Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -60,32 +60,6 @@
 static URISchemeRegistry::Add<TestScheme>
     X("test", "Test scheme for clangd lit tests.");
 
-TextEdit replacementToEdit(StringRef Code, const tooling::Replacement &R) {
-  Range ReplacementRange = {
-      offsetToPosition(Code, R.getOffset()),
-      offsetToPosition(Code, R.getOffset() + R.getLength())};
-  return {ReplacementRange, R.getReplacementText()};
-}
-
-std::vector<TextEdit>
-replacementsToEdits(StringRef Code,
-                    const std::vector<tooling::Replacement> &Replacements) {
-  // Turn the replacements into the format specified by the Language Server
-  // Protocol. Fuse them into one big JSON array.
-  std::vector<TextEdit> Edits;
-  for (const auto &R : Replacements)
-    Edits.push_back(replacementToEdit(Code, R));
-  return Edits;
-}
-
-std::vector<TextEdit> replacementsToEdits(StringRef Code,
-                                          const tooling::Replacements &Repls) {
-  std::vector<TextEdit> Edits;
-  for (const auto &R : Repls)
-    Edits.push_back(replacementToEdit(Code, R));
-  return Edits;
-}
-
 SymbolKindBitset defaultSymbolKinds() {
   SymbolKindBitset Defaults;
   for (size_t I = SymbolKindMin; I <= static_cast<size_t>(SymbolKind::Array);
@@ -291,7 +265,11 @@
           return replyError(ErrorCode::InternalError,
                             llvm::toString(Replacements.takeError()));
 
-        std::vector<TextEdit> Edits = replacementsToEdits(*Code, *Replacements);
+        // Turn the replacements into the format specified by the Language
+        // Server Protocol. Fuse them into one big JSON array.
+        std::vector<TextEdit> Edits;
+        for (const auto &R : *Replacements)
+          Edits.push_back(replacementToEdit(*Code, R));
         WorkspaceEdit WE;
         WE.changes = {{Params.textDocument.uri.uri(), Edits}};
         reply(WE);
Index: clangd/SourceCode.h
===================================================================
--- clangd/SourceCode.h
+++ clangd/SourceCode.h
@@ -15,6 +15,7 @@
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SOURCECODE_H
 #include "Protocol.h"
 #include "clang/Basic/SourceLocation.h"
+#include "clang/Tooling/Core/Replacement.h"
 
 namespace clang {
 class SourceManager;
@@ -55,6 +56,11 @@
 std::pair<llvm::StringRef, llvm::StringRef>
 splitQualifiedName(llvm::StringRef QName);
 
+TextEdit replacementToEdit(StringRef Code, const tooling::Replacement &R);
+
+std::vector<TextEdit> replacementsToEdits(StringRef Code,
+                                          const tooling::Replacements &Repls);
+
 } // namespace clangd
 } // namespace clang
 #endif
Index: clangd/SourceCode.cpp
===================================================================
--- clangd/SourceCode.cpp
+++ clangd/SourceCode.cpp
@@ -166,5 +166,20 @@
   return {QName.substr(0, Pos + 2), QName.substr(Pos + 2)};
 }
 
+TextEdit replacementToEdit(StringRef Code, const tooling::Replacement &R) {
+  Range ReplacementRange = {
+      offsetToPosition(Code, R.getOffset()),
+      offsetToPosition(Code, R.getOffset() + R.getLength())};
+  return {ReplacementRange, R.getReplacementText()};
+}
+
+std::vector<TextEdit> replacementsToEdits(StringRef Code,
+                                          const tooling::Replacements &Repls) {
+  std::vector<TextEdit> Edits;
+  for (const auto &R : Repls)
+    Edits.push_back(replacementToEdit(Code, R));
+  return Edits;
+}
+
 } // namespace clangd
 } // namespace clang
Index: clangd/index/SymbolCollector.cpp
===================================================================
--- clangd/index/SymbolCollector.cpp
+++ clangd/index/SymbolCollector.cpp
@@ -90,6 +90,44 @@
   return llvm::None;
 }
 
