Index: clang-tools-extra/trunk/clangd/ClangdLSPServer.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/ClangdLSPServer.cpp
+++ clang-tools-extra/trunk/clangd/ClangdLSPServer.cpp
@@ -84,6 +84,8 @@
       Params.capabilities.textDocument.completion.completionItem.snippetSupport;
   DiagOpts.EmbedFixesInDiagnostics =
       Params.capabilities.textDocument.publishDiagnostics.clangdFixSupport;
+  DiagOpts.SendDiagnosticCategory =
+      Params.capabilities.textDocument.publishDiagnostics.categorySupport;
 
   if (Params.capabilities.workspace && Params.capabilities.workspace->symbol &&
       Params.capabilities.workspace->symbol->symbolKind) {
@@ -506,7 +508,7 @@
         }
         LSPDiag["clangd_fixes"] = std::move(ClangdFixes);
       }
-      if (!Diag.category.empty())
+      if (DiagOpts.SendDiagnosticCategory && !Diag.category.empty())
         LSPDiag["category"] = Diag.category;
       DiagnosticsJSON.push_back(std::move(LSPDiag));
 
Index: clang-tools-extra/trunk/clangd/Diagnostics.h
===================================================================
--- clang-tools-extra/trunk/clangd/Diagnostics.h
+++ clang-tools-extra/trunk/clangd/Diagnostics.h
@@ -27,6 +27,12 @@
   /// If true, Clangd uses an LSP extension to embed the fixes with the
   /// diagnostics that are sent to the client.
   bool EmbedFixesInDiagnostics = false;
+
+  /// If true, Clangd uses an LSP extension to send the diagnostic's
+  /// category to the client. The category typically describes the compilation
+  /// stage during which the issue was produced, e.g. "Semantic Issue" or "Parse
+  /// Issue".
+  bool SendDiagnosticCategory = false;
 };
 
 /// Contains basic information about a diagnostic.
Index: clang-tools-extra/trunk/clangd/Protocol.h
===================================================================
--- clang-tools-extra/trunk/clangd/Protocol.h
+++ clang-tools-extra/trunk/clangd/Protocol.h
@@ -255,6 +255,10 @@
   /// Whether the client accepts diagnostics with fixes attached using the
   /// "clangd_fixes" extension.
   bool clangdFixSupport = false;
+
+  /// Whether the client accepts diagnostics with category attached to it
+  /// using the "category" extension.
+  bool categorySupport = false;
 };
 bool fromJSON(const llvm::json::Value &,
               PublishDiagnosticsClientCapabilities &);
Index: clang-tools-extra/trunk/clangd/Protocol.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/Protocol.cpp
+++ clang-tools-extra/trunk/clangd/Protocol.cpp
@@ -184,6 +184,7 @@
   if (!O)
     return false;
   O.map("clangdFixSupport", R.clangdFixSupport);
+  O.map("categorySupport", R.categorySupport);
   return true;
 }
 
Index: clang-tools-extra/trunk/test/clangd/compile-commands-path-in-initialize.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/compile-commands-path-in-initialize.test
+++ clang-tools-extra/trunk/test/clangd/compile-commands-path-in-initialize.test
@@ -23,7 +23,6 @@
 # CHECK-NEXT:   "params": {
 # CHECK-NEXT:     "diagnostics": [
 # CHECK-NEXT:       {
-# CHECK-NEXT:         "category": "#pragma message Directive",
 # CHECK-NEXT:         "message": "MACRO is one",
 ---
 {"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":"INPUT_DIR/build-2"}}}
@@ -31,7 +30,6 @@
 # CHECK-NEXT:   "params": {
 # CHECK-NEXT:     "diagnostics": [
 # CHECK-NEXT:       {
-# CHECK-NEXT:         "category": "#pragma message Directive",
 # CHECK-NEXT:         "message": "MACRO is two",
 ---
 {"jsonrpc":"2.0","id":10000,"method":"shutdown"}
Index: clang-tools-extra/trunk/test/clangd/compile-commands-path.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/compile-commands-path.test
+++ clang-tools-extra/trunk/test/clangd/compile-commands-path.test
@@ -23,7 +23,6 @@
 # CHECK-NEXT:   "params": {
 # CHECK-NEXT:     "diagnostics": [
 # CHECK-NEXT:       {
-# CHECK-NEXT:         "category": "#pragma message Directive",
 # CHECK-NEXT:         "message": "MACRO is not defined",
 ---
 {"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":"INPUT_DIR/build-1"}}}
@@ -31,7 +30,6 @@
 # CHECK-NEXT:   "params": {
 # CHECK-NEXT:     "diagnostics": [
 # CHECK-NEXT:       {
-# CHECK-NEXT:         "category": "#pragma message Directive",
 # CHECK-NEXT:         "message": "MACRO is one",
 ---
 {"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":"INPUT_DIR/build-2"}}}
@@ -39,7 +37,6 @@
 # CHECK-NEXT:   "params": {
 # CHECK-NEXT:     "diagnostics": [
 # CHECK-NEXT:       {
-# CHECK-NEXT:         "category": "#pragma message Directive",
 # CHECK-NEXT:         "message": "MACRO is two",
 ---
 {"jsonrpc":"2.0","id":10000,"method":"shutdown"}
Index: clang-tools-extra/trunk/test/clangd/diagnostic-category.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/diagnostic-category.test
+++ clang-tools-extra/trunk/test/clangd/diagnostic-category.test
@@ -0,0 +1,43 @@
+# RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{"textDocument":{"publishDiagnostics":{"categorySupport":true}}},"trace":"off"}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///foo.c","languageId":"c","version":1,"text":"struct Point {}; union Point p;"}}}
+#      CHECK:    "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:    "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:      {
+# CHECK-NEXT:        "category": "Semantic Issue",
+# CHECK-NEXT:        "message": "Use of 'Point' with tag type that does not match previous declaration\n\nfoo.c:1:8: note: previous use is here",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 22,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 17,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          }
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "severity": 1
+# CHECK-NEXT:      },
+# CHECK-NEXT:      {
+# CHECK-NEXT:        "message": "Previous use is here\n\nfoo.c:1:18: error: use of 'Point' with tag type that does not match previous declaration",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 12,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 7,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          }
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "severity": 3
+# CHECK-NEXT:      }
+# CHECK-NEXT:    ],
+# CHECK-NEXT:    "uri": "file://{{.*}}/foo.c"
+# CHECK-NEXT:  }
+---
+{"jsonrpc":"2.0","id":4,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}
Index: clang-tools-extra/trunk/test/clangd/diagnostics.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/diagnostics.test
+++ clang-tools-extra/trunk/test/clangd/diagnostics.test
@@ -6,7 +6,6 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "message": "Return type of 'main' is not 'int'",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
Index: clang-tools-extra/trunk/test/clangd/did-change-configuration-params.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/did-change-configuration-params.test
+++ clang-tools-extra/trunk/test/clangd/did-change-configuration-params.test
@@ -24,7 +24,6 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "message": "Variable 'i' is uninitialized when used here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
Index: clang-tools-extra/trunk/test/clangd/execute-command.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/execute-command.test
+++ clang-tools-extra/trunk/test/clangd/execute-command.test
@@ -6,7 +6,6 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "message": "Using the result of an assignment as a condition without parentheses",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
Index: clang-tools-extra/trunk/test/clangd/extra-flags.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/extra-flags.test
+++ clang-tools-extra/trunk/test/clangd/extra-flags.test
@@ -6,7 +6,6 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "message": "Variable 'i' is uninitialized when used here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
@@ -29,7 +28,6 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "message": "Variable 'i' is uninitialized when used here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
Index: clang-tools-extra/trunk/test/clangd/fixits-embed-in-diagnostic.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/fixits-embed-in-diagnostic.test
+++ clang-tools-extra/trunk/test/clangd/fixits-embed-in-diagnostic.test
@@ -6,7 +6,6 @@
 # CHECK-NEXT:    "params": {
 # CHECK-NEXT:     "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "clangd_fixes": [
 # CHECK-NEXT:          {
 # CHECK-NEXT:            "edit": {
Index: clang-tools-extra/trunk/test/clangd/fixits.test
===================================================================
--- clang-tools-extra/trunk/test/clangd/fixits.test
+++ clang-tools-extra/trunk/test/clangd/fixits.test
@@ -6,7 +6,6 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "category": "Semantic Issue",
 # CHECK-NEXT:        "message": "Using the result of an assignment as a condition without parentheses",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {