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
@@ -106,6 +106,7 @@
   std::vector<Note> Notes;
   /// *Alternative* fixes for this diagnostic, one should be chosen.
   std::vector<Fix> Fixes;
+  std::vector<DiagnosticTag> Tags;
 };
 llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const Diag &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
@@ -26,6 +26,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/Capacity.h"
@@ -35,6 +36,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cstddef>
+#include <vector>
 
 namespace clang {
 namespace clangd {
@@ -324,6 +326,61 @@
   OS.flush();
   return capitalize(std::move(Result));
 }
+
+std::vector<DiagnosticTag> getDiagnosticTags(unsigned DiagID) {
+  static const llvm::DenseSet<unsigned> DeprecatedDiags = {
+      diag::warn_access_decl_deprecated,
+      diag::warn_atl_uuid_deprecated,
+      diag::warn_deprecated,
+      diag::warn_deprecated_altivec_src_compat,
+      diag::warn_deprecated_comma_subscript,
+      diag::warn_deprecated_compound_assign_volatile,
+      diag::warn_deprecated_copy,
+      diag::warn_deprecated_copy_with_dtor,
+      diag::warn_deprecated_copy_with_user_provided_copy,
+      diag::warn_deprecated_copy_with_user_provided_dtor,
+      diag::warn_deprecated_def,
+      diag::warn_deprecated_increment_decrement_volatile,
+      diag::warn_deprecated_message,
+      diag::warn_deprecated_redundant_constexpr_static_def,
+      diag::warn_deprecated_register,
+      diag::warn_deprecated_simple_assign_volatile,
+      diag::warn_deprecated_string_literal_conversion,
+      diag::warn_deprecated_this_capture,
+      diag::warn_deprecated_volatile_param,
+      diag::warn_deprecated_volatile_return,
+      diag::warn_deprecated_volatile_structured_binding,
+      diag::warn_opencl_attr_deprecated_ignored,
+      diag::warn_property_method_deprecated,
+      diag::warn_vector_mode_deprecated,
+  };
+  static const llvm::DenseSet<unsigned> UnusedDiags = {
+      diag::warn_opencl_attr_deprecated_ignored,
+      diag::warn_pragma_attribute_unused,
+      diag::warn_unused_but_set_parameter,
+      diag::warn_unused_but_set_variable,
+      diag::warn_unused_comparison,
+      diag::warn_unused_const_variable,
+      diag::warn_unused_exception_param,
+      diag::warn_unused_function,
+      diag::warn_unused_label,
+      diag::warn_unused_lambda_capture,
+      diag::warn_unused_local_typedef,
+      diag::warn_unused_member_function,
+      diag::warn_unused_parameter,
+      diag::warn_unused_private_field,
+      diag::warn_unused_property_backing_ivar,
+      diag::warn_unused_template,
+      diag::warn_unused_variable,
+  };
+  std::vector<DiagnosticTag> Result;
+  if (DeprecatedDiags.contains(DiagID)) {
+    Result.push_back(DiagnosticTag::Deprecated);
+  } else if (UnusedDiags.contains(DiagID)) {
+    Result.push_back(DiagnosticTag::Unnecessary);
+  }
+  return Result;
+}
 } // namespace
 
 llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const DiagBase &D) {
@@ -461,6 +518,7 @@
       Main.relatedInformation->push_back(std::move(RelInfo));
     }
   }
+  Main.tags = D.Tags;
   OutFn(std::move(Main), D.Fixes);
 
   // If we didn't emit the notes as relatedLocations, emit separate diagnostics
@@ -517,6 +575,7 @@
         Diag.Name = std::string(Name);
       }
       Diag.Source = Diag::Clang;
+      Diag.Tags = getDiagnosticTags(Diag.ID);
       continue;
     }
     if (Tidy != nullptr) {
@@ -524,6 +583,7 @@
       if (!TidyDiag.empty()) {
         Diag.Name = std::move(TidyDiag);
         Diag.Source = Diag::ClangTidy;
+        // FIXME: Set diagnostic tags based on check names.
         // clang-tidy bakes the name into diagnostic messages. Strip it out.
         // It would be much nicer to make clang-tidy not do this.
         auto CleanMessage = [&](std::string &Msg) {
@@ -545,7 +605,7 @@
   // duplicated messages due to various reasons (e.g. the check doesn't handle
   // template instantiations well; clang-tidy alias checks).
   std::set<std::pair<Range, std::string>> SeenDiags;
-  llvm::erase_if(Output, [&](const Diag& D) {
+  llvm::erase_if(Output, [&](const Diag &D) {
     return !SeenDiags.emplace(D.Range, D.Message).second;
   });
   return std::move(Output);
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
@@ -811,6 +811,19 @@
 };
 llvm::json::Value toJSON(const DiagnosticRelatedInformation &);
 
