diff --git a/.clang-tidy b/.clang-tidy
index f279587e1c11..c3a53752672b 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -121,6 +121,10 @@ CheckOptions:
     value: CamelCase
   - key: readability-identifier-naming.NamespaceCase
     value: CamelCase
+  # Headers re-open LLVM and Clang namespaces to forward-declare their types,
+  # which is much cheaper to compile than including their headers.
+  - key: readability-identifier-naming.NamespaceIgnoredRegexp
+    value: '^(clang|llvm)$'
   - key: readability-identifier-naming.StructCase
     value: CamelCase
   - key: readability-identifier-naming.TemplateParameterCase
diff --git a/toolchain/check/cpp/access.cpp b/toolchain/check/cpp/access.cpp
index 8250e4af63b4..107a0303733a 100644
--- a/toolchain/check/cpp/access.cpp
+++ b/toolchain/check/cpp/access.cpp
@@ -4,6 +4,8 @@
 
 #include "toolchain/check/cpp/access.h"
 
+#include "clang/AST/Decl.h"
+
 namespace Carbon::Check {
 
 static auto CalculateEffectiveAccess(clang::DeclAccessPair access_pair)
diff --git a/toolchain/check/cpp/access.h b/toolchain/check/cpp/access.h
index 04b7d070dd85..a48507f2a200 100644
--- a/toolchain/check/cpp/access.h
+++ b/toolchain/check/cpp/access.h
@@ -5,6 +5,8 @@
 #ifndef CARBON_TOOLCHAIN_CHECK_CPP_ACCESS_H_
 #define CARBON_TOOLCHAIN_CHECK_CPP_ACCESS_H_
 
+#include "clang/AST/DeclAccessPair.h"
+#include "clang/Basic/Specifiers.h"
 #include "toolchain/sem_ir/name_scope.h"
 
 namespace Carbon::Check {
diff --git a/toolchain/check/cpp/call.h b/toolchain/check/cpp/call.h
index 816b894437bf..770bb4d4342d 100644
--- a/toolchain/check/cpp/call.h
+++ b/toolchain/check/cpp/call.h
@@ -8,6 +8,11 @@
 #include "toolchain/check/context.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class TemplateArgumentListInfo;
+class TemplateDecl;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Converts a call argument list into a Clang template argument list for a given
diff --git a/toolchain/check/cpp/constant.cpp b/toolchain/check/cpp/constant.cpp
index d16f52f9448d..3602cc9c0cb2 100644
--- a/toolchain/check/cpp/constant.cpp
+++ b/toolchain/check/cpp/constant.cpp
@@ -4,6 +4,8 @@
 
 #include "toolchain/check/cpp/constant.h"
 
+#include "clang/Basic/DiagnosticSema.h"
+#include "clang/Sema/Sema.h"
 #include "toolchain/check/cpp/import.h"
 #include "toolchain/check/cpp/location.h"
 #include "toolchain/check/cpp/type_mapping.h"
diff --git a/toolchain/check/cpp/constant.h b/toolchain/check/cpp/constant.h
index 52ad34c4d9e3..c55d164d65ca 100644
--- a/toolchain/check/cpp/constant.h
+++ b/toolchain/check/cpp/constant.h
@@ -11,6 +11,10 @@
 #include "toolchain/check/context.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class VarDecl;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Converts an `APValue` to a Carbon `ConstantId`.
diff --git a/toolchain/check/cpp/context.cpp b/toolchain/check/cpp/context.cpp
index aba3f6d51105..4593347bbc55 100644
--- a/toolchain/check/cpp/context.cpp
+++ b/toolchain/check/cpp/context.cpp
@@ -5,6 +5,8 @@
 #include "toolchain/check/cpp/context.h"
 
 #include "clang/AST/Mangle.h"
+#include "clang/Frontend/CompilerInstance.h"
+#include "clang/Parse/Parser.h"
 
 namespace Carbon::Check {
 
diff --git a/toolchain/check/cpp/context.h b/toolchain/check/cpp/context.h
index 2cce3d4f3f58..2ebc83ef1157 100644
--- a/toolchain/check/cpp/context.h
+++ b/toolchain/check/cpp/context.h
@@ -8,12 +8,18 @@
 #include <memory>
 
 #include "clang/Basic/SourceLocation.h"
-#include "clang/Frontend/CompilerInstance.h"
-#include "clang/Frontend/FrontendAction.h"
-#include "clang/Parse/Parser.h"
 #include "common/check.h"
 #include "llvm/ADT/SmallVector.h"
 
+namespace clang {
+class ASTContext;
+class CompilerInstance;
+class FunctionDecl;
+class MangleContext;
+class Parser;
+class Sema;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Context for C++ code during check.
diff --git a/toolchain/check/cpp/export.cpp b/toolchain/check/cpp/export.cpp
index faed1f9f032e..03cc847c00f6 100644
--- a/toolchain/check/cpp/export.cpp
+++ b/toolchain/check/cpp/export.cpp
@@ -6,6 +6,8 @@
 
 #include <optional>
 
+#include "clang/AST/ASTConsumer.h"
+#include "clang/Sema/Sema.h"
 #include "llvm/Support/Casting.h"
 #include "toolchain/check/cpp/access.h"
 #include "toolchain/check/cpp/import.h"
diff --git a/toolchain/check/cpp/export.h b/toolchain/check/cpp/export.h
index f064430baad3..0e6e95b9b8a2 100644
--- a/toolchain/check/cpp/export.h
+++ b/toolchain/check/cpp/export.h
@@ -9,6 +9,11 @@
 #include "toolchain/check/context.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class CXXDestructorDecl;
+class CXXRecordDecl;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Exports a Carbon name scope into C++ as a namespace or class, or returns the
diff --git a/toolchain/check/cpp/generate_ast.h b/toolchain/check/cpp/generate_ast.h
index ebf96532d0e7..b4ffeef0d49a 100644
--- a/toolchain/check/cpp/generate_ast.h
+++ b/toolchain/check/cpp/generate_ast.h
@@ -15,6 +15,10 @@
 #include "toolchain/parse/tree.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class CompilerInvocation;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Generates a Clang AST for the given C++ imports and sets it as the context's
diff --git a/toolchain/check/cpp/impl_lookup.cpp b/toolchain/check/cpp/impl_lookup.cpp
index 12e8774143a9..00e12cdbd729 100644
--- a/toolchain/check/cpp/impl_lookup.cpp
+++ b/toolchain/check/cpp/impl_lookup.cpp
@@ -5,7 +5,9 @@
 #include "toolchain/check/cpp/impl_lookup.h"
 
 #include "clang/Basic/SourceLocation.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/Overload.h"
 #include "clang/Sema/Sema.h"
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/context.h"
diff --git a/toolchain/check/cpp/import.cpp b/toolchain/check/cpp/import.cpp
index 81c976422baa..54e27a091dce 100644
--- a/toolchain/check/cpp/import.cpp
+++ b/toolchain/check/cpp/import.cpp
@@ -16,8 +16,11 @@
 #include "clang/AST/UnresolvedSet.h"
 #include "clang/AST/VTableBuilder.h"
 #include "clang/Frontend/CompilerInvocation.h"
+#include "clang/Lex/MacroInfo.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/Overload.h"
+#include "clang/Sema/Sema.h"
 #include "common/check.h"
 #include "common/ostream.h"
 #include "common/raw_string_ostream.h"
@@ -2375,7 +2378,7 @@ auto ImportCppOverloadSet(
     Context& context, SemIR::LocId loc_id, SemIR::NameScopeId scope_id,
     SemIR::NameId name_id, clang::CXXRecordDecl* naming_class,
     clang::UnresolvedSet<4>&& overload_set,
-    clang::OverloadCandidateSet::OperatorRewriteInfo operator_rewrite_info)
+    SemIR::CppOverloadSet::OperatorRewriteInfo operator_rewrite_info)
     -> SemIR::InstId {
   SemIR::CppOverloadSetId overload_set_id = context.cpp_overload_sets().Add(
       SemIR::CppOverloadSet{.name_id = name_id,
diff --git a/toolchain/check/cpp/import.h b/toolchain/check/cpp/import.h
index f94a04e0bffa..951ed22dac5c 100644
--- a/toolchain/check/cpp/import.h
+++ b/toolchain/check/cpp/import.h
@@ -5,6 +5,7 @@
 #ifndef CARBON_TOOLCHAIN_CHECK_CPP_IMPORT_H_
 #define CARBON_TOOLCHAIN_CHECK_CPP_IMPORT_H_
 
+#include "clang/AST/Type.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/StringRef.h"
@@ -16,6 +17,12 @@
 #include "toolchain/sem_ir/clang_decl.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class CompilerInvocation;
+class IdentifierInfo;
+class VarDecl;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Returns whether the given function is an object member function. This is true
@@ -84,7 +91,7 @@ auto ImportCppOverloadSet(
     Context& context, SemIR::LocId loc_id, SemIR::NameScopeId scope_id,
     SemIR::NameId name_id, clang::CXXRecordDecl* naming_class,
     clang::UnresolvedSet<4>&& overload_set,
-    clang::OverloadCandidateSet::OperatorRewriteInfo operator_rewrite_info)
+    SemIR::CppOverloadSet::OperatorRewriteInfo operator_rewrite_info)
     -> SemIR::InstId;
 
 // Looks up the given name in the Clang AST generated when importing C++ code
diff --git a/toolchain/check/cpp/location.cpp b/toolchain/check/cpp/location.cpp
index 2d5c9c5e0a8d..ca39927570ee 100644
--- a/toolchain/check/cpp/location.cpp
+++ b/toolchain/check/cpp/location.cpp
@@ -4,6 +4,8 @@
 
 #include "toolchain/check/cpp/location.h"
 
+#include "clang/AST/ASTContext.h"
+#include "clang/Basic/SourceManager.h"
 #include "toolchain/sem_ir/absolute_node_ref.h"
 #include "toolchain/sem_ir/ids.h"
 
diff --git a/toolchain/check/cpp/macros.cpp b/toolchain/check/cpp/macros.cpp
index 91b0570878ba..8267af39c8cd 100644
--- a/toolchain/check/cpp/macros.cpp
+++ b/toolchain/check/cpp/macros.cpp
@@ -6,6 +6,7 @@
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Expr.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Parse/Parser.h"
 #include "clang/Sema/Sema.h"
 #include "common/check.h"
diff --git a/toolchain/check/cpp/macros.h b/toolchain/check/cpp/macros.h
index 27b9a5a5f2d7..dbf046770852 100644
--- a/toolchain/check/cpp/macros.h
+++ b/toolchain/check/cpp/macros.h
@@ -7,6 +7,10 @@
 
 #include "toolchain/check/context.h"
 
+namespace clang {
+class MacroInfo;
+}  // namespace clang
+
 namespace Carbon::Check {
 
 // Tries to evaluate the given macro. The macro will be evaluated as a
diff --git a/toolchain/check/cpp/overload_resolution.cpp b/toolchain/check/cpp/overload_resolution.cpp
index b179a0733750..4ff5bf30b9b5 100644
--- a/toolchain/check/cpp/overload_resolution.cpp
+++ b/toolchain/check/cpp/overload_resolution.cpp
@@ -290,12 +290,15 @@ auto PerformCppOverloadResolution(
   clang::SourceLocation loc = GetCppLocation(context, loc_id);
 
   // Add candidate functions from the name lookup.
+  const auto& rewrite_info = overload_set.operator_rewrite_info;
   clang::OverloadCandidateSet candidate_set(
       loc,
-      overload_set.operator_rewrite_info.OriginalOperator
+      rewrite_info.original_operator
           ? clang::OverloadCandidateSet::CandidateSetKind::CSK_Operator
           : clang::OverloadCandidateSet::CandidateSetKind::CSK_Normal,
-      overload_set.operator_rewrite_info);
+      clang::OverloadCandidateSet::OperatorRewriteInfo(
+          rewrite_info.original_operator, rewrite_info.op_loc,
+          rewrite_info.allow_rewritten_candidates));
 
