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
@@ -357,6 +357,7 @@
   Context createProcessingContext(PathRef) const;
   config::Provider *ConfigProvider = nullptr;
 
+  const GlobalCompilationDatabase &CDB;
   const ThreadsafeFS &TFS;
 
   Path ResourceDir;
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
@@ -172,7 +172,7 @@
 ClangdServer::ClangdServer(const GlobalCompilationDatabase &CDB,
                            const ThreadsafeFS &TFS, const Options &Opts,
                            Callbacks *Callbacks)
-    : ConfigProvider(Opts.ConfigProvider), TFS(TFS),
+    : ConfigProvider(Opts.ConfigProvider), CDB(CDB), TFS(TFS),
       DynamicIdx(Opts.BuildDynamicSymbolIndex
                      ? new FileIndex(Opts.HeavyweightDynamicSymbolIndex,
                                      Opts.CollectMainFileRefs)
@@ -257,7 +257,7 @@
   Inputs.Contents = std::string(Contents);
   Inputs.Version = Version.str();
   Inputs.ForceRebuild = ForceRebuild;
-  Inputs.Opts = std::move(Opts);
+  Inputs.Opts = Opts;
   Inputs.Index = Index;
   Inputs.Opts.BuildRecoveryAST = BuildRecoveryAST;
   Inputs.Opts.PreserveRecoveryASTType = PreserveRecoveryASTType;
@@ -265,6 +265,23 @@
   // If we loaded Foo.h, we want to make sure Foo.cpp is indexed.
   if (NewFile && BackgroundIdx)
     BackgroundIdx->boostRelated(File);
+
+  if (NewFile) {
+    if (auto *InternalCDB = CDB.lookupCDB(File)) {
+      for (const auto &CDBFile : InternalCDB->getAllFiles()) {
+        if (CDBFile != File && !WorkScheduler.hasFile(CDBFile)) {
+          WithContext WithContext(createProcessingContext(CDBFile));
+          if (Config::current().AST.Build == Config::ASTPolicy::PreBuild) {
+            auto Buffer = llvm::MemoryBuffer::getFile(CDBFile);
+            Inputs.Contents = std::string(Buffer->get()->getBuffer());
+            WorkScheduler.update(CDBFile, Inputs, WantDiagnostics::No);
+            if (BackgroundIdx)
+              BackgroundIdx->boostRelated(CDBFile);
+          }
+        }
+      }
+    }
+  }
 }
 
 void ClangdServer::removeDocument(PathRef File) { WorkScheduler.remove(File); }
diff --git a/clang-tools-extra/clangd/Config.h b/clang-tools-extra/clangd/Config.h
--- a/clang-tools-extra/clangd/Config.h
+++ b/clang-tools-extra/clangd/Config.h
@@ -70,6 +70,13 @@
     // ::). All nested namespaces are affected as well.
     std::vector<std::string> FullyQualifiedNamespaces;
   } Style;
+
+  enum class ASTPolicy { PreBuild, OnDemand };
+  /// Controls AST prebuild behavior.
+  struct {
+    /// Whether this AST should be prebuild.
+    ASTPolicy Build = ASTPolicy::OnDemand;
+  } AST;
 };
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/ConfigCompile.cpp b/clang-tools-extra/clangd/ConfigCompile.cpp
--- a/clang-tools-extra/clangd/ConfigCompile.cpp
+++ b/clang-tools-extra/clangd/ConfigCompile.cpp
@@ -157,6 +157,7 @@
     compile(std::move(F.If));
     compile(std::move(F.CompileFlags));
     compile(std::move(F.Index));
+    compile(std::move(F.AST));
   }
 
   void compile(Fragment::IfBlock &&F) {
@@ -264,6 +265,18 @@
     }
   }
 
+  void compile(Fragment::ASTBlock &&F) {
+    if (F.Build) {
+      if (auto Val = compileEnum<Config::ASTPolicy>("Build", **F.Build)
+                         .map("OnDemand", Config::ASTPolicy::OnDemand)
+                         .map("PreBuild", Config::ASTPolicy::PreBuild)
+                         .value()) {
+        Out.Apply.push_back(
+            [Val](const Params &, Config &C) { C.AST.Build = *Val; });
+      }
+    }
+  }
+
   constexpr static llvm::SourceMgr::DiagKind Error = llvm::SourceMgr::DK_Error;
   constexpr static llvm::SourceMgr::DiagKind Warning =
       llvm::SourceMgr::DK_Warning;
diff --git a/clang-tools-extra/clangd/ConfigFragment.h b/clang-tools-extra/clangd/ConfigFragment.h
--- a/clang-tools-extra/clangd/ConfigFragment.h
+++ b/clang-tools-extra/clangd/ConfigFragment.h
@@ -174,6 +174,16 @@
     std::vector<Located<std::string>> FullyQualifiedNamespaces;
   };
   StyleBlock Style;
+
+  struct ASTBlock {
+    /// Controls whether clangd prebuilds the AST for the current file. When
+    /// opening a file for the first time. clangd iterates over all the files in
+    /// its compilation database and prebuilds the AST for those that have this
+    /// option set to "PreBuild". Legal values are "PreBuild" or "OnDemand".
+    /// Default is "OnDemand".
+    llvm::Optional<Located<std::string>> Build;
+  };
+  ASTBlock AST;
 };
 
