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
@@ -205,6 +205,7 @@
   Inputs.Opts = std::move(Opts);
   Inputs.Index = Index;
   Inputs.ClangTidyProvider = ClangTidyProvider;
+  Inputs.Modules = Modules;
   bool NewFile = WorkScheduler->update(File, Inputs, WantDiags);
   // If we loaded Foo.h, we want to make sure Foo.cpp is indexed.
   if (NewFile && BackgroundIdx)
diff --git a/clang-tools-extra/clangd/Compiler.h b/clang-tools-extra/clangd/Compiler.h
--- a/clang-tools-extra/clangd/Compiler.h
+++ b/clang-tools-extra/clangd/Compiler.h
@@ -16,6 +16,7 @@
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_COMPILER_H
 
 #include "GlobalCompilationDatabase.h"
+#include "Module.h"
 #include "TidyProvider.h"
 #include "index/Index.h"
 #include "support/ThreadsafeFS.h"
@@ -54,6 +55,7 @@
   const SymbolIndex *Index = nullptr;
   ParseOptions Opts = ParseOptions();
   TidyProviderRef ClangTidyProvider = {};
+  const ModuleSet *Modules = nullptr;
 };
 
 /// Builds compiler invocation that could be used to build AST or preamble.
diff --git a/clang-tools-extra/clangd/Diagnostics.h b/clang-tools-extra/clangd/Diagnostics.h
--- a/clang-tools-extra/clangd/Diagnostics.h
+++ b/clang-tools-extra/clangd/Diagnostics.h
@@ -9,6 +9,7 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_DIAGNOSTICS_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_DIAGNOSTICS_H
 
+#include "Module.h"
 #include "Protocol.h"
 #include "support/Path.h"
 #include "clang/Basic/Diagnostic.h"
@@ -145,6 +146,7 @@
   /// diagnostics, such as promoting warnings to errors, or ignoring
   /// diagnostics.
   void setLevelAdjuster(LevelAdjuster Adjuster) { this->Adjuster = Adjuster; }
+  void setDiagAugmenters(const ModuleSet *Modules) { this->Modules = Modules; }
 
 private:
   void flushLastDiag();
@@ -157,6 +159,7 @@
   llvm::Optional<FullSourceLoc> LastDiagLoc; // Valid only when LastDiag is set.
   bool LastDiagOriginallyError = false;      // Valid only when LastDiag is set.
   SourceManager *OrigSrcMgr = nullptr;
+  const ModuleSet *Modules = nullptr;
 
   llvm::DenseSet<std::pair<unsigned, unsigned>> IncludedErrorLocations;
   bool LastPrimaryDiagnosticWasSuppressed = false;
diff --git a/clang-tools-extra/clangd/Diagnostics.cpp b/clang-tools-extra/clangd/Diagnostics.cpp
--- a/clang-tools-extra/clangd/Diagnostics.cpp
+++ b/clang-tools-extra/clangd/Diagnostics.cpp
@@ -746,6 +746,12 @@
       LastDiag->Fixes.insert(LastDiag->Fixes.end(), ExtraFixes.begin(),
                              ExtraFixes.end());
     }
+    if (Modules) {
+      for (auto &M : *Modules) {
+        if (auto Action = M.augmentDiagFix(DiagLevel, Info))
+          LastDiag->Actions.emplace_back(std::move(*Action));
+      }
+    }
   } else {
     // Handle a note to an existing diagnostic.
 
diff --git a/clang-tools-extra/clangd/Module.h b/clang-tools-extra/clangd/Module.h
--- a/clang-tools-extra/clangd/Module.h
+++ b/clang-tools-extra/clangd/Module.h
@@ -9,9 +9,12 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULE_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULE_H
 
+#include "Protocol.h"
 #include "support/Function.h"
 #include "support/Threading.h"
+#include "clang/Basic/Diagnostic.h"
 #include "llvm/ADT/FunctionExtras.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/JSON.h"
@@ -91,6 +94,15 @@
   /// Called by the server when shutting down, and also by tests.
   virtual bool blockUntilIdle(Deadline) { return true; }
 
+  /// Called by auxilary threads as diagnostics encountered to generate fixes.
+  /// So this can be called concurrently from multiple threads. If a module
+  /// provides a code action with a custom, it should also register itself as
+  /// that commands handler.
+  virtual llvm::Optional<CodeAction>
+  augmentDiagFix(DiagnosticsEngine::Level L, const clang::Diagnostic &Diag) {
+    return llvm::None;
+  }
+
 protected:
   /// Accessors for modules to access shared server facilities they depend on.
   Facilities &facilities();
diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -262,6 +262,7 @@
   CI->getLangOpts()->DelayedTemplateParsing = false;
 
   StoreDiags ASTDiags;
+  ASTDiags.setDiagAugmenters(Inputs.Modules);
 
   llvm::Optional<PreamblePatch> Patch;
   if (Preamble) {
diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -326,6 +326,7 @@
   trace::Span Tracer("BuildPreamble");
   SPAN_ATTACH(Tracer, "File", FileName);
   StoreDiags PreambleDiagnostics;
+  PreambleDiagnostics.setDiagAugmenters(Inputs.Modules);
   llvm::IntrusiveRefCntPtr<DiagnosticsEngine> PreambleDiagsEngine =
       CompilerInstance::createDiagnostics(&CI.getDiagnosticOpts(),
                                           &PreambleDiagnostics, false);
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
@@ -652,6 +652,7 @@
         printArgv(Inputs.CompileCommand.CommandLine));
 
     StoreDiags CompilerInvocationDiagConsumer;
+    CompilerInvocationDiagConsumer.setDiagAugmenters(Inputs.Modules);
     std::vector<std::string> CC1Args;
     std::unique_ptr<CompilerInvocation> Invocation = buildCompilerInvocation(
         Inputs, CompilerInvocationDiagConsumer, &CC1Args);
@@ -716,6 +717,7 @@
         IdleASTs.take(this, &ASTAccessForRead);
     if (!AST) {
       StoreDiags CompilerInvocationDiagConsumer;
+      CompilerInvocationDiagConsumer.setDiagAugmenters(FileInputs.Modules);
       std::unique_ptr<CompilerInvocation> Invocation =
           buildCompilerInvocation(FileInputs, CompilerInvocationDiagConsumer);
       // Try rebuilding the AST.