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
@@ -165,6 +165,8 @@
     bool FoldingRanges = false;
 
     FeatureModuleSet *FeatureModules = nullptr;
+    /// If true, use the dirty buffer contents when building Preambles.
+    bool UseDirtyPreambles = false;
 
     explicit operator TUScheduler::Options() const;
   };
@@ -390,6 +392,9 @@
 private:
   FeatureModuleSet *FeatureModules;
   const GlobalCompilationDatabase &CDB;
+  const ThreadsafeFS &getPreambleFS() const {
+    return UseDirtyPreambles ? *DirtyFS : TFS;
+  }
   const ThreadsafeFS &TFS;
 
   Path ResourceDir;
@@ -409,6 +414,8 @@
   // When set, provides clang-tidy options for a specific file.
   TidyProviderRef ClangTidyProvider;
 
+  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
@@ -156,6 +156,7 @@
     : FeatureModules(Opts.FeatureModules), CDB(CDB), TFS(TFS),
       DynamicIdx(Opts.BuildDynamicSymbolIndex ? new FileIndex() : nullptr),
       ClangTidyProvider(Opts.ClangTidyProvider),
+      UseDirtyPreambles(Opts.UseDirtyPreambles),
       WorkspaceRoot(Opts.WorkspaceRoot),
       Transient(Opts.ImplicitCancellation ? TUScheduler::InvalidateOnUpdate
                                           : TUScheduler::NoInvalidation),
@@ -228,7 +229,7 @@
 
   // 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 = std::move(ActualVersion);
   Inputs.ForceRebuild = ForceRebuild;
@@ -368,7 +369,7 @@
         SpecFuzzyFind->CachedReq = CachedCompletionFuzzyFindRequestByFile[File];
       }
     }
-    ParseInputs ParseInput{IP->Command, &TFS, IP->Contents.str()};
+    ParseInputs ParseInput{IP->Command, &getPreambleFS(), IP->Contents.str()};
     ParseInput.Index = Index;
 
     CodeCompleteOpts.MainFileSignals = IP->Signals;
@@ -416,7 +417,7 @@
     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;
     CB(clangd::signatureHelp(File, Pos, *PreambleData, ParseInput,
                              DocumentationFormat));
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
@@ -501,6 +501,12 @@
     init(true),
 };
 
+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",
@@ -938,6 +944,7 @@
     ClangTidyOptProvider = combine(std::move(Providers));
     Opts.ClangTidyProvider = ClangTidyOptProvider;
   }
+  Opts.UseDirtyPreambles = UseDirtyPreambles;
   Opts.QueryDriverGlobs = std::move(QueryDriverGlobs);
   Opts.TweakFilter = [&](const Tweak &T) {
     if (T.hidden() && !HiddenFeatures)