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
@@ -156,6 +156,9 @@
     /// Enable preview of FoldingRanges feature.
     bool FoldingRanges = false;
 
+    /// If true, use the dirty buffer contents when building Preambles.
+    bool UseDirtyPreambles = false;
+
     explicit operator TUScheduler::Options() const;
   };
   // Sensible default options for use in tests.
@@ -358,6 +361,10 @@
                   ArrayRef<tooling::Range> Ranges,
                   Callback<tooling::Replacements> CB);
 
+  const ThreadsafeFS &getPreambleFS() const {
+    return UseDirtyPreambles ? DirtyFS : TFS;
+  }
+
   /// Derives a context for a task processing the specified source file.
   /// This includes the current configuration (see Options::ConfigProvider).
   /// The empty string means no particular file is the target.
@@ -399,6 +406,8 @@
   // If true, preserve the type for recovery AST.
   bool PreserveRecoveryASTType = false;
 
+  bool UseDirtyPreambles = false;
+
   // GUARDED_BY(CachedCompletionFuzzyFindRequestMutex)
   llvm::StringMap<llvm::Optional<FuzzyFindRequest>>
       CachedCompletionFuzzyFindRequestByFile;
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
@@ -149,6 +149,7 @@
       SuggestMissingIncludes(Opts.SuggestMissingIncludes),
       BuildRecoveryAST(Opts.BuildRecoveryAST),
       PreserveRecoveryASTType(Opts.PreserveRecoveryASTType),
+      UseDirtyPreambles(Opts.UseDirtyPreambles),
       WorkspaceRoot(Opts.WorkspaceRoot),
       // Pass a callback into `WorkScheduler` to extract symbols from a newly
       // parsed file and rebuild the file index synchronously each time an AST
@@ -209,7 +210,7 @@
 
   // Compile command is set asynchronously during update, as it can be slow.
   ParseInputs Inputs;
-  Inputs.TFS = &TFS;
+  Inputs.TFS = &getPreambleFS();
   Inputs.Contents = std::string(Contents);
   Inputs.Version = Version.str();
   Inputs.ForceRebuild = ForceRebuild;
@@ -253,7 +254,7 @@
         SpecFuzzyFind->CachedReq = CachedCompletionFuzzyFindRequestByFile[File];
       }
     }
-    ParseInputs ParseInput{IP->Command, &TFS, IP->Contents.str()};
+    ParseInputs ParseInput{IP->Command, &getPreambleFS(), IP->Contents.str()};
     ParseInput.Index = Index;
     ParseInput.Opts.BuildRecoveryAST = BuildRecoveryAST;
     ParseInput.Opts.PreserveRecoveryASTType = PreserveRecoveryASTType;
@@ -300,7 +301,7 @@
     if (!PreambleData)
       return CB(error("Failed to parse includes"));
 
-    ParseInputs ParseInput{IP->Command, &TFS, IP->Contents.str()};
+    ParseInputs ParseInput{IP->Command, &getPreambleFS(), IP->Contents.str()};
     ParseInput.Index = Index;
     ParseInput.Opts.BuildRecoveryAST = BuildRecoveryAST;
     ParseInput.Opts.PreserveRecoveryASTType = PreserveRecoveryASTType;
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
@@ -493,6 +493,12 @@
     init(ClangdServer::Options().CollectMainFileRefs),
 };
 
+opt<bool> UseDirtyPreambles{"use-dirty-preambles", cat(Misc),
+                            desc("Use files open in the editor when building "
+                                 "pre-ambles instead of reading from the disk"),
+                            Hidden,
+                            init(ClangdServer::Options().UseDirtyPreambles)};
+
 #if defined(__GLIBC__) && CLANGD_MALLOC_TRIM
 opt<bool> EnableMallocTrim{
     "malloc-trim",
@@ -873,6 +879,7 @@
     Opts.ClangTidyProvider = ClangTidyOptProvider;
   }
   Opts.AsyncPreambleBuilds = AsyncPreamble;
+  Opts.UseDirtyPreambles = UseDirtyPreambles;
   Opts.SuggestMissingIncludes = SuggestMissingIncludes;
   Opts.QueryDriverGlobs = std::move(QueryDriverGlobs);
   Opts.TweakFilter = [&](const Tweak &T) {