diff --git a/CMakeLists.txt b/CMakeLists.txt
index fa116816..01f49968 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -180,7 +180,6 @@ file(GLOB SOURCES src/*.cc src/*.h src/serializers/*.cc src/serializers/*.h
 target_sources(ccls PRIVATE third_party/siphash.cc)
 
 target_sources(ccls PRIVATE
-  src/clang_complete.cc
   src/clang_tu.cc
   src/config.cc
   src/filesystem.cc
@@ -197,6 +196,7 @@ target_sources(ccls PRIVATE
   src/position.cc
   src/project.cc
   src/query.cc
+  src/sema_manager.cc
   src/serializer.cc
   src/test.cc
   src/utils.cc
diff --git a/src/config.hh b/src/config.hh
index 57a06b6c..694ed8de 100644
--- a/src/config.hh
+++ b/src/config.hh
@@ -228,6 +228,10 @@ struct Config {
     std::vector<std::string> whitelist;
   } index;
 
+  struct Session {
+    int maxNum = 10;
+  } session;
+
   struct WorkspaceSymbol {
     int caseSensitivity = 1;
     // Maximum workspace search results.
@@ -261,12 +265,13 @@ MAKE_REFLECT_STRUCT(Config::Index, blacklist, comments, initialBlacklist,
                     initialWhitelist, multiVersion, multiVersionBlacklist,
                     multiVersionWhitelist, onChange, threads, trackDependency,
                     whitelist);
+MAKE_REFLECT_STRUCT(Config::Session, maxNum);
 MAKE_REFLECT_STRUCT(Config::WorkspaceSymbol, caseSensitivity, maxNum, sort);
 MAKE_REFLECT_STRUCT(Config::Xref, maxNum);
 MAKE_REFLECT_STRUCT(Config, compilationDatabaseCommand,
                     compilationDatabaseDirectory, cacheDirectory, cacheFormat,
                     clang, client, codeLens, completion, diagnostics, highlight,
-                    index, workspaceSymbol, xref);
+                    index, session, workspaceSymbol, xref);
 
 extern Config *g_config;
 
diff --git a/src/indexer.cc b/src/indexer.cc
index 94286959..61540e27 100644
--- a/src/indexer.cc
+++ b/src/indexer.cc
@@ -3,11 +3,11 @@
 
 #include "indexer.hh"
 
-#include "clang_complete.hh"
 #include "clang_tu.hh"
 #include "log.hh"
 #include "pipeline.hh"
 #include "platform.hh"
+#include "sema_manager.hh"
 #include "serializer.hh"
 
 #include <clang/AST/AST.h>
@@ -1219,10 +1219,11 @@ void Init() {
 }
 
 std::vector<std::unique_ptr<IndexFile>>
-Index(CompletionManager *completion, WorkingFiles *wfiles, VFS *vfs,
+Index(SemaManager *manager, WorkingFiles *wfiles, VFS *vfs,
       const std::string &opt_wdir, const std::string &file,
       const std::vector<const char *> &args,
-      const std::vector<std::pair<std::string, std::string>> &remapped, bool &ok) {
+      const std::vector<std::pair<std::string, std::string>> &remapped,
+      bool &ok) {
   ok = true;
   auto PCH = std::make_shared<PCHContainerOperations>();
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS = llvm::vfs::getRealFileSystem();
@@ -1243,7 +1244,7 @@ Index(CompletionManager *completion, WorkingFiles *wfiles, VFS *vfs,
     bool done_remap = false;
 #if 0
     std::shared_ptr<CompletionSession> session =
-      completion->TryGetSession(file, false, false);
+      manager->TryGetSession(file, false, false);
     if (session)
       if (auto preamble = session->GetPreamble()) {
         Bufs.push_back(llvm::MemoryBuffer::getMemBuffer(buf));
diff --git a/src/indexer.hh b/src/indexer.hh
index e435b39d..a9f493d2 100644
--- a/src/indexer.hh
+++ b/src/indexer.hh
@@ -312,14 +312,14 @@ struct IndexFile {
   std::string ToString();
 };
 
-struct CompletionManager;
+struct SemaManager;
 struct WorkingFiles;
 struct VFS;
 
 namespace idx {
 void Init();
 std::vector<std::unique_ptr<IndexFile>>
-Index(CompletionManager *complete, WorkingFiles *wfiles, VFS *vfs,
+Index(SemaManager *complete, WorkingFiles *wfiles, VFS *vfs,
       const std::string &opt_wdir, const std::string &file,
       const std::vector<const char *> &args,
       const std::vector<std::pair<std::string, std::string>> &remapped,
diff --git a/src/message_handler.hh b/src/message_handler.hh
index 030f626d..b00eb383 100644
--- a/src/message_handler.hh
+++ b/src/message_handler.hh
@@ -12,7 +12,7 @@
 #include <vector>
 
 namespace ccls {
-struct CompletionManager;
+struct SemaManager;
 struct VFS;
 struct IncludeComplete;
 struct Project;
@@ -193,7 +193,7 @@ struct ReplyOnce {
 };
 
 struct MessageHandler {
-  CompletionManager *clang_complete = nullptr;
+  SemaManager *manager = nullptr;
   DB *db = nullptr;
   IncludeComplete *include_complete = nullptr;
   Project *project = nullptr;
diff --git a/src/messages/ccls_call.cc b/src/messages/ccls_call.cc
index 32078759..02e451a7 100644
--- a/src/messages/ccls_call.cc
+++ b/src/messages/ccls_call.cc
@@ -189,11 +189,10 @@ void MessageHandler::ccls_call(Reader &reader, ReplyOnce &reply) {
              param.levels);
   } else {
     QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-    if (!file)
+    WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+    if (!wf)
       return;
-    WorkingFile *working_file = wfiles->GetFileByFilename(file->def->path);
-    for (SymbolRef sym :
-         FindSymbolsAtLocation(working_file, file, param.position)) {
+    for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position)) {
       if (sym.kind == Kind::Func) {
         result = BuildInitial(this, sym.usr, param.callee, param.callType,
                               param.qualified, param.levels);
diff --git a/src/messages/ccls_inheritance.cc b/src/messages/ccls_inheritance.cc
index b806fede..ff84d6ce 100644
--- a/src/messages/ccls_inheritance.cc
+++ b/src/messages/ccls_inheritance.cc
@@ -137,9 +137,8 @@ void Inheritance(MessageHandler *m, Param &param, ReplyOnce &reply) {
     QueryFile *file = m->FindFile(reply, param.textDocument.uri.GetPath());
     if (!file)
       return;
-    WorkingFile *wfile = m->wfiles->GetFileByFilename(file->def->path);
-
-    for (SymbolRef sym : FindSymbolsAtLocation(wfile, file, param.position))
+    WorkingFile *wf = m->wfiles->GetFile(file->def->path);
+    for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position))
       if (sym.kind == Kind::Func || sym.kind == Kind::Type) {
         result = BuildInitial(m, sym, param.derived, param.qualified,
                               param.levels);
diff --git a/src/messages/ccls_member.cc b/src/messages/ccls_member.cc
index 6fd80529..f4a6d1f4 100644
--- a/src/messages/ccls_member.cc
+++ b/src/messages/ccls_member.cc
@@ -270,11 +270,10 @@ void MessageHandler::ccls_member(Reader &reader, ReplyOnce &reply) {
       result.reset();
   } else {
     QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-    if (!file)
+    WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+    if (!wf)
       return;
-    WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
-    for (SymbolRef sym :
-         FindSymbolsAtLocation(wfile, file, param.position)) {
+    for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position)) {
       switch (sym.kind) {
       case Kind::Func:
       case Kind::Type:
diff --git a/src/messages/ccls_navigate.cc b/src/messages/ccls_navigate.cc
index dad671a5..6da450f1 100644
--- a/src/messages/ccls_navigate.cc
+++ b/src/messages/ccls_navigate.cc
@@ -31,14 +31,13 @@ void MessageHandler::ccls_navigate(Reader &reader,
   Param param;
   Reflect(reader, param);
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
-
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
   Position ls_pos = param.position;
-  if (wfile && wfile->index_lines.size())
-    if (auto line = wfile->GetIndexPosFromBufferPos(ls_pos.line,
-                                                    &ls_pos.character, false))
+  if (wf->index_lines.size())
+    if (auto line =
+            wf->GetIndexPosFromBufferPos(ls_pos.line, &ls_pos.character, false))
       ls_pos.line = *line;
   Pos pos{(int16_t)ls_pos.line, (int16_t)ls_pos.character};
 
@@ -85,7 +84,7 @@ void MessageHandler::ccls_navigate(Reader &reader,
   }
   std::vector<Location> result;
   if (res)
-    if (auto ls_range = GetLsRange(wfile, *res)) {
+    if (auto ls_range = GetLsRange(wf, *res)) {
       Location &ls_loc = result.emplace_back();
       ls_loc.uri = param.textDocument.uri;
       ls_loc.range = *ls_range;
diff --git a/src/messages/ccls_reload.cc b/src/messages/ccls_reload.cc
index 13db9bf8..9d60366c 100644
--- a/src/messages/ccls_reload.cc
+++ b/src/messages/ccls_reload.cc
@@ -1,10 +1,10 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
 #include "message_handler.hh"
 #include "pipeline.hh"
 #include "project.hh"
+#include "sema_manager.hh"
 #include "working_files.hh"
 
