diff --git a/clang-tools-extra/clangd/CMakeLists.txt b/clang-tools-extra/clangd/CMakeLists.txt
--- a/clang-tools-extra/clangd/CMakeLists.txt
+++ b/clang-tools-extra/clangd/CMakeLists.txt
@@ -36,6 +36,7 @@
   CollectMacros.cpp
   CompileCommands.cpp
   Compiler.cpp
+  Config.cpp
   ConfigYAML.cpp
   Diagnostics.cpp
   DraftStore.cpp
diff --git a/clang-tools-extra/clangd/CompileCommands.cpp b/clang-tools-extra/clangd/CompileCommands.cpp
--- a/clang-tools-extra/clangd/CompileCommands.cpp
+++ b/clang-tools-extra/clangd/CompileCommands.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "CompileCommands.h"
+#include "Config.h"
 #include "support/Logger.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Tooling/ArgumentsAdjusters.h"
@@ -182,6 +183,10 @@
 }
 
 void CommandMangler::adjust(std::vector<std::string> &Cmd) const {
+  // FIXME: remove const_cast once unique_function is const-compatible.
+  for (auto &Edit : const_cast<Config &>(Config::current()).CompileFlags.Edits)
+    Edit(Cmd);
+
   // Check whether the flag exists, either as -flag or -flag=*
   auto Has = [&](llvm::StringRef Flag) {
     for (llvm::StringRef Arg : Cmd) {
diff --git a/clang-tools-extra/clangd/Config.h b/clang-tools-extra/clangd/Config.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/Config.h
@@ -0,0 +1,63 @@
+//===--- Config.h - User configuration of clangd behavior --------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Various clangd features have configurable behaviour (or can be disabled).
+// This file defines "resolved" configuration seen by features within clangd.
+// For example, settings may vary per-file, the resolved Config only contains
+// settings that apply to the current file.
+//
+// This is distinct from how the config is specified by the user (Fragment)
+// interpreted (CompiledFragment), and combined (Provider).
+// ConfigFragment.h describes the steps to add a new configuration option.
+//
+// Because this structure is shared throughout clangd, it's a potential source
+// of layering problems. Config should be expressed in terms of simple
+// vocubulary types where possible.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONFIG_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONFIG_H
+
+#include "support/Context.h"
+#include "llvm/ADT/FunctionExtras.h"
+#include <string>
+#include <vector>
+
+namespace clang {
+namespace clangd {
+
+/// Settings that express user/project preferences and control clangd behavior.
+///
+/// Generally, features should consume Config::current() and the caller is
+/// responsible for setting it appropriately. In practice these callers are
+/// ClangdServer, TUScheduler, and BackgroundQueue.
+struct Config {
+  /// Returns the Config of the current Context, or an empty configuration.
+  static const Config &current();
+  /// Context key which can be used to set the current Config.
+  static clangd::Key<Config> Key;
+
+  Config() = default;
+  Config(const Config &) = delete;
+  Config &operator=(const Config &) = delete;
+  Config(Config &&) = default;
+  Config &operator=(Config &&) = default;
+
+  /// Controls how the compile command for the current file is determined.
+  struct {
+    // Edits to apply to the compile command, in sequence.
+    // FIXME: these functions need to be const-callable. For now, const_cast.
+    std::vector<llvm::unique_function<void(std::vector<std::string> &)>> Edits;
+  } CompileFlags;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif
diff --git a/clang-tools-extra/clangd/Config.cpp b/clang-tools-extra/clangd/Config.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/Config.cpp
@@ -0,0 +1,25 @@
+//===--- Config.cpp - User configuration of clangd behavior ---------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Config.h"
+#include "support/Context.h"
+
+namespace clang {
+namespace clangd {
+
+Key<Config> Config::Key;
+
+const Config &Config::current() {
+  if (const Config *C = Context::current().get(Key))
+    return *C;
+  static Config Default;
+  return Default;
+}
+
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp b/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
--- a/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
@@ -7,7 +7,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "CompileCommands.h"
+#include "Config.h"
 #include "TestFS.h"
+#include "support/Context.h"
 
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringExtras.h"
@@ -185,6 +187,26 @@
 }
 #endif
 
+TEST(CommandMangler, ConfigEdits) {
+  auto Mangler = CommandMangler::forTests();
+  std::vector<std::string> Cmd = {"clang++", "foo.cc"};
+  {
+    Config Cfg;
+    Cfg.CompileFlags.Edits.push_back([](std::vector<std::string> &Argv) {
+      for (auto &Arg : Argv)
+        for (char &C : Arg)
+          C = llvm::toUpper(C);
+    });
+    Cfg.CompileFlags.Edits.push_back(
+        [](std::vector<std::string> &Argv) { Argv.push_back("--hello"); });
+    WithContextValue WithConfig(Config::Key, std::move(Cfg));
+    Mangler.adjust(Cmd);
+  }
+  // Edits are applied in given order and before other mangling.
+  EXPECT_THAT(Cmd,
+              ElementsAre("CLANG++", "FOO.CC", "--hello", "-fsyntax-only"));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang