diff --git a/envoy/config/core/v3/substitution_format_string.proto b/envoy/config/core/v3/substitution_format_string.proto
index c3a213a1..7259725e 100644
--- a/envoy/config/core/v3/substitution_format_string.proto
+++ b/envoy/config/core/v3/substitution_format_string.proto
@@ -106,7 +106,8 @@ message SubstitutionFormatString {
   //
   //   content_type: "text/html; charset=UTF-8"
   //
-  string content_type = 4;
+  string content_type = 4
+      [(validate.rules).string = {well_known_regex: HTTP_HEADER_VALUE strict: false}];
 
   // Specifies a collection of Formatter plugins that can be called from the access log configuration.
   // See the formatters extensions documentation for details.