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
@@ -20,12 +20,14 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringSet.h"
+#include "llvm/Support/JSON.h"
 #include "llvm/Support/SourceMgr.h"
 #include <cassert>
 #include <functional>
 #include <memory>
 #include <string>
 #include <utility>
+#include <vector>
 
 namespace clang {
 namespace tidy {
@@ -70,6 +72,10 @@
   // diags from the main file.
   bool InsideMainFile = false;
   unsigned ID; // e.g. member of clang::diag, or clang-tidy assigned ID.
+  // Feature modules can make use of this field to propagate data from a
+  // diagnostic to a CodeAction request. Each module should only append to the
+  // list.
+  llvm::json::Object OpaqueData;
 };
 llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const DiagBase &D);
 
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
@@ -476,6 +476,10 @@
       Res.message = noteMessage(D, Note, Opts);
       OutFn(std::move(Res), llvm::ArrayRef<Fix>());
     }
+
+  // FIXME: Get rid of the copies here by taking in a mutable clangd::Diag.
+  for (auto &Entry : D.OpaqueData)
+    Main.data.insert({Entry.first, Entry.second});
 }
 
 int getSeverity(DiagnosticsEngine::Level L) {
diff --git a/clang-tools-extra/clangd/Protocol.h b/clang-tools-extra/clangd/Protocol.h
--- a/clang-tools-extra/clangd/Protocol.h
+++ b/clang-tools-extra/clangd/Protocol.h
@@ -844,6 +844,13 @@
   /// Only with capability textDocument.publishDiagnostics.codeActionsInline.
   /// (These actions can also be obtained using textDocument/codeAction).
   llvm::Optional<std::vector<CodeAction>> codeActions;
+
+  /// A data entry field that is preserved between a
+  /// `textDocument/publishDiagnostics` notification
+  /// and`textDocument/codeAction` request.
+  /// Mutating users should associate their data with a unique key they can use
+  /// to retrieve later on.
+  llvm::json::Object data;
 };
 llvm::json::Value toJSON(const Diagnostic &);
 
diff --git a/clang-tools-extra/clangd/Protocol.cpp b/clang-tools-extra/clangd/Protocol.cpp
--- a/clang-tools-extra/clangd/Protocol.cpp
+++ b/clang-tools-extra/clangd/Protocol.cpp
@@ -600,6 +600,8 @@
     Diag["source"] = D.source;
   if (D.relatedInformation)
     Diag["relatedInformation"] = *D.relatedInformation;
+  if (!D.data.empty())
+    Diag["data"] = llvm::json::Object(D.data);
   // FIXME: workaround for older gcc/clang
   return std::move(Diag);
 }
@@ -607,7 +609,11 @@
 bool fromJSON(const llvm::json::Value &Params, Diagnostic &R,
               llvm::json::Path P) {
   llvm::json::ObjectMapper O(Params, P);
-  return O && O.map("range", R.range) && O.map("message", R.message) &&
+  if (!O)
+    return false;
+  if (auto *Data = Params.getAsObject()->getObject("data"))
+    R.data = *Data;
+  return O.map("range", R.range) && O.map("message", R.message) &&
          mapOptOrNull(Params, "severity", R.severity, P) &&
          mapOptOrNull(Params, "category", R.category, P) &&
          mapOptOrNull(Params, "code", R.code, P) &&
diff --git a/clang-tools-extra/clangd/refactor/Tweak.h b/clang-tools-extra/clangd/refactor/Tweak.h
--- a/clang-tools-extra/clangd/refactor/Tweak.h
+++ b/clang-tools-extra/clangd/refactor/Tweak.h
@@ -26,6 +26,7 @@
 #include "index/Index.h"
 #include "support/Path.h"
 #include "clang/Tooling/Core/Replacement.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"