 #include <queue>
@@ -28,7 +28,7 @@ void MessageHandler::ccls_reload(Reader &reader) {
     vfs->Clear();
     db->clear();
     project->Index(wfiles, RequestId());
-    clang_complete->FlushAllSessions();
+    manager->Clear();
     return;
   }
 }
diff --git a/src/messages/ccls_vars.cc b/src/messages/ccls_vars.cc
index 51537768..4890e331 100644
--- a/src/messages/ccls_vars.cc
+++ b/src/messages/ccls_vars.cc
@@ -20,13 +20,12 @@ void MessageHandler::ccls_vars(Reader &reader, ReplyOnce &reply) {
   Param param;
   Reflect(reader, param);
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
-  WorkingFile *working_file = wfiles->GetFileByFilename(file->def->path);
 
   std::vector<Location> result;
-  for (SymbolRef sym :
-       FindSymbolsAtLocation(working_file, file, param.position)) {
+  for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position)) {
     Usr usr = sym.usr;
     switch (sym.kind) {
     default:
diff --git a/src/messages/initialize.cc b/src/messages/initialize.cc
index a38903c8..2e96bde8 100644
--- a/src/messages/initialize.cc
+++ b/src/messages/initialize.cc
@@ -1,7 +1,7 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
+#include "sema_manager.hh"
 #include "filesystem.hh"
 #include "include_complete.hh"
 #include "log.hh"
@@ -229,7 +229,7 @@ void *Indexer(void *arg_) {
   delete arg;
   std::string name = "indexer" + std::to_string(idx);
   set_thread_name(name.c_str());
-  pipeline::Indexer_Main(h->clang_complete, h->vfs, h->project, h->wfiles);
+  pipeline::Indexer_Main(h->manager, h->vfs, h->project, h->wfiles);
   return nullptr;
 }
 } // namespace
@@ -325,6 +325,8 @@ void Initialize(MessageHandler *m, InitializeParam &param, ReplyOnce &reply) {
 
   LOG_S(INFO) << "dispatch initial index requests";
   m->project->Index(m->wfiles, reply.id);
+
+  m->manager->sessions.SetCapacity(g_config->session.maxNum);
 }
 
 void MessageHandler::initialize(Reader &reader, ReplyOnce &reply) {
diff --git a/src/messages/textDocument_code.cc b/src/messages/textDocument_code.cc
index 17395b75..abd94c1b 100644
--- a/src/messages/textDocument_code.cc
+++ b/src/messages/textDocument_code.cc
@@ -21,13 +21,12 @@ MAKE_REFLECT_STRUCT(CodeAction, title, kind, edit);
 }
 void MessageHandler::textDocument_codeAction(CodeActionParam &param,
                                              ReplyOnce &reply) {
-  WorkingFile *wf =
-      wfiles->GetFileByFilename(param.textDocument.uri.GetPath());
+  WorkingFile *wf = wfiles->GetFile(param.textDocument.uri.GetPath());
   if (!wf)
     return;
   std::vector<CodeAction> result;
   std::vector<Diagnostic> diagnostics;
-  wfiles->DoAction([&]() { diagnostics = wf->diagnostics_; });
+  wfiles->WithLock([&]() { diagnostics = wf->diagnostics; });
   for (Diagnostic &diag : diagnostics)
     if (diag.fixits_.size() &&
         (param.range.Intersects(diag.range) ||
@@ -85,9 +84,8 @@ void MessageHandler::textDocument_codeLens(TextDocumentParam &param,
   std::string path = param.textDocument.uri.GetPath();
 
   QueryFile *file = FindFile(reply, path);
-  WorkingFile *wfile =
-      file ? wfiles->GetFileByFilename(file->def->path) : nullptr;
-  if (!wfile) {
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf) {
     return;
   }
 
@@ -95,7 +93,7 @@ void MessageHandler::textDocument_codeLens(TextDocumentParam &param,
                  bool force_display = false) {
     if (!num && !force_display)
       return;
-    std::optional<lsRange> ls_range = GetLsRange(wfile, range);
+    std::optional<lsRange> ls_range = GetLsRange(wf, range);
     if (!ls_range)
       return;
     CodeLens &code_lens = result.emplace_back();
diff --git a/src/messages/textDocument_completion.cc b/src/messages/textDocument_completion.cc
index bce205a7..99e03856 100644
--- a/src/messages/textDocument_completion.cc
+++ b/src/messages/textDocument_completion.cc
@@ -1,12 +1,12 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
 #include "fuzzy_match.hh"
 #include "include_complete.hh"
 #include "log.hh"
 #include "message_handler.hh"
 #include "pipeline.hh"
+#include "sema_manager.hh"
 #include "working_files.hh"
 
 #include <clang/Sema/CodeCompleteConsumer.h>
@@ -439,7 +439,7 @@ void MessageHandler::textDocument_completion(CompletionParam &param,
   static CompleteConsumerCache<std::vector<CompletionItem>> cache;
   CompletionList result;
   std::string path = param.textDocument.uri.GetPath();
-  WorkingFile *file = wfiles->GetFileByFilename(path);
+  WorkingFile *file = wfiles->GetFile(path);
   if (!file) {
     return;
   }
@@ -511,7 +511,7 @@ void MessageHandler::textDocument_completion(CompletionParam &param,
   }
 #endif
 
-  CompletionManager::OnComplete callback =
+  SemaManager::OnComplete callback =
       [filter, path, begin_pos, end_pos, reply,
        buffer_line](CodeCompleteConsumer *OptConsumer) {
         if (!OptConsumer)
@@ -536,11 +536,9 @@ void MessageHandler::textDocument_completion(CompletionParam &param,
     cache.WithLock([&]() { Consumer.ls_items = cache.result; });
     callback(&Consumer);
   } else {
-    clang_complete->completion_request_.PushBack(
-        std::make_unique<CompletionManager::CompletionRequest>(
-            reply.id, param.textDocument, begin_pos,
-            std::make_unique<CompletionConsumer>(CCOpts, false), CCOpts,
-            callback));
+    manager->comp_tasks.PushBack(std::make_unique<SemaManager::CompTask>(
+        reply.id, param.textDocument.uri.GetPath(), begin_pos,
+        std::make_unique<CompletionConsumer>(CCOpts, false), CCOpts, callback));
   }
 }
 } // namespace ccls
diff --git a/src/messages/textDocument_definition.cc b/src/messages/textDocument_definition.cc
index c1e3f806..b8017492 100644
--- a/src/messages/textDocument_definition.cc
+++ b/src/messages/textDocument_definition.cc
@@ -37,15 +37,15 @@ void MessageHandler::textDocument_definition(TextDocumentPositionParam &param,
                                              ReplyOnce &reply) {
   int file_id;
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath(), &file_id);
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
 
   std::vector<Location> result;
   Maybe<Use> on_def;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
   Position &ls_pos = param.position;
 
-  for (SymbolRef sym : FindSymbolsAtLocation(wfile, file, ls_pos, true)) {
+  for (SymbolRef sym : FindSymbolsAtLocation(wf, file, ls_pos, true)) {
     // Special cases which are handled:
     //  - symbol has declaration but no definition (ie, pure virtual)
     //  - goto declaration while in definition of recursive type
@@ -96,7 +96,7 @@ void MessageHandler::textDocument_definition(TextDocumentPositionParam &param,
     // Find the best match of the identifier at point.
     if (!range) {
       Position position = param.position;
-      const std::string &buffer = wfile->buffer_content;
+      const std::string &buffer = wf->buffer_content;
       std::string_view query = LexIdentifierAroundPos(position, buffer);
       std::string_view short_query = query;
       {
@@ -160,7 +160,7 @@ void MessageHandler::textDocument_typeDefinition(
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
   if (!file)
     return;
-  WorkingFile *working_file = wfiles->GetFileByFilename(file->def->path);
+  WorkingFile *working_file = wfiles->GetFile(file->def->path);
 
   std::vector<Location> result;
   auto Add = [&](const QueryType &type) {
diff --git a/src/messages/textDocument_did.cc b/src/messages/textDocument_did.cc
index 5ef8adb4..7a6bf1a2 100644
--- a/src/messages/textDocument_did.cc
+++ b/src/messages/textDocument_did.cc
@@ -1,11 +1,11 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
 #include "include_complete.hh"
 #include "message_handler.hh"
 #include "pipeline.hh"
 #include "project.hh"
+#include "sema_manager.hh"
 #include "working_files.hh"
 
 namespace ccls {
@@ -14,31 +14,31 @@ void MessageHandler::textDocument_didChange(TextDocumentDidChangeParam &param) {
   wfiles->OnChange(param);
   if (g_config->index.onChange)
     pipeline::Index(path, {}, IndexMode::OnChange);
-  clang_complete->NotifyView(path);
+  manager->OnView(path);
   if (g_config->diagnostics.onChange >= 0)
-    clang_complete->DiagnosticsUpdate(path, g_config->diagnostics.onChange);
+    manager->ScheduleDiag(path, g_config->diagnostics.onChange);
 }
 
 void MessageHandler::textDocument_didClose(TextDocumentParam &param) {
   std::string path = param.textDocument.uri.GetPath();
-  wfiles->OnClose(param.textDocument);
-  clang_complete->OnClose(path);
+  wfiles->OnClose(path);
+  manager->OnClose(path);
 }
 
 void MessageHandler::textDocument_didOpen(DidOpenTextDocumentParam &param) {
   std::string path = param.textDocument.uri.GetPath();
-  WorkingFile *working_file = wfiles->OnOpen(param.textDocument);
+  WorkingFile *wf = wfiles->OnOpen(param.textDocument);
   if (std::optional<std::string> cached_file_contents =
           pipeline::LoadIndexedContent(path))
-    working_file->SetIndexContent(*cached_file_contents);
+    wf->SetIndexContent(*cached_file_contents);
 
