diff --git a/src/backends/contextfree.h b/src/backends/contextfree.h
index b387e55..ab04ab5 100644
--- a/src/backends/contextfree.h
+++ b/src/backends/contextfree.h
@@ -1,3 +1,8 @@
+/*
+ * NOTE: This is an internal header and installed for use by extensions. The
+ * API is not guaranteed stable.
+*/
+
 // This is an internal header; it provides macros to make desugaring cleaner.
 #include <assert.h>
 #include "../internal.h"
diff --git a/src/backends/regex.h b/src/backends/regex.h
index a84904d..4ea85a8 100644
--- a/src/backends/regex.h
+++ b/src/backends/regex.h
@@ -1,3 +1,8 @@
+/*
+ * NOTE: This is an internal header and installed for use by extensions. The
+ * API is not guaranteed stable.
+*/
+
 // Internal defs
 #ifndef HAMMER_BACKEND_REGEX__H
 #define HAMMER_BACKEND_REGEX__H
diff --git a/src/internal.h b/src/internal.h
index c402da5..89cb380 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -15,6 +15,11 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  */
 
+/*
+ * NOTE: This is an internal header and installed for use by extensions. The
+ * API is not guaranteed stable.
+*/
+
 #ifndef HAMMER_INTERNAL__H
 #define HAMMER_INTERNAL__H
 #include <stdint.h>
diff --git a/src/parsers/parser_internal.h b/src/parsers/parser_internal.h
index aeb202b..ec97dd1 100644
--- a/src/parsers/parser_internal.h
+++ b/src/parsers/parser_internal.h
@@ -1,3 +1,8 @@
+/*
+ * NOTE: This is an internal header and installed for use by extensions. The
+ * API is not guaranteed stable.
+*/
+
 #ifndef HAMMER_PARSE_INTERNAL__H
 #define HAMMER_PARSE_INTERNAL__H
 #include "../hammer.h"