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
@@ -526,6 +526,9 @@
     }
   }
   Main.tags = D.Tags;
+  // 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});
   OutFn(std::move(Main), D.Fixes);
 
   // If we didn't emit the notes as relatedLocations, emit separate diagnostics
@@ -540,10 +543,6 @@
       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/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -23,6 +23,7 @@
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticSema.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/JSON.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/TargetSelect.h"
 #include "gmock/gmock.h"
@@ -1012,6 +1013,7 @@
   D.Severity = DiagnosticsEngine::Error;
   D.File = "foo/bar/main.cpp";
   D.AbsFile = std::string(MainFile.file());
+  D.OpaqueData["test"] = "bar";
 
   clangd::Note NoteInMain;
   NoteInMain.Message = "declared somewhere in the main file";
@@ -1050,6 +1052,7 @@
 ../foo/baz/header.h:10:11:
 note: declared somewhere in the header file)";
   MainLSP.tags = {DiagnosticTag::Unnecessary};
+  MainLSP.data = D.OpaqueData;
 
   clangd::Diagnostic NoteInMainLSP;
   NoteInMainLSP.range = NoteInMain.Range;