   ReplyOnce reply;
   QueryFile *file = FindFile(reply, path);
   if (file) {
-    EmitSkippedRanges(working_file, *file);
-    EmitSemanticHighlight(db, working_file, *file);
+    EmitSkippedRanges(wf, *file);
+    EmitSemanticHighlight(db, wf, *file);
   }
-  include_complete->AddFile(working_file->filename);
+  include_complete->AddFile(wf->filename);
 
   // Submit new index request if it is not a header file or there is no
   // pending index request.
@@ -47,12 +47,12 @@ void MessageHandler::textDocument_didOpen(DidOpenTextDocumentParam &param) {
       !pipeline::pending_index_requests)
     pipeline::Index(path, {}, IndexMode::Normal);
 
-  clang_complete->NotifyView(path);
+  manager->OnView(path);
 }
 
 void MessageHandler::textDocument_didSave(TextDocumentParam &param) {
   const std::string &path = param.textDocument.uri.GetPath();
   pipeline::Index(path, {}, IndexMode::Normal);
-  clang_complete->NotifySave(path);
+  manager->OnSave(path);
 }
 } // namespace ccls
diff --git a/src/messages/textDocument_document.cc b/src/messages/textDocument_document.cc
index eaddd7ca..c4b1cf88 100644
--- a/src/messages/textDocument_document.cc
+++ b/src/messages/textDocument_document.cc
@@ -33,14 +33,13 @@ void MessageHandler::textDocument_documentHighlight(
     TextDocumentPositionParam &param, ReplyOnce &reply) {
   int file_id;
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath(), &file_id);
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
 
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
   std::vector<DocumentHighlight> result;
-
   std::vector<SymbolRef> syms =
-      FindSymbolsAtLocation(wfile, file, param.position, true);
+      FindSymbolsAtLocation(wf, file, param.position, true);
   for (auto [sym, refcnt] : file->symbol2refcnt) {
     if (refcnt <= 0)
       continue;
@@ -78,7 +77,7 @@ MAKE_REFLECT_STRUCT(DocumentLink, range, target);
 void MessageHandler::textDocument_documentLink(TextDocumentParam &param,
                                                ReplyOnce &reply) {
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  WorkingFile *wf = file ? wfiles->GetFileByFilename(file->def->path) : nullptr;
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
   if (!wf) {
     reply.Error(ErrorCode::InternalError, "not opened");
     return;
@@ -153,10 +152,8 @@ void MessageHandler::textDocument_documentSymbol(Reader &reader,
 
   int file_id;
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath(), &file_id);
-  if (!file)
-    return;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
-  if (!wfile)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
 
   if (param.startLine >= 0) {
@@ -181,14 +178,14 @@ void MessageHandler::textDocument_documentSymbol(Reader &reader,
         continue;
       auto &ds = r.first->second;
       ds = std::make_unique<DocumentSymbol>();
-      if (auto range = GetLsRange(wfile, sym.range)) {
+      if (auto range = GetLsRange(wf, sym.range)) {
         ds->selectionRange = *range;
         ds->range = ds->selectionRange;
         // For a macro expansion, M(name), we may use `M` for extent and `name`
         // for spell, do the check as selectionRange must be a subrange of
         // range.
         if (sym.extent.Valid())
-          if (auto range1 = GetLsRange(wfile, sym.extent);
+          if (auto range1 = GetLsRange(wf, sym.extent);
               range1 && range1->Includes(*range))
             ds->range = *range1;
       }
diff --git a/src/messages/textDocument_foldingRange.cc b/src/messages/textDocument_foldingRange.cc
index da3f5247..f5a5e4a7 100644
--- a/src/messages/textDocument_foldingRange.cc
+++ b/src/messages/textDocument_foldingRange.cc
@@ -22,7 +22,7 @@ void MessageHandler::textDocument_foldingRange(TextDocumentParam &param,
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
   if (!file)
     return;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
+  WorkingFile *wfile = wfiles->GetFile(file->def->path);
   if (!wfile)
     return;
   std::vector<FoldingRange> result;
diff --git a/src/messages/textDocument_formatting.cc b/src/messages/textDocument_formatting.cc
index c82b4f9b..9713b952 100644
--- a/src/messages/textDocument_formatting.cc
+++ b/src/messages/textDocument_formatting.cc
@@ -69,41 +69,35 @@ void Format(ReplyOnce &reply, WorkingFile *wfile, tooling::Range range) {
 void MessageHandler::textDocument_formatting(DocumentFormattingParam &param,
                                              ReplyOnce &reply) {
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
-  if (!wfile)
-    return;
-  Format(reply, wfile, {0, (unsigned)wfile->buffer_content.size()});
+  Format(reply, wf, {0, (unsigned)wf->buffer_content.size()});
 }
 
 void MessageHandler::textDocument_onTypeFormatting(
     DocumentOnTypeFormattingParam &param, ReplyOnce &reply) {
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
-  if (!wfile)
-    return;
-  std::string_view code = wfile->buffer_content;
+  std::string_view code = wf->buffer_content;
   int pos = GetOffsetForPosition(param.position, code);
   auto lbrace = code.find_last_of('{', pos);
   if (lbrace == std::string::npos)
     lbrace = pos;
-  Format(reply, wfile, {(unsigned)lbrace, unsigned(pos - lbrace)});
+  Format(reply, wf, {(unsigned)lbrace, unsigned(pos - lbrace)});
 }
 
 void MessageHandler::textDocument_rangeFormatting(
     DocumentRangeFormattingParam &param, ReplyOnce &reply) {
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
-  if (!wfile)
-    return;
-  std::string_view code = wfile->buffer_content;
+  std::string_view code = wf->buffer_content;
   int begin = GetOffsetForPosition(param.range.start, code),
       end = GetOffsetForPosition(param.range.end, code);
-  Format(reply, wfile, {(unsigned)begin, unsigned(end - begin)});
+  Format(reply, wf, {(unsigned)begin, unsigned(end - begin)});
 }
 } // namespace ccls
diff --git a/src/messages/textDocument_hover.cc b/src/messages/textDocument_hover.cc
index f0a71f1c..bb81df2f 100644
--- a/src/messages/textDocument_hover.cc
+++ b/src/messages/textDocument_hover.cc
@@ -82,15 +82,14 @@ GetHover(DB *db, LanguageId lang, SymbolRef sym, int file_id) {
 void MessageHandler::textDocument_hover(TextDocumentPositionParam &param,
                                         ReplyOnce &reply) {
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
-
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
   Hover result;
 
-  for (SymbolRef sym : FindSymbolsAtLocation(wfile, file, param.position)) {
+  for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position)) {
     std::optional<lsRange> ls_range =
-        GetLsRange(wfiles->GetFileByFilename(file->def->path), sym.range);
+        GetLsRange(wfiles->GetFile(file->def->path), sym.range);
     if (!ls_range)
       continue;
 
diff --git a/src/messages/textDocument_references.cc b/src/messages/textDocument_references.cc
index 410554dd..58abf6b2 100644
--- a/src/messages/textDocument_references.cc
+++ b/src/messages/textDocument_references.cc
@@ -33,10 +33,8 @@ void MessageHandler::textDocument_references(Reader &reader, ReplyOnce &reply) {
   ReferenceParam param;
   Reflect(reader, param);
   QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
-  if (!file)
-    return;
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
-  if (!wfile)
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
   for (auto &folder : param.folders)
     EnsureEndsInSlash(folder);
@@ -46,7 +44,7 @@ void MessageHandler::textDocument_references(Reader &reader, ReplyOnce &reply) {
   std::unordered_set<Use> seen_uses;
   int line = param.position.line;
 
-  for (SymbolRef sym : FindSymbolsAtLocation(wfile, file, param.position)) {
+  for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position)) {
     // Found symbol. Return references.
     std::unordered_set<Usr> seen;
     seen.insert(sym.usr);
@@ -95,7 +93,7 @@ void MessageHandler::textDocument_references(Reader &reader, ReplyOnce &reply) {
     // = 0,
     // use the current filename.
     std::string path;
-    if (line == 0 || line >= (int)wfile->buffer_lines.size() - 1)
+    if (line == 0 || line >= (int)wf->buffer_lines.size() - 1)
       path = file->def->path;
     for (const IndexInclude &include : file->def->includes)
       if (include.line == param.position.line) {
diff --git a/src/messages/textDocument_rename.cc b/src/messages/textDocument_rename.cc
index 18abde67..83725113 100644
--- a/src/messages/textDocument_rename.cc
+++ b/src/messages/textDocument_rename.cc
@@ -26,7 +26,7 @@ WorkspaceEdit BuildWorkspaceEdit(DB *db, WorkingFiles *wfiles, SymbolRef sym,
       const std::string &path = file.def->path;
       path_to_edit[file_id].textDocument.uri = DocumentUri::FromPath(path);
 
-      WorkingFile *working_file = wfiles->GetFileByFilename(path);
+      WorkingFile *working_file = wfiles->GetFile(path);
       if (working_file)
         path_to_edit[file_id].textDocument.version = working_file->version;
     }
@@ -44,14 +44,13 @@ WorkspaceEdit BuildWorkspaceEdit(DB *db, WorkingFiles *wfiles, SymbolRef sym,
 } // namespace
 
 void MessageHandler::textDocument_rename(RenameParam &param, ReplyOnce &reply) {
-  int file_id;
-  QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath(), &file_id);
-  if (!file)
+  QueryFile *file = FindFile(reply, param.textDocument.uri.GetPath());
+  WorkingFile *wf = file ? wfiles->GetFile(file->def->path) : nullptr;
+  if (!wf)
     return;
 