 } // namespace config
diff --git a/clang-tools-extra/clangd/ConfigYAML.cpp b/clang-tools-extra/clangd/ConfigYAML.cpp
--- a/clang-tools-extra/clangd/ConfigYAML.cpp
+++ b/clang-tools-extra/clangd/ConfigYAML.cpp
@@ -40,6 +40,7 @@
     Dict.handle("CompileFlags", [&](Node &N) { parse(F.CompileFlags, N); });
     Dict.handle("Index", [&](Node &N) { parse(F.Index, N); });
     Dict.handle("Style", [&](Node &N) { parse(F.Style, N); });
+    Dict.handle("AST", [&](Node &N) { parse(F.AST, N); });
     Dict.parse(N);
     return !(N.failed() || HadError);
   }
@@ -89,6 +90,12 @@
     Dict.parse(N);
   }
 
+  void parse(Fragment::ASTBlock &F, Node &N) {
+    DictParser Dict("AST", this);
+    Dict.handle("Build", [&](Node &N) { F.Build = scalarValue(N, "Build"); });
+    Dict.parse(N);
+  }
+
   // Helper for parsing mapping nodes (dictionaries).
   // We don't use YamlIO as we want to control over unknown keys.
   class DictParser {
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.h b/clang-tools-extra/clangd/GlobalCompilationDatabase.h
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.h
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.h
@@ -40,6 +40,8 @@
   virtual llvm::Optional<tooling::CompileCommand>
   getCompileCommand(PathRef File) const = 0;
 
+  virtual tooling::CompilationDatabase *lookupCDB(PathRef File) const = 0;
+
   /// Finds the closest project to \p File.
   virtual llvm::Optional<ProjectInfo> getProjectInfo(PathRef File) const {
     return llvm::None;
@@ -76,6 +78,8 @@
   llvm::Optional<tooling::CompileCommand>
   getCompileCommand(PathRef File) const override;
 
+  virtual tooling::CompilationDatabase *lookupCDB(PathRef File) const override;
+
   /// Returns the path to first directory containing a compilation database in
   /// \p File's parents.
   llvm::Optional<ProjectInfo> getProjectInfo(PathRef File) const override;
@@ -132,6 +136,9 @@
 
   llvm::Optional<tooling::CompileCommand>
   getCompileCommand(PathRef File) const override;
+
+  tooling::CompilationDatabase *lookupCDB(PathRef File) const override;
+
   tooling::CompileCommand getFallbackCommand(PathRef File) const override;
   /// Project info is gathered purely from the inner compilation database to
   /// ensure consistency.
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
@@ -68,6 +68,20 @@
 
 llvm::Optional<tooling::CompileCommand>
 DirectoryBasedGlobalCompilationDatabase::getCompileCommand(PathRef File) const {
+  auto *CDB = lookupCDB(File);
+  if (!CDB) {
+    return llvm::None;
+  }
+
+  auto Candidates = CDB->getCompileCommands(File);
+  if (!Candidates.empty())
+    return std::move(Candidates.front());
+
+  return None;
+}
+
+tooling::CompilationDatabase *
+DirectoryBasedGlobalCompilationDatabase::lookupCDB(PathRef File) const {
   CDBLookupRequest Req;
   Req.FileName = File;
   Req.ShouldBroadcast = true;
@@ -75,14 +89,10 @@
   auto Res = lookupCDB(Req);
   if (!Res) {
     log("Failed to find compilation database for {0}", File);
-    return llvm::None;
+    return nullptr;
   }
 
-  auto Candidates = Res->CDB->getCompileCommands(File);
-  if (!Candidates.empty())
-    return std::move(Candidates.front());
-
-  return None;
+  return Res->CDB;
 }
 
 // For platforms where paths are case-insensitive (but case-preserving),
@@ -270,6 +280,10 @@
   return Cmd;
 }
 