+enum DiagnosticTag {
+  /// Unused or unnecessary code.
+  ///
+  /// Clients are allowed to render diagnostics with this tag faded out instead
+  /// of having an error squiggle.
+  Unnecessary = 1,
+  /// Deprecated or obsolete code.
+  ///
+  /// Clients are allowed to rendered diagnostics with this tag strike through.
+  Deprecated = 2,
+};
+llvm::json::Value toJSON(DiagnosticTag Tag);
+
 struct CodeAction;
 struct Diagnostic {
   /// The range at which the message applies.
@@ -830,6 +843,9 @@
   /// The diagnostic's message.
   std::string message;
 
+  /// Additional metadata about the diagnostic.
+  std::vector<DiagnosticTag> tags;
+
   /// An array of related diagnostic information, e.g. when symbol-names within
   /// a scope collide all definitions can be marked via this property.
   llvm::Optional<std::vector<DiagnosticRelatedInformation>> relatedInformation;
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
@@ -602,6 +602,8 @@
     Diag["relatedInformation"] = *D.relatedInformation;
   if (!D.data.empty())
     Diag["data"] = llvm::json::Object(D.data);
+  if (!D.tags.empty())
+    Diag["tags"] = D.tags;
   // FIXME: workaround for older gcc/clang
   return std::move(Diag);
 }
@@ -1450,5 +1452,6 @@
   return OS;
 }
 
+llvm::json::Value toJSON(DiagnosticTag Tag) { return static_cast<int>(Tag); }
 } // namespace clangd
 } // namespace clang
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
@@ -65,6 +65,11 @@
   return Field(&Diag::Notes, UnorderedElementsAre(NoteMatcher1, NoteMatcher2));
 }
 
+::testing::Matcher<const Diag &>
+WithTag(::testing::Matcher<DiagnosticTag> TagMatcher) {
+  return Field(&Diag::Tags, Contains(TagMatcher));
+}
+
 MATCHER_P2(Diag, Range, Message,
            "Diag at " + llvm::to_string(Range) + " = [" + Message + "]") {
   return arg.Range == Range && arg.Message == Message;
@@ -665,6 +670,7 @@
 
   clangd::Diag D;
   D.ID = clang::diag::err_undeclared_var_use;
+  D.Tags = {DiagnosticTag::Unnecessary};
   D.Name = "undeclared_var_use";
   D.Source = clangd::Diag::Clang;
   D.Message = "something terrible happened";
@@ -710,6 +716,7 @@
 
 ../foo/baz/header.h:10:11:
 note: declared somewhere in the header file)";
+  MainLSP.tags = {DiagnosticTag::Unnecessary};
 
   clangd::Diagnostic NoteInMainLSP;
   NoteInMainLSP.range = NoteInMain.Range;
@@ -1419,6 +1426,24 @@
         testing::Contains(Diag(Code.range(), "no newline at end of file")));
   }
 }
+
+TEST(Diagnostics, Tags) {
+  TestTU TU;
+  TU.ExtraArgs = {"-Wunused", "-Wdeprecated"};
+  Annotations Test(R"cpp(
+  void bar() __attribute__((deprecated));
+  void foo() {
+    int $unused[[x]];
+    $deprecated[[bar]]();
+  })cpp");
+  TU.Code = Test.code().str();
+  EXPECT_THAT(*TU.build().getDiagnostics(),
+              UnorderedElementsAre(
+                  AllOf(Diag(Test.range("unused"), "unused variable 'x'"),
+                        WithTag(DiagnosticTag::Unnecessary)),
+                  AllOf(Diag(Test.range("deprecated"), "'bar' is deprecated"),
+                        WithTag(DiagnosticTag::Deprecated))));
+}
 } // namespace
 } // namespace clangd
 } // namespace clang