-  WorkingFile *wfile = wfiles->GetFileByFilename(file->def->path);
   WorkspaceEdit result;
-  for (SymbolRef sym : FindSymbolsAtLocation(wfile, file, param.position)) {
+  for (SymbolRef sym : FindSymbolsAtLocation(wf, file, param.position)) {
     result = BuildWorkspaceEdit(db, wfiles, sym, param.newName);
     break;
   }
diff --git a/src/messages/textDocument_signatureHelp.cc b/src/messages/textDocument_signatureHelp.cc
index 3d33c780..f51be49c 100644
--- a/src/messages/textDocument_signatureHelp.cc
+++ b/src/messages/textDocument_signatureHelp.cc
@@ -1,9 +1,9 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
 #include "message_handler.hh"
 #include "pipeline.hh"
+#include "sema_manager.hh"
 
 #include <clang/Sema/Sema.h>
 
@@ -144,14 +144,14 @@ void MessageHandler::textDocument_signatureHelp(
 
   std::string path = param.textDocument.uri.GetPath();
   Position begin_pos = param.position;
-  if (WorkingFile *file = wfiles->GetFileByFilename(path)) {
+  if (WorkingFile *file = wfiles->GetFile(path)) {
     std::string completion_text;
     Position end_pos = param.position;
     begin_pos = file->FindStableCompletionSource(param.position,
                                                  &completion_text, &end_pos);
   }
 
-  CompletionManager::OnComplete callback =
+  SemaManager::OnComplete callback =
       [reply, path, begin_pos](CodeCompleteConsumer *OptConsumer) {
         if (!OptConsumer)
           return;
@@ -175,11 +175,10 @@ void MessageHandler::textDocument_signatureHelp(
     cache.WithLock([&]() { Consumer.ls_sighelp = cache.result; });
     callback(&Consumer);
   } else {
-    clang_complete->completion_request_.PushBack(
-        std::make_unique<CompletionManager::CompletionRequest>(
-            reply.id, param.textDocument, param.position,
-            std::make_unique<SignatureHelpConsumer>(CCOpts, false), CCOpts,
-            callback));
+    manager->comp_tasks.PushBack(std::make_unique<SemaManager::CompTask>(
+        reply.id, param.textDocument.uri.GetPath(), param.position,
+        std::make_unique<SignatureHelpConsumer>(CCOpts, false), CCOpts,
+        callback));
   }
 }
 } // namespace ccls
diff --git a/src/messages/workspace.cc b/src/messages/workspace.cc
index 3f14ed89..cc36578b 100644
--- a/src/messages/workspace.cc
+++ b/src/messages/workspace.cc
@@ -1,7 +1,7 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
+#include "sema_manager.hh"
 #include "fuzzy_match.hh"
 #include "log.hh"
 #include "message_handler.hh"
@@ -25,29 +25,28 @@ void MessageHandler::workspace_didChangeConfiguration(EmptyParam &) {
     project->Load(folder);
   project->Index(wfiles, RequestId());
 
-  clang_complete->FlushAllSessions();
+  manager->Clear();
 };
 
 void MessageHandler::workspace_didChangeWatchedFiles(
     DidChangeWatchedFilesParam &param) {
   for (auto &event : param.changes) {
     std::string path = event.uri.GetPath();
-    IndexMode mode = wfiles->GetFileByFilename(path)
-                         ? IndexMode::Normal
-                         : IndexMode::NonInteractive;
+    IndexMode mode =
+        wfiles->GetFile(path) ? IndexMode::Normal : IndexMode::NonInteractive;
     switch (event.type) {
     case FileChangeType::Created:
     case FileChangeType::Changed: {
       pipeline::Index(path, {}, mode);
       if (mode == IndexMode::Normal)
-        clang_complete->NotifySave(path);
+        manager->OnSave(path);
       else
-        clang_complete->OnClose(path);
+        manager->OnClose(path);
       break;
     }
     case FileChangeType::Deleted:
       pipeline::Index(path, {}, mode);
-      clang_complete->OnClose(path);
+      manager->OnClose(path);
       break;
     }
   }
@@ -79,7 +78,7 @@ void MessageHandler::workspace_didChangeWorkspaceFolders(
 
   project->Index(wfiles, RequestId());
 
-  clang_complete->FlushAllSessions();
+  manager->Clear();
 }
 
 namespace {
diff --git a/src/pipeline.cc b/src/pipeline.cc
index 805e8c2e..5f534bf2 100644
--- a/src/pipeline.cc
+++ b/src/pipeline.cc
@@ -3,7 +3,6 @@
 
 #include "pipeline.hh"
 
-#include "clang_complete.hh"
 #include "config.hh"
 #include "include_complete.hh"
 #include "log.hh"
@@ -13,6 +12,7 @@
 #include "platform.hh"
 #include "project.hh"
 #include "query.hh"
+#include "sema_manager.hh"
 #include "serializers/json.hh"
 
 #include <rapidjson/document.h>
@@ -167,7 +167,7 @@ std::mutex &GetFileMutex(const std::string &path) {
   return mutexes[std::hash<std::string>()(path) % N_MUTEXES];
 }
 
-bool Indexer_Parse(CompletionManager *completion, WorkingFiles *wfiles,
+bool Indexer_Parse(SemaManager *completion, WorkingFiles *wfiles,
                    Project *project, VFS *vfs, const GroupMatch &matcher) {
   std::optional<Index_Request> opt_request = index_request->TryPopFront();
   if (!opt_request)
@@ -364,11 +364,11 @@ void Init() {
   for_stdout = new ThreadedQueue<std::string>(stdout_waiter);
 }
 
-void Indexer_Main(CompletionManager *completion, VFS *vfs, Project *project,
+void Indexer_Main(SemaManager *manager, VFS *vfs, Project *project,
                   WorkingFiles *wfiles) {
   GroupMatch matcher(g_config->index.whitelist, g_config->index.blacklist);
   while (true)
-    if (!Indexer_Parse(completion, wfiles, project, vfs, matcher))
+    if (!Indexer_Parse(manager, wfiles, project, vfs, matcher))
       indexer_waiter->Wait(index_request);
 }
 
@@ -376,13 +376,13 @@ void Main_OnIndexed(DB *db, WorkingFiles *wfiles, IndexUpdate *update) {
   if (update->refresh) {
     LOG_S(INFO)
         << "loaded project. Refresh semantic highlight for all working file.";
-    std::lock_guard<std::mutex> lock(wfiles->files_mutex);
-    for (auto &f : wfiles->files) {
-      std::string filename = LowerPathIfInsensitive(f->filename);
-      if (db->name2file_id.find(filename) == db->name2file_id.end())
+    std::lock_guard lock(wfiles->mutex);
+    for (auto &[f, wf] : wfiles->files) {
+      std::string path = LowerPathIfInsensitive(f);
+      if (db->name2file_id.find(path) == db->name2file_id.end())
         continue;
-      QueryFile &file = db->files[db->name2file_id[filename]];
-      EmitSemanticHighlight(db, f.get(), file);
+      QueryFile &file = db->files[db->name2file_id[path]];
+      EmitSemanticHighlight(db, wf.get(), file);
     }
     return;
   }
@@ -395,7 +395,7 @@ void Main_OnIndexed(DB *db, WorkingFiles *wfiles, IndexUpdate *update) {
   // Update indexed content, skipped ranges, and semantic highlighting.
   if (update->files_def_update) {
     auto &def_u = *update->files_def_update;
-    if (WorkingFile *wfile = wfiles->GetFileByFilename(def_u.first.path)) {
+    if (WorkingFile *wfile = wfiles->GetFile(def_u.first.path)) {
       // FIXME With index.onChange: true, use buffer_content only for
       // request.path
       wfile->SetIndexContent(g_config->index.onChange ? wfile->buffer_content
@@ -483,7 +483,7 @@ void MainLoop() {
   WorkingFiles wfiles;
   VFS vfs;
 
-  CompletionManager clang_complete(
+  SemaManager manager(
       &project, &wfiles,
       [&](std::string path, std::vector<Diagnostic> diagnostics) {
         PublishDiagnosticParam params;
@@ -509,7 +509,7 @@ void MainLoop() {
   handler.project = &project;
   handler.vfs = &vfs;
   handler.wfiles = &wfiles;
-  handler.clang_complete = &clang_complete;
+  handler.manager = &manager;
   handler.include_complete = &include_complete;
 
   bool has_indexed = false;
@@ -545,12 +545,16 @@ void Standalone(const std::string &root) {
   Project project;
   WorkingFiles wfiles;
   VFS vfs;
+  SemaManager manager(
+      nullptr, nullptr, [&](std::string, std::vector<Diagnostic>) {},
+      [](RequestId id) {});
   IncludeComplete complete(&project);
 
   MessageHandler handler;
   handler.project = &project;
   handler.wfiles = &wfiles;
   handler.vfs = &vfs;
+  handler.manager = &manager;
   handler.include_complete = &complete;
 