+tooling::CompilationDatabase *OverlayCDB::lookupCDB(PathRef File) const {
+  return Base ? Base->lookupCDB(File) : nullptr;
+}
+
 tooling::CompileCommand OverlayCDB::getFallbackCommand(PathRef File) const {
   auto Cmd = Base ? Base->getFallbackCommand(File)
                   : GlobalCompilationDatabase::getFallbackCommand(File);
diff --git a/clang-tools-extra/clangd/QueryDriverDatabase.cpp b/clang-tools-extra/clangd/QueryDriverDatabase.cpp
--- a/clang-tools-extra/clangd/QueryDriverDatabase.cpp
+++ b/clang-tools-extra/clangd/QueryDriverDatabase.cpp
@@ -277,6 +277,10 @@
     return addSystemIncludes(*Cmd, SystemIncludes);
   }
 
+  tooling::CompilationDatabase *lookupCDB(PathRef File) const override {
+    return Base->lookupCDB(File);
+  }
+
   llvm::Optional<ProjectInfo> getProjectInfo(PathRef File) const override {
     return Base->getProjectInfo(File);
   }
diff --git a/clang-tools-extra/clangd/TUScheduler.h b/clang-tools-extra/clangd/TUScheduler.h
--- a/clang-tools-extra/clangd/TUScheduler.h
+++ b/clang-tools-extra/clangd/TUScheduler.h
@@ -231,6 +231,8 @@
   /// Returns true if the file was not previously tracked.
   bool update(PathRef File, ParseInputs Inputs, WantDiagnostics WD);
 
+  bool hasFile(PathRef File);
+
   /// Remove \p File from the list of tracked files and schedule removal of its
   /// resources. Pending diagnostics for closed files may not be delivered, even
   /// if requested with WantDiags::Auto or WantDiags::Yes.
diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -627,11 +627,15 @@
       FileInputs = Inputs;
     }
 
-    log("ASTWorker building file {0} version {1} with command {2}\n[{3}]\n{4}",
+    log("ASTWorker building file {0} version {1}", FileName, Inputs.Version);
+    vlog("ASTWorker building file {0} version {1} with command {2}\n[{3}]\n{4}",
         FileName, Inputs.Version, Inputs.CompileCommand.Heuristic,
         Inputs.CompileCommand.Directory,
         llvm::join(Inputs.CompileCommand.CommandLine, " "));
 
+    if (!InputsAreTheSame)
+      log("ASTWorker inputs are not the same, cached AST is invalidated");
+
     StoreDiags CompilerInvocationDiagConsumer;
     std::vector<std::string> CC1Args;
     std::unique_ptr<CompilerInvocation> Invocation = buildCompilerInvocation(
@@ -1289,6 +1293,8 @@
   return NewFile;
 }
 
+bool TUScheduler::hasFile(PathRef File) { return Files[File] != nullptr; }
+
 void TUScheduler::remove(PathRef File) {
   bool Removed = Files.erase(File);
   if (!Removed)
diff --git a/clang-tools-extra/clangd/unittests/ClangdTests.cpp b/clang-tools-extra/clangd/unittests/ClangdTests.cpp
--- a/clang-tools-extra/clangd/unittests/ClangdTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ClangdTests.cpp
@@ -1123,6 +1123,10 @@
                                       FileName, std::move(CommandLine), "")};
     }
 
+    tooling::CompilationDatabase *lookupCDB(PathRef File) const override {
+      return nullptr;
+    }
+
     std::vector<std::string> ExtraClangFlags;
 
   private:
diff --git a/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp b/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
--- a/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
+++ b/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
@@ -70,6 +70,10 @@
       return None;
     }
 
+    tooling::CompilationDatabase *lookupCDB(PathRef File) const override {
+      return nullptr;
+    }
+
     tooling::CompileCommand
     getFallbackCommand(llvm::StringRef File) const override {
       return cmd(File, "-DA=2");
diff --git a/clang-tools-extra/clangd/unittests/TestFS.h b/clang-tools-extra/clangd/unittests/TestFS.h
--- a/clang-tools-extra/clangd/unittests/TestFS.h
+++ b/clang-tools-extra/clangd/unittests/TestFS.h
@@ -65,6 +65,8 @@
   llvm::Optional<tooling::CompileCommand>
   getCompileCommand(PathRef File) const override;
 
+  tooling::CompilationDatabase *lookupCDB(PathRef File) const override;
+
   llvm::Optional<ProjectInfo> getProjectInfo(PathRef File) const override;
 
   std::vector<std::string> ExtraClangFlags;
diff --git a/clang-tools-extra/clangd/unittests/TestFS.cpp b/clang-tools-extra/clangd/unittests/TestFS.cpp
--- a/clang-tools-extra/clangd/unittests/TestFS.cpp
+++ b/clang-tools-extra/clangd/unittests/TestFS.cpp
@@ -71,6 +71,11 @@
                                   FileName, std::move(CommandLine), "")};
 }
 
+tooling::CompilationDatabase *
+MockCompilationDatabase::lookupCDB(PathRef File) const {
+  return nullptr;
+}
+
 const char *testRoot() {
 #ifdef _WIN32
   return "C:\\clangd-test";