   AddOverloadCandidates(context, candidate_set,
                         overload_set.candidate_functions, template_arg_ids,
diff --git a/toolchain/check/cpp/overload_resolution.h b/toolchain/check/cpp/overload_resolution.h
index 39453a780819..49d7fe4ee9b7 100644
--- a/toolchain/check/cpp/overload_resolution.h
+++ b/toolchain/check/cpp/overload_resolution.h
@@ -5,6 +5,7 @@
 #ifndef CARBON_TOOLCHAIN_CHECK_CPP_OVERLOAD_RESOLUTION_H_
 #define CARBON_TOOLCHAIN_CHECK_CPP_OVERLOAD_RESOLUTION_H_
 
+#include "clang/Sema/Overload.h"
 #include "toolchain/check/context.h"
 #include "toolchain/sem_ir/function.h"
 #include "toolchain/sem_ir/ids.h"
diff --git a/toolchain/check/cpp/type_mapping.cpp b/toolchain/check/cpp/type_mapping.cpp
index 86d0d8ae34d3..4ff4974559ee 100644
--- a/toolchain/check/cpp/type_mapping.cpp
+++ b/toolchain/check/cpp/type_mapping.cpp
@@ -11,7 +11,9 @@
 
 #include "clang/AST/Type.h"
 #include "clang/Basic/TargetInfo.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/Sema.h"
 #include "llvm/ADT/SmallVector.h"
 #include "toolchain/base/int.h"
 #include "toolchain/base/kind_switch.h"
diff --git a/toolchain/lower/context.cpp b/toolchain/lower/context.cpp
index 37d0b5db6deb..bc805a0b5ab5 100644
--- a/toolchain/lower/context.cpp
+++ b/toolchain/lower/context.cpp
@@ -4,6 +4,7 @@
 
 #include "toolchain/lower/context.h"
 
+#include "clang/Basic/SourceManager.h"
 #include "common/check.h"
 #include "common/growing_range.h"
 #include "common/raw_string_ostream.h"
diff --git a/toolchain/sem_ir/BUILD b/toolchain/sem_ir/BUILD
index 2ac528de5a8b..3dcafff73a97 100644
--- a/toolchain/sem_ir/BUILD
+++ b/toolchain/sem_ir/BUILD
@@ -88,6 +88,7 @@ cc_library(
         "class.cpp",
         "constant.cpp",
         "core_interface.cpp",
+        "cpp_file.cpp",
         "cpp_initializer_list.cpp",
         "cpp_overload_set.cpp",
         "facet_type_info.cpp",
@@ -178,6 +179,7 @@ cc_library(
         "//toolchain/parse:node_kind",
         "//toolchain/parse:tree",
         "@llvm-project//clang:ast",
+        "@llvm-project//clang:basic",
         "@llvm-project//clang:frontend",
         "@llvm-project//clang:sema",
         "@llvm-project//llvm:Support",
@@ -302,6 +304,7 @@ cc_library(
         ":typed_insts",
         "//toolchain/diagnostics:emitter",
         "//toolchain/parse:tree",
+        "@llvm-project//clang:basic",
         "@llvm-project//clang:frontend",
         "@llvm-project//llvm:Support",
     ],
diff --git a/toolchain/sem_ir/clang_decl.cpp b/toolchain/sem_ir/clang_decl.cpp
index 8fc830892fe2..6c96abae11ae 100644
--- a/toolchain/sem_ir/clang_decl.cpp
+++ b/toolchain/sem_ir/clang_decl.cpp
@@ -57,6 +57,21 @@ auto ClangDeclSignature::Print(llvm::raw_ostream& out) const -> void {
   out << "}";
 }
 
+auto ClangDeclKey::ForFunctionDecl(clang::FunctionDecl* decl,
+                                   ClangDeclSignatureId signature_id)
+    -> ClangDeclKey {
+  return ClangDeclKey(decl, signature_id, UncheckedTag());
+}
+
+auto ClangDeclKey::ForNonFunctionDecl(clang::Decl* decl) -> ClangDeclKey {
+  CARBON_CHECK(!isa<clang::FunctionDecl>(decl));
+  return ClangDeclKey(decl, ClangDeclSignatureId::None, UncheckedTag());
+}
+
+ClangDeclKey::ClangDeclKey(clang::Decl* decl, ClangDeclSignatureId signature_id,
+                           UncheckedTag /*_*/)
+    : decl(decl->getCanonicalDecl()), signature_id(signature_id) {}
+
 auto ClangDeclKey::Print(llvm::raw_ostream& out) const -> void {
   RawStringOstream decl_stream;
   auto policy = decl->getASTContext().getPrintingPolicy();
diff --git a/toolchain/sem_ir/clang_decl.h b/toolchain/sem_ir/clang_decl.h
index f65d33390ea4..7179eccb06ea 100644
--- a/toolchain/sem_ir/clang_decl.h
+++ b/toolchain/sem_ir/clang_decl.h
@@ -7,12 +7,16 @@
 
 #include <concepts>
 
-#include "clang/AST/Decl.h"
 #include "common/hashtable_key_context.h"
 #include "common/ostream.h"
 #include "toolchain/base/canonical_value_store.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class Decl;
+class FunctionDecl;
+}  // namespace clang
+
 namespace Carbon::SemIR {
 
 // Information about how to form the Carbon function signature from the Clang
@@ -126,16 +130,11 @@ struct ClangDeclKey : public Printable<ClangDeclKey> {
   // count is required.
   static auto ForFunctionDecl(clang::FunctionDecl* decl,
                               ClangDeclSignatureId signature_id)
-      -> ClangDeclKey {
-    return ClangDeclKey(decl, signature_id, UncheckedTag());
-  }
+      -> ClangDeclKey;
 
   // Factory function for clang declaration that is dynamically known to not be
   // a function declaration.
-  static auto ForNonFunctionDecl(clang::Decl* decl) -> ClangDeclKey {
-    CARBON_CHECK(!isa<clang::FunctionDecl>(decl));
-    return ClangDeclKey(decl, ClangDeclSignatureId::None, UncheckedTag());
-  }
+  static auto ForNonFunctionDecl(clang::Decl* decl) -> ClangDeclKey;
 
   auto Print(llvm::raw_ostream& out) const -> void;
 
@@ -163,8 +162,7 @@ struct ClangDeclKey : public Printable<ClangDeclKey> {
     explicit UncheckedTag() = default;
   };
   ClangDeclKey(clang::Decl* decl, ClangDeclSignatureId signature_id,
-               UncheckedTag /*_*/)
-      : decl(decl->getCanonicalDecl()), signature_id(signature_id) {}
+               UncheckedTag /*_*/);
 };
 
 // A Clang declaration mapped to a Carbon instruction.
diff --git a/toolchain/sem_ir/cpp_file.cpp b/toolchain/sem_ir/cpp_file.cpp
new file mode 100644
index 000000000000..9477ea901c13
--- /dev/null
+++ b/toolchain/sem_ir/cpp_file.cpp
@@ -0,0 +1,57 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/sem_ir/cpp_file.h"
+
+#include "clang/AST/Mangle.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Frontend/CompilerInstance.h"
+#include "common/check.h"
+
+namespace Carbon::SemIR {
+
+CppFile::CppFile(std::unique_ptr<clang::CompilerInstance> clang,
+                 llvm::LLVMContext* llvm_context)
+    : clang_(std::move(clang)), llvm_context_(llvm_context) {}
+
+CppFile::~CppFile() = default;
+
+auto CppFile::diagnostic_options() const -> const clang::DiagnosticOptions& {
+  return clang_->getDiagnostics().getDiagnosticOptions();
+}
+
+auto CppFile::lang_options() const -> const clang::LangOptions& {
+  return clang_->getLangOpts();
+}
+
+auto CppFile::source_manager() -> clang::SourceManager& {
+  return clang_->getSourceManager();
+}
+
+auto CppFile::source_manager() const -> const clang::SourceManager& {
+  return clang_->getSourceManager();
+}
+
+auto CppFile::diagnostics() const -> clang::DiagnosticsEngine& {
+  return clang_->getDiagnostics();
+}
+
+auto CppFile::ast_context() -> clang::ASTContext& {
+  return clang_->getASTContext();
+}
+
+auto CppFile::ast_context() const -> const clang::ASTContext& {
+  return clang_->getASTContext();
+}
+
+auto CppFile::CreateMangleContext() -> void {
+  CARBON_CHECK(!mangle_context_);
+  mangle_context_.reset(ast_context().createMangleContext());
+}
+
+auto CppFile::mangle_context() const -> clang::MangleContext& {
+  return *mangle_context_;
+}
+
+}  // namespace Carbon::SemIR
diff --git a/toolchain/sem_ir/cpp_file.h b/toolchain/sem_ir/cpp_file.h
index f3ab7a3043a0..3968ffdc83e8 100644
--- a/toolchain/sem_ir/cpp_file.h
+++ b/toolchain/sem_ir/cpp_file.h
@@ -7,16 +7,20 @@
 
 #include <memory>
 
-#include "clang/AST/Mangle.h"
-#include "clang/Basic/CodeGenOptions.h"
-#include "clang/Basic/Diagnostic.h"
-#include "clang/CodeGen/ModuleBuilder.h"
-#include "clang/Frontend/CompilerInstance.h"
-#include "clang/Lex/PreprocessorOptions.h"
-#include "common/check.h"
-#include "llvm/ADT/IntrusiveRefCntPtr.h"
-#include "llvm/IR/Module.h"
-#include "llvm/Support/FileSystem.h"
+namespace clang {
+class ASTContext;
+class CodeGenerator;
+class CompilerInstance;
+class DiagnosticOptions;
+class DiagnosticsEngine;
+class LangOptions;
+class MangleContext;
+class SourceManager;
+}  // namespace clang
+
+namespace llvm {
+class LLVMContext;
+}  // namespace llvm
 
 namespace Carbon::SemIR {
 
@@ -25,46 +29,29 @@ namespace Carbon::SemIR {
 class CppFile {
  public:
   explicit CppFile(std::unique_ptr<clang::CompilerInstance> clang,
-                   llvm::LLVMContext* llvm_context)
-      : clang_(std::move(clang)), llvm_context_(llvm_context) {}
+                   llvm::LLVMContext* llvm_context);
+  ~CppFile();
 
   // Access to compilation options.
-  auto diagnostic_options() const -> const clang::DiagnosticOptions& {
-    return clang_->getDiagnostics().getDiagnosticOptions();
-  }
-  auto lang_options() const -> const clang::LangOptions& {
-    return clang_->getLangOpts();
-  }
+  auto diagnostic_options() const -> const clang::DiagnosticOptions&;
+  auto lang_options() const -> const clang::LangOptions&;
 
   // Access to Clang's compilation environment.
-  auto source_manager() -> clang::SourceManager& {
-    return clang_->getSourceManager();
-  }
-  auto source_manager() const -> const clang::SourceManager& {
-    return clang_->getSourceManager();
-  }
+  auto source_manager() -> clang::SourceManager&;
+  auto source_manager() const -> const clang::SourceManager&;
   // TODO: This doesn't really belong here, but is currently used by lowering
   // because Clang's code generation may produce diagnostics.
-  auto diagnostics() const -> clang::DiagnosticsEngine& {
-    return clang_->getDiagnostics();
-  }
+  auto diagnostics() const -> clang::DiagnosticsEngine&;
 
   // Access to layers of Clang's C++ representation.
-  auto ast_context() -> clang::ASTContext& { return clang_->getASTContext(); }
-  auto ast_context() const -> const clang::ASTContext& {
-    return clang_->getASTContext();
-  }
+  auto ast_context() -> clang::ASTContext&;
+  auto ast_context() const -> const clang::ASTContext&;
 
   // Creates the mangle context for this file's C++ AST. Must be called once the
   // AST context is available (after the frontend begins the source file) and
   // before `mangle_context()` is used.
-  auto CreateMangleContext() -> void {
-    CARBON_CHECK(!mangle_context_);
-    mangle_context_.reset(ast_context().createMangleContext());
-  }
-  auto mangle_context() const -> clang::MangleContext& {
-    return *mangle_context_;
-  }
+  auto CreateMangleContext() -> void;
+  auto mangle_context() const -> clang::MangleContext&;
 
   auto llvm_context() const -> llvm::LLVMContext* { return llvm_context_; }
   auto SetCodeGenerator(clang::CodeGenerator* code_generator) -> void {
diff --git a/toolchain/sem_ir/cpp_overload_set.h b/toolchain/sem_ir/cpp_overload_set.h
index 31716a3f4740..f5385669974a 100644
--- a/toolchain/sem_ir/cpp_overload_set.h
+++ b/toolchain/sem_ir/cpp_overload_set.h
@@ -5,17 +5,36 @@
 #ifndef CARBON_TOOLCHAIN_SEM_IR_CPP_OVERLOAD_SET_H_
 #define CARBON_TOOLCHAIN_SEM_IR_CPP_OVERLOAD_SET_H_
 