   StandaloneInitialize(handler, root);
diff --git a/src/pipeline.hh b/src/pipeline.hh
index 86eae6c2..47a40b7b 100644
--- a/src/pipeline.hh
+++ b/src/pipeline.hh
@@ -13,7 +13,7 @@
 #include <vector>
 
 namespace ccls {
-struct CompletionManager;
+struct SemaManager;
 struct GroupMatch;
 struct Project;
 struct WorkingFiles;
@@ -44,7 +44,7 @@ extern int64_t tick;
 void Init();
 void LaunchStdin();
 void LaunchStdout();
-void Indexer_Main(CompletionManager *completion, VFS *vfs, Project *project,
+void Indexer_Main(SemaManager *manager, VFS *vfs, Project *project,
                   WorkingFiles *wfiles);
 void MainLoop();
 void Standalone(const std::string &root);
diff --git a/src/project.cc b/src/project.cc
index 0286fa26..aac2694c 100644
--- a/src/project.cc
+++ b/src/project.cc
@@ -473,8 +473,7 @@ void Project::Index(WorkingFiles *wfiles, RequestId id) {
         std::string reason;
         if (match.Matches(entry.filename, &reason) &&
             match_i.Matches(entry.filename, &reason)) {
-          bool interactive =
-              wfiles->GetFileByFilename(entry.filename) != nullptr;
+          bool interactive = wfiles->GetFile(entry.filename) != nullptr;
           pipeline::Index(
               entry.filename, entry.args,
               interactive ? IndexMode::Normal : IndexMode::NonInteractive, id);
diff --git a/src/query.cc b/src/query.cc
index d015229a..5cc6cf8f 100644
--- a/src/query.cc
+++ b/src/query.cc
@@ -657,8 +657,7 @@ DocumentUri GetLsDocumentUri(DB *db, int file_id) {
 std::optional<Location> GetLsLocation(DB *db, WorkingFiles *wfiles, Use use) {
   std::string path;
   DocumentUri uri = GetLsDocumentUri(db, use.file_id, &path);
-  std::optional<lsRange> range =
-      GetLsRange(wfiles->GetFileByFilename(path), use.range);
+  std::optional<lsRange> range = GetLsRange(wfiles->GetFile(path), use.range);
   if (!range)
     return std::nullopt;
   return Location{uri, *range};
diff --git a/src/clang_complete.cc b/src/sema_manager.cc
similarity index 69%
rename from src/clang_complete.cc
rename to src/sema_manager.cc
index 8bb23b1a..f22ed9ae 100644
--- a/src/clang_complete.cc
+++ b/src/sema_manager.cc
@@ -1,7 +1,7 @@
 // Copyright 2017-2018 ccls Authors
 // SPDX-License-Identifier: Apache-2.0
 
-#include "clang_complete.hh"
+#include "sema_manager.hh"
 
 #include "clang_tu.hh"
 #include "filesystem.hh"
@@ -69,6 +69,8 @@ TextEdit ToTextEdit(const clang::SourceManager &SM, const clang::LangOptions &L,
   return edit;
 }
 
+using IncludeStructure = std::vector<std::pair<std::string, int64_t>>;
+
 struct PreambleStatCache {
   llvm::StringMap<ErrorOr<llvm::vfs::Status>> Cache;
 
@@ -120,11 +122,13 @@ struct PreambleStatCache {
 };
 
 struct PreambleData {
-  PreambleData(clang::PrecompiledPreamble P, std::vector<Diag> diags,
+  PreambleData(clang::PrecompiledPreamble P, IncludeStructure includes,
+               std::vector<Diag> diags,
                std::unique_ptr<PreambleStatCache> stat_cache)
-      : Preamble(std::move(P)), diags(std::move(diags)),
-        stat_cache(std::move(stat_cache)) {}
+      : Preamble(std::move(P)), includes(std::move(includes)),
+        diags(std::move(diags)), stat_cache(std::move(stat_cache)) {}
   clang::PrecompiledPreamble Preamble;
+  IncludeStructure includes;
   std::vector<Diag> diags;
   std::unique_ptr<PreambleStatCache> stat_cache;
 };
@@ -161,7 +165,40 @@ CharSourceRange DiagnosticRange(const clang::Diagnostic &D, const LangOptions &L
   return R;
 }
 
+class StoreInclude : public PPCallbacks {
+  const SourceManager &SM;
+  IncludeStructure &out;
+  DenseSet<const FileEntry *> Seen;
 
+public:
+  StoreInclude(const SourceManager &SM, IncludeStructure &out)
+      : SM(SM), out(out) {}
+  void InclusionDirective(SourceLocation HashLoc, const Token &IncludeTok,
+                          StringRef FileName, bool IsAngled,
+                          CharSourceRange FilenameRange, const FileEntry *File,
+                          StringRef SearchPath, StringRef RelativePath,
+                          const clang::Module *Imported
+#if LLVM_VERSION_MAJOR >= 7
+                          , SrcMgr::CharacteristicKind FileKind
+#endif
+                          ) override {
+    (void)SM;
+    if (File && Seen.insert(File).second)
+      out.emplace_back(PathFromFileEntry(*File), File->getModificationTime());
+  }
+};
+
+class CclsPreambleCallbacks : public PreambleCallbacks {
+public:
+  void BeforeExecute(CompilerInstance &CI) override {
+    SM = &CI.getSourceManager();
+  }
+  std::unique_ptr<PPCallbacks> createPPCallbacks() override {
+    return std::make_unique<StoreInclude>(*SM, includes);
+  }
+  SourceManager *SM = nullptr;
+  IncludeStructure includes;
+};
 
 class StoreDiags : public DiagnosticConsumer {
   const LangOptions *LangOpts;
@@ -250,7 +287,7 @@ public:
 };
 
 std::unique_ptr<CompilerInstance> BuildCompilerInstance(
-    CompletionSession &session, std::unique_ptr<CompilerInvocation> CI,
+    Session &session, std::unique_ptr<CompilerInvocation> CI,
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS, DiagnosticConsumer &DC,
     const PreambleData *preamble, const std::string &path,
     std::unique_ptr<llvm::MemoryBuffer> &Buf) {
@@ -291,16 +328,17 @@ bool Parse(CompilerInstance &Clang) {
   return true;
 }
 
-void BuildPreamble(CompletionSession &session, CompilerInvocation &CI,
+void BuildPreamble(Session &session, CompilerInvocation &CI,
                    IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS,
-                   const std::string &main,
+                   const SemaManager::PreambleTask &task,
                    std::unique_ptr<PreambleStatCache> stat_cache) {
   std::shared_ptr<PreambleData> OldP = session.GetPreamble();
-  std::string content = session.wfiles->GetContent(main);
+  std::string content = session.wfiles->GetContent(task.path);
   std::unique_ptr<llvm::MemoryBuffer> Buf =
       llvm::MemoryBuffer::getMemBuffer(content);
   auto Bounds = ComputePreambleBounds(*CI.getLangOpts(), Buf.get(), 0);
-  if (OldP && OldP->Preamble.CanReuse(CI, Buf.get(), Bounds, FS.get()))
+  if (!task.from_diag && OldP &&
+      OldP->Preamble.CanReuse(CI, Buf.get(), Bounds, FS.get()))
     return;
   // -Werror makes warnings issued as errors, which stops parsing
   // prematurely because of -ferror-limit=. This also works around the issue
@@ -312,70 +350,88 @@ void BuildPreamble(CompletionSession &session, CompilerInvocation &CI,
   CI.getLangOpts()->CommentOpts.ParseAllComments = g_config->index.comments > 1;
   CI.getLangOpts()->RetainCommentsFromSystemHeaders = true;
 
-  StoreDiags DC(main);
+  StoreDiags DC(task.path);
   IntrusiveRefCntPtr<DiagnosticsEngine> DE =
       CompilerInstance::createDiagnostics(&CI.getDiagnosticOpts(), &DC, false);
-  PreambleCallbacks PP;
+  if (OldP) {
+    std::lock_guard lock(session.wfiles->mutex);
+    for (auto &include : OldP->includes)
+      if (WorkingFile *wf = session.wfiles->GetFileUnlocked(include.first))
+        CI.getPreprocessorOpts().addRemappedFile(
+            include.first,
+            llvm::MemoryBuffer::getMemBufferCopy(wf->buffer_content).release());
+  }
+
+  CclsPreambleCallbacks PC;
   if (auto NewPreamble = PrecompiledPreamble::Build(
-          CI, Buf.get(), Bounds, *DE, FS, session.PCH, true, PP)) {
+          CI, Buf.get(), Bounds, *DE, FS, session.PCH, true, PC)) {
+    assert(!CI.getPreprocessorOpts().RetainRemappedFileBuffers);
+    if (OldP) {
+      auto &old_includes = OldP->includes;
+      auto it = old_includes.begin();
+      std::sort(PC.includes.begin(), PC.includes.end());
+      for (auto &include : PC.includes)
+        if (include.second == 0) {
+          while (it != old_includes.end() && it->first < include.first)
+            ++it;
+          if (it == old_includes.end())
+            break;
+          include.second = it->second;
+        }
+    }
+
     std::lock_guard lock(session.mutex);
     session.preamble = std::make_shared<PreambleData>(
-        std::move(*NewPreamble), DC.Take(), std::move(stat_cache));
+        std::move(*NewPreamble), std::move(PC.includes), DC.Take(),
+        std::move(stat_cache));
   }
 }
 
-void *CompletionPreloadMain(void *manager_) {
-  auto *manager = static_cast<CompletionManager*>(manager_);
-  set_thread_name("comp-preload");
+void *PreambleMain(void *manager_) {
+  auto *manager = static_cast<SemaManager *>(manager_);
+  set_thread_name("preamble");
   while (true) {
-    auto request = manager->preload_requests_.Dequeue();
+    SemaManager::PreambleTask task = manager->preamble_tasks.Dequeue();
 
-    bool is_open = false;
-    std::shared_ptr<CompletionSession> session =
-        manager->TryGetSession(request.path, true, &is_open);
-    if (!session)
-      continue;
+    bool created = false;
+    std::shared_ptr<Session> session =
+        manager->EnsureSession(task.path, &created);
 
-    const auto &args = session->file.args;
     auto stat_cache = std::make_unique<PreambleStatCache>();
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS =
         stat_cache->Producer(session->FS);
     if (std::unique_ptr<CompilerInvocation> CI =
-            BuildCompilerInvocation(args, FS))
-      BuildPreamble(*session, *CI, FS, request.path, std::move(stat_cache));
+            BuildCompilerInvocation(session->file.args, FS))
+      BuildPreamble(*session, *CI, FS, task, std::move(stat_cache));
 
