::Effect>> Effect;
diff --git a/clang-tools-extra/clangd/refactor/Tweak.h b/clang-tools-extra/clangd/refactor/Tweak.h
--- a/clang-tools-extra/clangd/refactor/Tweak.h
+++ b/clang-tools-extra/clangd/refactor/Tweak.h
@@ -48,7 +48,8 @@
   /// Input to prepare and apply tweaks.
   struct Selection {
     Selection(const SymbolIndex *Index, ParsedAST &AST, unsigned RangeBegin,
-              unsigned RangeEnd, SelectionTree ASTSelection);
+              unsigned RangeEnd, SelectionTree ASTSelection,
+              llvm::vfs::FileSystem *VFS);
     /// The text of the active document.
     llvm::StringRef Code;
     /// The Index for handling codebase related queries.
@@ -64,6 +65,11 @@
     unsigned SelectionEnd;
     /// The AST nodes that were selected.
     SelectionTree ASTSelection;
+    /// File system used to access source code (for cross-file tweaks).
+    /// This can be used to overlay the "dirty" contents of files open in the
+    /// editor, which (in case of headers) may not match the saved contents used
+    /// for building the AST.
+    llvm::vfs::FileSystem *FS = nullptr;
     // FIXME: provide a way to get sources and ASTs for other files.
   };
 
diff --git a/clang-tools-extra/clangd/refactor/Tweak.cpp b/clang-tools-extra/clangd/refactor/Tweak.cpp
--- a/clang-tools-extra/clangd/refactor/Tweak.cpp
+++ b/clang-tools-extra/clangd/refactor/Tweak.cpp
@@ -47,9 +47,12 @@
 
 Tweak::Selection::Selection(const SymbolIndex *Index, ParsedAST &AST,
                             unsigned RangeBegin, unsigned RangeEnd,
-                            SelectionTree ASTSelection)
+                            SelectionTree ASTSelection,
+                            llvm::vfs::FileSystem *FS)
     : Index(Index), AST(&AST), SelectionBegin(RangeBegin),
-      SelectionEnd(RangeEnd), ASTSelection(std::move(ASTSelection)) {
+      SelectionEnd(RangeEnd), ASTSelection(std::move(ASTSelection)),
+      FS(FS ? FS
+            : &AST.getSourceManager().getFileManager().getVirtualFileSystem()) {
   auto &SM = AST.getSourceManager();
   Code = SM.getBufferData(SM.getMainFileID());
   Cursor = SM.getComposedLoc(SM.getMainFileID(), RangeBegin);
diff --git a/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp b/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp
--- a/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp
+++ b/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp
@@ -63,10 +63,9 @@
 }
 
 llvm::Optional<Path> getSourceFile(llvm::StringRef FileName,
-                                   const Tweak::Selection &Sel) {
-  if (auto Source = getCorrespondingHeaderOrSource(
-          FileName,
-          &Sel.AST->getSourceManager().getFileManager().getVirtualFileSystem()))
+                                   const Tweak::Selection &Sel,
+                                   llvm::vfs::FileSystem *FS) {
+  if (auto Source = getCorrespondingHeaderOrSource(FileName, FS))
     return *Source;
   return getCorrespondingHeaderOrSource(FileName, *Sel.AST, Sel.Index);
 }
@@ -403,13 +402,14 @@
     if (!MainFileName)
       return error("Couldn't get absolute path for main file.");
 
-    auto CCFile = getSourceFile(*MainFileName, Sel);
+    auto *FS = Sel.FS;
+    assert(FS && "FS Must be set in apply");
+
+    auto CCFile = getSourceFile(*MainFileName, Sel, FS);
+
     if (!CCFile)
       return error("Couldn't find a suitable implementation file.");
-
-    auto &FS =
-        Sel.AST->getSourceManager().getFileManager().getVirtualFileSystem();
-    auto Buffer = FS.getBufferForFile(*CCFile);
+    auto Buffer = FS->getBufferForFile(*CCFile);
     // FIXME: Maybe we should consider creating the implementation file if it
     // doesn't exist?
     if (!Buffer)
diff --git a/clang-tools-extra/clangd/tool/Check.cpp b/clang-tools-extra/clangd/tool/Check.cpp
--- a/clang-tools-extra/clangd/tool/Check.cpp
+++ b/clang-tools-extra/clangd/tool/Check.cpp
@@ -203,7 +203,8 @@
       vlog("  {0} {1}", Pos, Tok.text(AST->getSourceManager()));
       auto Tree = SelectionTree::createRight(AST->getASTContext(),
                                              AST->getTokens(), Start, End);
-      Tweak::Selection Selection(&Index, *AST, Start, End, std::move(Tree));
+      Tweak::Selection Selection(&Index, *AST, Start, End, std::move(Tree),
+                                 nullptr);
       for (const auto &T : prepareTweaks(Selection, Opts.TweakFilter)) {
         auto Result = T->apply(Selection);
         if (!Result) {
diff --git a/clang-tools-extra/clangd/unittests/tweaks/TweakTesting.cpp b/clang-tools-extra/clangd/unittests/tweaks/TweakTesting.cpp
--- a/clang-tools-extra/clangd/unittests/tweaks/TweakTesting.cpp
+++ b/clang-tools-extra/clangd/unittests/tweaks/TweakTesting.cpp
@@ -74,7 +74,8 @@
   SelectionTree::createEach(AST.getASTContext(), AST.getTokens(), Range.first,
                             Range.second, [&](SelectionTree ST) {
                               Tweak::Selection S(Index, AST, Range.first,
-                                                 Range.second, std::move(ST));
+                                                 Range.second, std::move(ST),
+                                                 nullptr);
                               if (auto T = prepareTweak(TweakID, S)) {
                                 Result = (*T)->apply(S);
                                 return true;