diff --git a/packages/cxx-gen-ast/src/kwgen.ts b/packages/cxx-gen-ast/src/kwgen.ts
index 9f3ae17b..f1e37998 100644
--- a/packages/cxx-gen-ast/src/kwgen.ts
+++ b/packages/cxx-gen-ast/src/kwgen.ts
@@ -25,13 +25,17 @@ export default function kwgen(options: Options) {
   } = options;
 
   let out: string[] = [];
-  const emit = (s: string) => out.push(s);
+  const emit = (s: string = "") => out.push(s);
 
   if (copyright !== undefined) {
     emit("// Generated file by: kwgen.ts");
     emit(copyright);
   }
 
+  emit();
+  emit("#pragma once");
+  emit();
+
   const keywordsByLength = Map.groupBy(keywords, (s) => s.length);
 
   const lengths = Array.from(keywordsByLength.keys());
diff --git a/src/parser/cxx/literals.h b/src/parser/cxx/literals.h
index 79eb43af..939a9038 100644
--- a/src/parser/cxx/literals.h
+++ b/src/parser/cxx/literals.h
@@ -18,6 +18,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 #include <cxx/cxx_fwd.h>
 
 #include <cstdint>
diff --git a/src/parser/cxx/literals_fwd.h b/src/parser/cxx/literals_fwd.h
index cb2a26e8..aea78271 100644
--- a/src/parser/cxx/literals_fwd.h
+++ b/src/parser/cxx/literals_fwd.h
@@ -18,6 +18,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 #include <cxx/cxx_fwd.h>
 
 namespace cxx {
diff --git a/src/parser/cxx/name_lookup.h b/src/parser/cxx/name_lookup.h
index 73103366..c6e06700 100644
--- a/src/parser/cxx/name_lookup.h
+++ b/src/parser/cxx/name_lookup.h
@@ -18,6 +18,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 #include <cxx/ast_fwd.h>
 #include <cxx/names_fwd.h>
 #include <cxx/symbols_fwd.h>
diff --git a/src/parser/cxx/private/c_keywords-priv.h b/src/parser/cxx/private/c_keywords-priv.h
index a9dfd092..6af5d0da 100644
--- a/src/parser/cxx/private/c_keywords-priv.h
+++ b/src/parser/cxx/private/c_keywords-priv.h
@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 static inline auto classifyC2(const char* s) -> cxx::TokenKind {
   if (s[0] == 'd') {
     if (s[1] == 'o') {
diff --git a/src/parser/cxx/private/keywords-priv.h b/src/parser/cxx/private/keywords-priv.h
index e7001b0c..3337b860 100644
--- a/src/parser/cxx/private/keywords-priv.h
+++ b/src/parser/cxx/private/keywords-priv.h
@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 static inline auto classify2(const char* s) -> cxx::TokenKind {
   if (s[0] == 'd') {
     if (s[1] == 'o') {
diff --git a/src/parser/cxx/private/pp_directives-priv.h b/src/parser/cxx/private/pp_directives-priv.h
index f6cbc0b8..865cef53 100644
--- a/src/parser/cxx/private/pp_directives-priv.h
+++ b/src/parser/cxx/private/pp_directives-priv.h
@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 enum class PreprocessorDirectiveKind {
   T_IDENTIFIER,
   T_DEFINE,
diff --git a/src/parser/cxx/symbol_instantiation.h b/src/parser/cxx/symbol_instantiation.h
index c09d9cf2..e5c4467f 100644
--- a/src/parser/cxx/symbol_instantiation.h
+++ b/src/parser/cxx/symbol_instantiation.h
@@ -18,6 +18,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 #include <cxx/names_fwd.h>
 #include <cxx/symbols_fwd.h>
 #include <cxx/types_fwd.h>