-    int debounce =
-        is_open ? g_config->diagnostics.onOpen : g_config->diagnostics.onSave;
-    if (debounce >= 0) {
-      TextDocumentIdentifier document;
-      document.uri = DocumentUri::FromPath(request.path);
-      manager->DiagnosticsUpdate(request.path, debounce);
+    if (task.from_diag) {
+      manager->ScheduleDiag(task.path, 0);
+    } else {
+      int debounce =
+          created ? g_config->diagnostics.onOpen : g_config->diagnostics.onSave;
+      if (debounce >= 0)
+        manager->ScheduleDiag(task.path, debounce);
     }
   }
   return nullptr;
 }
 
 void *CompletionMain(void *manager_) {
-  auto *manager = static_cast<CompletionManager *>(manager_);
+  auto *manager = static_cast<SemaManager *>(manager_);
   set_thread_name("comp");
   while (true) {
-    // Fetching the completion request blocks until we have a request.
-    std::unique_ptr<CompletionManager::CompletionRequest> request =
-        manager->completion_request_.Dequeue();
+    std::unique_ptr<SemaManager::CompTask> task = manager->comp_tasks.Dequeue();
 
     // Drop older requests if we're not buffering.
     while (g_config->completion.dropOldRequests &&
-           !manager->completion_request_.IsEmpty()) {
-      manager->on_dropped_(request->id);
-      request->Consumer.reset();
-      request->on_complete(nullptr);
-      request = manager->completion_request_.Dequeue();
+           !manager->comp_tasks.IsEmpty()) {
+      manager->on_dropped_(task->id);
+      task->Consumer.reset();
+      task->on_complete(nullptr);
+      task = manager->comp_tasks.Dequeue();
     }
 
-    std::string path = request->document.uri.GetPath();
-
-    std::shared_ptr<CompletionSession> session =
-        manager->TryGetSession(path, false);
+    std::shared_ptr<Session> session = manager->EnsureSession(task->path);
     std::shared_ptr<PreambleData> preamble = session->GetPreamble();
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS =
         preamble ? preamble->stat_cache->Consumer(session->FS) : session->FS;
@@ -384,34 +440,34 @@ void *CompletionMain(void *manager_) {
     if (!CI)
       continue;
     auto &FOpts = CI->getFrontendOpts();
-    FOpts.CodeCompleteOpts = request->CCOpts;
-    FOpts.CodeCompletionAt.FileName = path;
-    FOpts.CodeCompletionAt.Line = request->position.line + 1;
-    FOpts.CodeCompletionAt.Column = request->position.character + 1;
+    FOpts.CodeCompleteOpts = task->CCOpts;
+    FOpts.CodeCompletionAt.FileName = task->path;
+    FOpts.CodeCompletionAt.Line = task->position.line + 1;
+    FOpts.CodeCompletionAt.Column = task->position.character + 1;
     FOpts.SkipFunctionBodies = true;
     CI->getLangOpts()->CommentOpts.ParseAllComments = true;
 
     DiagnosticConsumer DC;
-    std::string content = manager->wfiles_->GetContent(path);
+    std::string content = manager->wfiles->GetContent(task->path);
     auto Buf = llvm::MemoryBuffer::getMemBuffer(content);
     bool in_preamble =
-        GetOffsetForPosition(
-            {request->position.line, request->position.character}, content) <
+        GetOffsetForPosition({task->position.line, task->position.character},
+                             content) <
         ComputePreambleBounds(*CI->getLangOpts(), Buf.get(), 0).Size;
     if (in_preamble)
       preamble.reset();
     auto Clang = BuildCompilerInstance(*session, std::move(CI), FS, DC,
-                                       preamble.get(), path, Buf);
+                                       preamble.get(), task->path, Buf);
     if (!Clang)
       continue;
 
     Clang->getPreprocessorOpts().SingleFileParseMode = in_preamble;
-    Clang->setCodeCompletionConsumer(request->Consumer.release());
+    Clang->setCodeCompletionConsumer(task->Consumer.release());
     if (!Parse(*Clang))
       continue;
     Buf.release();
 
-    request->on_complete(&Clang->getCodeCompletionConsumer());
+    task->on_complete(&Clang->getCodeCompletionConsumer());
   }
   return nullptr;
 }
@@ -445,25 +501,38 @@ void printDiag(llvm::raw_string_ostream &OS, const DiagBase &d) {
 }
 
 void *DiagnosticMain(void *manager_) {
-  auto *manager = static_cast<CompletionManager*>(manager_);
+  auto *manager = static_cast<SemaManager *>(manager_);
   set_thread_name("diag");
   while (true) {
-    CompletionManager::DiagnosticRequest request =
-        manager->diagnostic_request_.Dequeue();
-    const std::string &path = request.path;
-    int64_t wait = request.wait_until -
+    SemaManager::DiagTask task = manager->diag_tasks.Dequeue();
+    int64_t wait = task.wait_until -
                    chrono::duration_cast<chrono::milliseconds>(
                        chrono::high_resolution_clock::now().time_since_epoch())
                        .count();
     if (wait > 0)
-      std::this_thread::sleep_for(chrono::duration<int64_t, std::milli>(
-          std::min(wait, request.debounce)));
+      std::this_thread::sleep_for(
+          chrono::duration<int64_t, std::milli>(std::min(wait, task.debounce)));
 
-    std::shared_ptr<CompletionSession> session =
-        manager->TryGetSession(path, false);
+    std::shared_ptr<Session> session = manager->EnsureSession(task.path);
     std::shared_ptr<PreambleData> preamble = session->GetPreamble();
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS =
         preamble ? preamble->stat_cache->Consumer(session->FS) : session->FS;
+    if (preamble) {
+      bool rebuild = false;
+      {
+        std::lock_guard lock(manager->wfiles->mutex);
+        for (auto &include : preamble->includes)
+          if (WorkingFile *wf = manager->wfiles->GetFileUnlocked(include.first);
+              wf && include.second < wf->timestamp) {
+            include.second = wf->timestamp;
+            rebuild = true;
+          }
+      }
+      if (rebuild) {
+        manager->preamble_tasks.PushBack({task.path, true}, true);
+        continue;
+      }
+    }
 
     std::unique_ptr<CompilerInvocation> CI =
         BuildCompilerInvocation(session->file.args, FS);
@@ -474,11 +543,11 @@ void *DiagnosticMain(void *manager_) {
       CI->getDiagnosticOpts().Warnings.push_back("no-unused-function");
     CI->getDiagnosticOpts().IgnoreWarnings = false;
     CI->getLangOpts()->SpellChecking = g_config->diagnostics.spellChecking;
-    StoreDiags DC(path);
-    std::string content = manager->wfiles_->GetContent(path);
+    StoreDiags DC(task.path);
+    std::string content = manager->wfiles->GetContent(task.path);
     auto Buf = llvm::MemoryBuffer::getMemBuffer(content);
     auto Clang = BuildCompilerInstance(*session, std::move(CI), FS, DC,
-                                       preamble.get(), path, Buf);
+                                       preamble.get(), task.path, Buf);
     if (!Clang)
       continue;
     if (!Parse(*Clang))
@@ -541,36 +610,32 @@ void *DiagnosticMain(void *manager_) {
     }
 
     {
-      std::lock_guard lock(session->wfiles->files_mutex);
-      if (WorkingFile *wfile = session->wfiles->GetFileByFilenameNoLock(path))
-        wfile->diagnostics_ = ls_diags;
+      std::lock_guard lock(manager->wfiles->mutex);
+      if (WorkingFile *wf = manager->wfiles->GetFileUnlocked(task.path))
+        wf->diagnostics = ls_diags;
     }
-    manager->on_diagnostic_(path, ls_diags);
+    manager->on_diagnostic_(task.path, ls_diags);
   }
   return nullptr;
 }
 
 } // namespace
 
-std::shared_ptr<PreambleData> CompletionSession::GetPreamble() {
+std::shared_ptr<PreambleData> Session::GetPreamble() {
   std::lock_guard<std::mutex> lock(mutex);
   return preamble;
 }
 
-CompletionManager::CompletionManager(Project *project, WorkingFiles *wfiles,
-                                     OnDiagnostic on_diagnostic,
-                                     OnDropped on_dropped)
-    : project_(project), wfiles_(wfiles), on_diagnostic_(on_diagnostic),
-      on_dropped_(on_dropped), preloads(kMaxPreloadedSessions),
-      sessions(kMaxCompletionSessions),
-      PCH(std::make_shared<PCHContainerOperations>()) {
+SemaManager::SemaManager(Project *project, WorkingFiles *wfiles,
+                         OnDiagnostic on_diagnostic, OnDropped on_dropped)
+    : project_(project), wfiles(wfiles), on_diagnostic_(on_diagnostic),
+      on_dropped_(on_dropped), PCH(std::make_shared<PCHContainerOperations>()) {
+  SpawnThread(ccls::PreambleMain, this);
   SpawnThread(ccls::CompletionMain, this);
-  SpawnThread(ccls::CompletionPreloadMain, this);
   SpawnThread(ccls::DiagnosticMain, this);
 }
 
-void CompletionManager::DiagnosticsUpdate(const std::string &path,
-                                          int debounce) {
+void SemaManager::ScheduleDiag(const std::string &path, int debounce) {
   static GroupMatch match(g_config->diagnostics.whitelist,
                           g_config->diagnostics.blacklist);
   if (!match.Matches(path))
@@ -590,78 +655,42 @@ void CompletionManager::DiagnosticsUpdate(const std::string &path,
     }
   }
   if (flag)
-    diagnostic_request_.PushBack({path, now + debounce, debounce}, false);
+    diag_tasks.PushBack({path, now + debounce, debounce}, false);
 }
 
-void CompletionManager::NotifyView(const std::string &path) {
-  // Only reparse the file if we create a new CompletionSession.
-  if (EnsureCompletionOrCreatePreloadSession(path))
-    preload_requests_.PushBack(PreloadRequest{path}, true);
+void SemaManager::OnView(const std::string &path) {
+  std::lock_guard lock(mutex);
+  if (!sessions.Get(path))
+    preamble_tasks.PushBack(PreambleTask{path}, true);
 }
 
-void CompletionManager::NotifySave(const std::string &filename) {
-  EnsureCompletionOrCreatePreloadSession(filename);
-  preload_requests_.PushBack(PreloadRequest{filename}, true);
+void SemaManager::OnSave(const std::string &path) {
+  preamble_tasks.PushBack(PreambleTask{path}, true);
 }
 
-void CompletionManager::OnClose(const std::string &filename) {
-  std::lock_guard<std::mutex> lock(sessions_lock_);
-  preloads.Take(filename);
-  sessions.Take(filename);
+void SemaManager::OnClose(const std::string &path) {
+  std::lock_guard lock(mutex);
+  sessions.Take(path);
 }
 
-bool CompletionManager::EnsureCompletionOrCreatePreloadSession(
-    const std::string &path) {
-  std::lock_guard<std::mutex> lock(sessions_lock_);
-  if (preloads.Get(path) || sessions.Get(path))
-    return false;
-
-  // No CompletionSession, create new one.
-  auto session = std::make_shared<ccls::CompletionSession>(
-      project_->FindEntry(path, false), wfiles_, PCH);
-  if (session->file.filename != path) {
-    session->inferred = true;
-    session->file.filename = path;
-  }
-  preloads.Insert(path, session);
-  LOG_S(INFO) << "create preload session for " << path;
-  return true;
-}
-
-std::shared_ptr<ccls::CompletionSession>
-CompletionManager::TryGetSession(const std::string &path, bool preload,
-                                 bool *is_open) {
-  std::lock_guard<std::mutex> lock(sessions_lock_);
-  std::shared_ptr<ccls::CompletionSession> session = preloads.Get(path);
-
-  if (session) {
-    if (!preload) {
-      preloads.Take(path);
-      sessions.Insert(path, session);
-      if (is_open)
-        *is_open = true;
-    }
-    return session;
-  }
-
-  session = sessions.Get(path);
-  if (!session && !preload) {
-    session = std::make_shared<ccls::CompletionSession>(
-        project_->FindEntry(path, false), wfiles_, PCH);
-    sessions.Insert(path, session);
+std::shared_ptr<ccls::Session>
+SemaManager::EnsureSession(const std::string &path, bool *created) {
+  std::lock_guard lock(mutex);
+  std::shared_ptr<ccls::Session> session = sessions.Get(path);
+  if (!session) {
+    session = std::make_shared<ccls::Session>(
+        project_->FindEntry(path, false), wfiles, PCH);
     LOG_S(INFO) << "create session for " << path;
-    if (is_open)
-      *is_open = true;
+    sessions.Insert(path, session);
+    if (created)
+      *created = true;
   }
-
   return session;
 }
 
-void CompletionManager::FlushAllSessions() {
-  LOG_S(INFO) << "flush all clang complete sessions";
-  std::lock_guard<std::mutex> lock(sessions_lock_);
-
-  preloads.Clear();
+void SemaManager::Clear() {
+  LOG_S(INFO) << "clear all sessions";
+  std::lock_guard lock(mutex);
   sessions.Clear();
 }
 } // namespace ccls
diff --git a/src/clang_complete.hh b/src/sema_manager.hh
similarity index 55%
rename from src/clang_complete.hh
rename to src/sema_manager.hh
index 0a7b8e22..8a9f42a8 100644
--- a/src/clang_complete.hh
+++ b/src/sema_manager.hh
@@ -42,8 +42,6 @@ TextEdit ToTextEdit(const clang::SourceManager &SM,
                       const clang::FixItHint &FixIt);
 
 template <typename K, typename V> struct LruCache {
-  LruCache(int capacity) : capacity(capacity) {}
-
   std::shared_ptr<V> Get(const K &key) {
     for (auto it = items.begin(); it != items.end(); ++it)
       if (it->first == key) {
@@ -69,14 +67,14 @@ template <typename K, typename V> struct LruCache {
     items.emplace(items.begin(), key, std::move(value));
   }
   void Clear() { items.clear(); }
+  void SetCapacity(int cap) { capacity = cap; }
 
 private:
   std::vector<std::pair<K, std::shared_ptr<V>>> items;
-  int capacity;
+  int capacity = 1;
 };
 
-struct CompletionSession
-    : public std::enable_shared_from_this<CompletionSession> {
+struct Session {
   std::mutex mutex;
   std::shared_ptr<PreambleData> preamble;
 
@@ -89,14 +87,14 @@ struct CompletionSession
       llvm::vfs::getRealFileSystem();
   std::shared_ptr<clang::PCHContainerOperations> PCH;
 
-  CompletionSession(const Project::Entry &file, WorkingFiles *wfiles,
+  Session(const Project::Entry &file, WorkingFiles *wfiles,
                     std::shared_ptr<clang::PCHContainerOperations> PCH)
       : file(file), wfiles(wfiles), PCH(PCH) {}
 
   std::shared_ptr<PreambleData> GetPreamble();
 };
 
-struct CompletionManager {
+struct SemaManager {
   using OnDiagnostic = std::function<void(
       std::string path, std::vector<Diagnostic> diagnostics)>;
   // If OptConsumer is nullptr, the request has been cancelled.
@@ -104,91 +102,57 @@ struct CompletionManager {
       std::function<void(clang::CodeCompleteConsumer *OptConsumer)>;
   using OnDropped = std::function<void(RequestId request_id)>;
 
-  struct PreloadRequest {
+  struct PreambleTask {
     std::string path;
+    bool from_diag = false;
   };
-  struct CompletionRequest {
-    CompletionRequest(const RequestId &id,
-                      const TextDocumentIdentifier &document,
-                      const Position &position,
-                      std::unique_ptr<clang::CodeCompleteConsumer> Consumer,
-                      clang::CodeCompleteOptions CCOpts,
-                      const OnComplete &on_complete)
-        : id(id), document(document), position(position),
-          Consumer(std::move(Consumer)), CCOpts(CCOpts),
-          on_complete(on_complete) {}
+  struct CompTask {
+    CompTask(const RequestId &id, const std::string &path,
+             const Position &position,
+             std::unique_ptr<clang::CodeCompleteConsumer> Consumer,
+             clang::CodeCompleteOptions CCOpts, const OnComplete &on_complete)
+        : id(id), path(path), position(position), Consumer(std::move(Consumer)),
+          CCOpts(CCOpts), on_complete(on_complete) {}
 
     RequestId id;
-    TextDocumentIdentifier document;
+    std::string path;
     Position position;
     std::unique_ptr<clang::CodeCompleteConsumer> Consumer;
     clang::CodeCompleteOptions CCOpts;
     OnComplete on_complete;
   };
-  struct DiagnosticRequest {
+  struct DiagTask {
     std::string path;
     int64_t wait_until;
     int64_t debounce;
   };
 
-  CompletionManager(Project *project, WorkingFiles *wfiles,
+  SemaManager(Project *project, WorkingFiles *wfiles,
                     OnDiagnostic on_diagnostic, OnDropped on_dropped);
 
-  // Request a diagnostics update.
-  void DiagnosticsUpdate(const std::string &path, int debounce);
-
-  // Notify the completion manager that |filename| has been viewed and we
-  // should begin preloading completion data.
-  void NotifyView(const std::string &path);
-  // Notify the completion manager that |filename| has been saved. This
-  // triggers a reparse.
-  void NotifySave(const std::string &path);
-  // Notify the completion manager that |filename| has been closed. Any existing
-  // completion session will be dropped.
+  void ScheduleDiag(const std::string &path, int debounce);
+  void OnView(const std::string &path);
+  void OnSave(const std::string &path);
   void OnClose(const std::string &path);
-
-  // Ensures there is a completion or preloaded session. Returns true if a new
-  // session was created.
-  bool EnsureCompletionOrCreatePreloadSession(const std::string &path);
-  // Tries to find an edit session for |filename|. This will move the session
-  // from view to edit.
-  std::shared_ptr<ccls::CompletionSession>
-  TryGetSession(const std::string &path, bool preload, bool *is_open = nullptr);
-
-  // Flushes all saved sessions
-  void FlushAllSessions(void);
-
-  // TODO: make these configurable.
-  const int kMaxPreloadedSessions = 10;
-  const int kMaxCompletionSessions = 5;
+  std::shared_ptr<ccls::Session> EnsureSession(const std::string &path,
+                                               bool *created = nullptr);
+  void Clear(void);
 
   // Global state.
   Project *project_;
-  WorkingFiles *wfiles_;
+  WorkingFiles *wfiles;
   OnDiagnostic on_diagnostic_;
   OnDropped on_dropped_;
 
-  using LruSessionCache = LruCache<std::string, ccls::CompletionSession>;
-
-  // CompletionSession instances which are preloaded, ie, files which the user
-  // has viewed but not requested code completion for.
-  LruSessionCache preloads;
-  // CompletionSession instances which the user has actually performed
-  // completion on. This is more rare so these instances tend to stay alive
-  // much longer than the ones in |preloaded_sessions_|.
-  LruSessionCache sessions;
-  // Mutex which protects |view_sessions_| and |edit_sessions_|.
-  std::mutex sessions_lock_;
+  std::mutex mutex;
+  LruCache<std::string, ccls::Session> sessions;
 
   std::mutex diag_mutex;
   std::unordered_map<std::string, int64_t> next_diag;
 
-  // Request a code completion at the given location.
-  ThreadedQueue<std::unique_ptr<CompletionRequest>> completion_request_;
-  ThreadedQueue<DiagnosticRequest> diagnostic_request_;
-  // Parse requests. The path may already be parsed, in which case it should be
-  // reparsed.
-  ThreadedQueue<PreloadRequest> preload_requests_;
+  ThreadedQueue<std::unique_ptr<CompTask>> comp_tasks;
+  ThreadedQueue<DiagTask> diag_tasks;
+  ThreadedQueue<PreambleTask> preamble_tasks;
 
   std::shared_ptr<clang::PCHContainerOperations> PCH;
 };
@@ -198,16 +162,14 @@ struct CompletionManager {
 // that happens.
 template <typename T>
 struct CompleteConsumerCache {
-  // NOTE: Make sure to access these variables under |WithLock|.
-  std::optional<std::string> path;
-  std::optional<Position> position;
+  std::mutex mutex;
+  std::string path;
+  Position position;
   T result;
 
-  std::mutex mutex;
-
-  void WithLock(std::function<void()> action) {
+  template <typename Fn> void WithLock(Fn &&fn) {
     std::lock_guard lock(mutex);
-    action();
+    fn();
   }
   bool IsCacheValid(const std::string path, Position position) {
     std::lock_guard lock(mutex);
diff --git a/src/test.cc b/src/test.cc
index 98e3ca48..261da2c2 100644
--- a/src/test.cc
+++ b/src/test.cc
@@ -3,7 +3,7 @@
 
 #include "test.hh"
 
-#include "clang_complete.hh"
+#include "sema_manager.hh"
 #include "filesystem.hh"
 #include "indexer.hh"
 #include "pipeline.hh"
@@ -276,9 +276,9 @@ bool RunIndexTests(const std::string &filter_path, bool enable_update) {
   bool update_all = false;
   // FIXME: show diagnostics in STL/headers when running tests. At the moment
   // this can be done by conRequestIdex index(1, 1);
-  CompletionManager completion(nullptr, nullptr,
-                               [&](std::string, std::vector<Diagnostic>) {},
-                               [](RequestId id) {});
+  SemaManager completion(
+      nullptr, nullptr, [&](std::string, std::vector<Diagnostic>) {},
+      [](RequestId id) {});
   GetFilesInFolder(
       "index_tests", true /*recursive*/, true /*add_folder_to_path*/,
       [&](const std::string &path) {
diff --git a/src/working_files.cc b/src/working_files.cc
index f2b29556..37d76c0c 100644
--- a/src/working_files.cc
+++ b/src/working_files.cc
@@ -9,9 +9,11 @@
 #include <clang/Basic/CharInfo.h>
 
 #include <algorithm>
+#include <chrono>
 #include <climits>
 #include <numeric>
 #include <sstream>
+namespace chrono = std::chrono;
 
 using namespace clang;
 using namespace llvm;
@@ -352,57 +354,53 @@ WorkingFile::FindStableCompletionSource(Position position,
   return GetPositionForOffset(buffer_content, i);
 }
 
-WorkingFile *WorkingFiles::GetFileByFilename(const std::string &filename) {
-  std::lock_guard<std::mutex> lock(files_mutex);
-  return GetFileByFilenameNoLock(filename);
+WorkingFile *WorkingFiles::GetFile(const std::string &path) {
+  std::lock_guard lock(mutex);
+  return GetFileUnlocked(path);
 }
 
-WorkingFile *
-WorkingFiles::GetFileByFilenameNoLock(const std::string &filename) {
-  for (auto &file : files) {
-    if (file->filename == filename)
-      return file.get();
-  }
-  return nullptr;
+WorkingFile *WorkingFiles::GetFileUnlocked(const std::string &path) {
+  auto it = files.find(path);
+  return it != files.end() ? it->second.get() : nullptr;
 }
 
-std::string WorkingFiles::GetContent(const std::string &filename) {
-  std::lock_guard<std::mutex> lock(files_mutex);
-  for (auto &file : files)
-    if (file->filename == filename)
-      return file->buffer_content;
-  return "";
+std::string WorkingFiles::GetContent(const std::string &path) {
+  std::lock_guard lock(mutex);
+  auto it = files.find(path);
+  return it != files.end() ? it->second->buffer_content : "";
 }
 
 WorkingFile *WorkingFiles::OnOpen(const TextDocumentItem &open) {
-  std::lock_guard<std::mutex> lock(files_mutex);
+  std::lock_guard lock(mutex);
 
-  std::string filename = open.uri.GetPath();
+  std::string path = open.uri.GetPath();
   std::string content = open.text;
 
-  // The file may already be open.
-  if (WorkingFile *file = GetFileByFilenameNoLock(filename)) {
-    file->version = open.version;
-    file->buffer_content = content;
-    file->OnBufferContentUpdated();
-    return file;
+  auto &wf = files[path];
+  if (wf) {
+    wf->version = open.version;
+    wf->buffer_content = content;
+    wf->OnBufferContentUpdated();
+  } else {
+    wf = std::make_unique<WorkingFile>(path, content);
   }
-
-  files.push_back(std::make_unique<WorkingFile>(filename, content));
-  return files[files.size() - 1].get();
+  return wf.get();
 }
 
 void WorkingFiles::OnChange(const TextDocumentDidChangeParam &change) {
-  std::lock_guard<std::mutex> lock(files_mutex);
+  std::lock_guard lock(mutex);
 
-  std::string filename = change.textDocument.uri.GetPath();
-  WorkingFile *file = GetFileByFilenameNoLock(filename);
+  std::string path = change.textDocument.uri.GetPath();
+  WorkingFile *file = GetFileUnlocked(path);
   if (!file) {
-    LOG_S(WARNING) << "Could not change " << filename
-                   << " because it was not open";
+    LOG_S(WARNING) << "Could not change " << path << " because it was not open";
     return;
   }
 
+  file->timestamp = chrono::duration_cast<chrono::seconds>(
+                        chrono::high_resolution_clock::now().time_since_epoch())
+                        .count();
+
   // version: number | null
   if (change.textDocument.version)
     file->version = *change.textDocument.version;
@@ -428,20 +426,9 @@ void WorkingFiles::OnChange(const TextDocumentDidChangeParam &change) {
   }
 }
 
-void WorkingFiles::OnClose(const TextDocumentIdentifier &close) {
-  std::lock_guard<std::mutex> lock(files_mutex);
-
-  std::string filename = close.uri.GetPath();
-
-  for (int i = 0; i < files.size(); ++i) {
-    if (files[i]->filename == filename) {
-      files.erase(files.begin() + i);
-      return;
-    }
-  }
-
-  LOG_S(WARNING) << "Could not close " << filename
-                 << " because it was not open";
+void WorkingFiles::OnClose(const std::string &path) {
+  std::lock_guard lock(mutex);
+  files.erase(path);
 }
 
 // VSCode (UTF-16) disagrees with Emacs lsp-mode (UTF-8) on how to represent
diff --git a/src/working_files.hh b/src/working_files.hh
index 5e5f9e61..00f21371 100644
--- a/src/working_files.hh
+++ b/src/working_files.hh
@@ -9,9 +9,11 @@
 #include <mutex>
 #include <optional>
 #include <string>
+#include <unordered_map>
 
 namespace ccls {
 struct WorkingFile {
+  int64_t timestamp = 0;
   int version = 0;
   std::string filename;
 
@@ -29,9 +31,7 @@ struct WorkingFile {
   std::vector<int> index_to_buffer;
   std::vector<int> buffer_to_index;
   // A set of diagnostics that have been reported for this file.
-  // NOTE: _ is appended because it must be accessed under the WorkingFiles
-  // lock!
-  std::vector<Diagnostic> diagnostics_;
+  std::vector<Diagnostic> diagnostics;
 
   WorkingFile(const std::string &filename, const std::string &buffer_content);
 
@@ -67,38 +67,21 @@ private:
 };
 
 struct WorkingFiles {
-  struct Snapshot {
-    struct File {
-      std::string filename;
-      std::string content;
-    };
+  WorkingFile *GetFile(const std::string &path);
+  WorkingFile *GetFileUnlocked(const std::string &path);
+  std::string GetContent(const std::string &path);
 
-    std::vector<File> files;
-  };
-
-  //
-  // :: IMPORTANT :: All methods in this class are guarded by a single lock.
-  //
-
-  // Find the file with the given filename.
-  WorkingFile *GetFileByFilename(const std::string &filename);
-  WorkingFile *GetFileByFilenameNoLock(const std::string &filename);
-  std::string GetContent(const std::string &filename);
-
-  // Run |action| under the lock.
-  template <typename Fn> void DoAction(Fn &&fn) {
-    std::lock_guard<std::mutex> lock(files_mutex);
+  template <typename Fn> void WithLock(Fn &&fn) {
+    std::lock_guard lock(mutex);
     fn();
   }
 
   WorkingFile *OnOpen(const TextDocumentItem &open);
   void OnChange(const TextDocumentDidChangeParam &change);
-  void OnClose(const TextDocumentIdentifier &close);
+  void OnClose(const std::string &close);
 
-  // Use unique_ptrs so we can handout WorkingFile ptrs and not have them
-  // invalidated if we resize files.
-  std::vector<std::unique_ptr<WorkingFile>> files;
-  std::mutex files_mutex; // Protects |files|.
+  std::mutex mutex;
+  std::unordered_map<std::string, std::unique_ptr<WorkingFile>> files;
 };
 
 int GetOffsetForPosition(Position pos, std::string_view content);