+// All proto generated headers should start with this line.
+static const char *PROTO_HEADER_COMMENT =
+    "// Generated by the protocol buffer compiler.  DO NOT EDIT!";
+
+// Checks whether the decl is a private symbol in a header generated by
+// protobuf compiler.
+// To identify whether a proto header is actually generated by proto compiler,
+// we check whether it starts with PROTO_HEADER_COMMENT.
+// FIXME: make filtering extensible when there are more use cases for symbol
+// filters.
+bool isPrivateProtoDecl(const NamedDecl &ND) {
+  const auto &SM = ND.getASTContext().getSourceManager();
+  auto Loc = findNameLoc(&ND);
+  auto FileName = SM.getFilename(Loc);
+  if (!FileName.endswith(".proto.h") && !FileName.endswith(".pb.h"))
+    return false;
+  auto FID = SM.getFileID(Loc);
+  // Double check that this is an actual protobuf header.
+  if (!SM.getBufferData(FID).startswith(PROTO_HEADER_COMMENT))
+    return false;
+
+  // If ND does not have an identifier/name, it must be private.
+  if (ND.getIdentifier() == nullptr)
+    return true;
+  auto Name = ND.getIdentifier()->getName();
+  if (!Name.contains('_'))
+    return false;
+  // Nested proto entities (e.g. Message::Nested) have top-level decls
+  // that shouldn't be used (Message_Nested). Ignore them completely.
+  // The nested entities are dangling type aliases, we may want to reconsider
+  // including them in the future.
+  // For enum constants, SOME_ENUM_CONSTANT is not private and should be
+  // indexed. Outer_INNER is private. This heuristic relies on naming style, it
+  // will include OUTER_INNER and exclude some_enum_constant.
+  return (ND.getKind() != Decl::EnumConstant) ||
+         std::any_of(Name.begin(), Name.end(), islower);
+}
+
 bool shouldFilterDecl(const NamedDecl *ND, ASTContext *ASTCtx,
                       const SymbolCollector::Options &Opts) {
   using namespace clang::ast_matchers;
@@ -130,6 +168,9 @@
           .empty())
     return true;
 
+  // Avoid indexing internal symbols in protobuf generated headers.
+  if (isPrivateProtoDecl(*ND))
+    return true;
   return false;
 }
 
Index: unittests/clangd/SymbolCollectorTests.cpp
===================================================================
--- unittests/clangd/SymbolCollectorTests.cpp
+++ unittests/clangd/SymbolCollectorTests.cpp
@@ -697,6 +697,40 @@
                            AllOf(QName("pörk"), DeclRange(Header.range()))));
 }
 
+TEST_F(SymbolCollectorTest, FilterPrivateProtoSymbols) {
+  TestHeaderName = testPath("x.proto.h");
+  const std::string Header =
+      R"(// Generated by the protocol buffer compiler.  DO NOT EDIT!
+         namespace nx {
+           class Top_Level {};
+           class TopLevel {};
+           enum Kind {
+             KIND_OK,
+             Kind_Not_Ok,
+           };
+           bool operator<(int x, int y);
+         })";
+  runSymbolCollector(Header, /*Main=*/"");
+  EXPECT_THAT(Symbols, UnorderedElementsAre(QName("nx"), QName("nx::TopLevel"),
+                                            QName("nx::Kind"),
+                                            QName("nx::KIND_OK")));
+}
+
+TEST_F(SymbolCollectorTest, DoubleCheckProtoHeaderComment) {
+  TestHeaderName = testPath("x.proto.h");
+  const std::string Header = R"(
+  namespace nx {
+    class Top_Level {};
+    enum Kind {
+      Kind_Fine
+    };
+  }
+  )";
+  runSymbolCollector(Header, /*Main=*/"");
+  EXPECT_THAT(Symbols,
+              UnorderedElementsAre(QName("nx"), QName("nx::Top_Level"),
+                                   QName("nx::Kind"), QName("nx::Kind_Fine")));
+}
 
 } // namespace
 } // namespace clangd