-#include "clang/AST/Decl.h"
 #include "clang/AST/UnresolvedSet.h"
-#include "clang/Sema/Overload.h"
+#include "clang/Basic/OperatorKinds.h"
+#include "clang/Basic/SourceLocation.h"
 #include "common/ostream.h"
 #include "toolchain/base/value_store.h"
 #include "toolchain/sem_ir/ids.h"
 
+namespace clang {
+class CXXRecordDecl;
+}  // namespace clang
+
 namespace Carbon::SemIR {
 
 // An overloaded C++ function.
 struct CppOverloadSet : public Printable<CppOverloadSet> {
+  // Information about operator rewrites to consider when adding operator
+  // functions to a candidate set.
+  //
+  // This mirrors `clang::OverloadCandidateSet::OperatorRewriteInfo` so that
+  // this header doesn't need `clang/Sema/Overload.h`; the use sites construct
+  // the Clang type from these fields.
+  struct OperatorRewriteInfo {
+    // The original operator as written in the source.
+    clang::OverloadedOperatorKind original_operator = clang::OO_None;
+    // The source location of the operator.
+    clang::SourceLocation op_loc;
+    // Whether we should include rewritten candidates in the overload set.
+    bool allow_rewritten_candidates = false;
+  };
+
   // The function's name.
   NameId name_id;
 
@@ -32,7 +51,7 @@ struct CppOverloadSet : public Printable<CppOverloadSet> {
 
   /// Information about operator rewrites to consider when adding operator
   /// functions to a candidate set.
-  clang::OverloadCandidateSet::OperatorRewriteInfo operator_rewrite_info;
+  OperatorRewriteInfo operator_rewrite_info;
 
   auto Print(llvm::raw_ostream& out) const -> void {
     out << "name: " << name_id << ", parent_scope: " << parent_scope_id;
diff --git a/toolchain/sem_ir/diagnostic_loc_converter.cpp b/toolchain/sem_ir/diagnostic_loc_converter.cpp
index aab353d9d3ed..fc80f86420d1 100644
--- a/toolchain/sem_ir/diagnostic_loc_converter.cpp
+++ b/toolchain/sem_ir/diagnostic_loc_converter.cpp
@@ -4,6 +4,7 @@
 
 #include "toolchain/sem_ir/diagnostic_loc_converter.h"
 
+#include "clang/Basic/SourceManager.h"
 #include "clang/Frontend/DiagnosticRenderer.h"
 
 namespace Carbon::SemIR {
diff --git a/toolchain/sem_ir/name_scope.h b/toolchain/sem_ir/name_scope.h
index a0ceea38957e..f7f17207813d 100644
--- a/toolchain/sem_ir/name_scope.h
+++ b/toolchain/sem_ir/name_scope.h
@@ -5,7 +5,6 @@
 #ifndef CARBON_TOOLCHAIN_SEM_IR_NAME_SCOPE_H_
 #define CARBON_TOOLCHAIN_SEM_IR_NAME_SCOPE_H_
 
-#include "clang/AST/DeclBase.h"
 #include "common/map.h"
 #include "toolchain/sem_ir/clang_decl.h"
 #include "toolchain/sem_ir/ids.h"
diff --git a/toolchain/sem_ir/type_info.cpp b/toolchain/sem_ir/type_info.cpp
index fedceb28a523..064f8ad7468f 100644
--- a/toolchain/sem_ir/type_info.cpp
+++ b/toolchain/sem_ir/type_info.cpp
@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include "clang/AST/ASTContext.h"
 #include "common/raw_string_ostream.h"
 #include "toolchain/sem_ir/file.h"