diff --git a/BUILD b/BUILD
index bc8c50266..914eaa892 100644
--- a/BUILD
+++ b/BUILD
@@ -243,6 +243,7 @@ proto_library(
         "//envoy/extensions/filters/udp/dns_filter/v3alpha:pkg",
         "//envoy/extensions/filters/udp/udp_proxy/v3:pkg",
         "//envoy/extensions/health_checkers/redis/v3:pkg",
+        "//envoy/extensions/http/header_formatters/preserve_case/v3:pkg",
         "//envoy/extensions/internal_redirect/allow_listed_routes/v3:pkg",
         "//envoy/extensions/internal_redirect/previous_routes/v3:pkg",
         "//envoy/extensions/internal_redirect/safe_cross_scheme/v3:pkg",
diff --git a/envoy/config/core/v3/protocol.proto b/envoy/config/core/v3/protocol.proto
index 8c7693c8a..ac2a4907b 100644
--- a/envoy/config/core/v3/protocol.proto
+++ b/envoy/config/core/v3/protocol.proto
@@ -2,6 +2,7 @@ syntax = "proto3";
 
 package envoy.config.core.v3;
 
+import "envoy/config/core/v3/extension.proto";
 import "envoy/type/v3/percent.proto";
 
 import "google/protobuf/duration.proto";
@@ -118,6 +119,7 @@ message Http1ProtocolOptions {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.api.v2.core.Http1ProtocolOptions";
 
+  // [#next-free-field: 9]
   message HeaderKeyFormat {
     option (udpa.annotations.versioning).previous_message_type =
         "envoy.api.v2.core.Http1ProtocolOptions.HeaderKeyFormat";
@@ -136,6 +138,11 @@ message Http1ProtocolOptions {
       // Note that while this results in most headers following conventional casing, certain headers
       // are not covered. For example, the "TE" header will be formatted as "Te".
       ProperCaseWords proper_case_words = 1;
+
+      // Configuration for stateful formatter extensions that allow using received headers to
+      // affect the output of encoding headers. E.g., preserving case during proxying.
+      // [#extension-category: envoy.http.stateful_header_formatters]
+      TypedExtensionConfig stateful_formatter = 8;
     }
   }
 
diff --git a/envoy/config/core/v4alpha/protocol.proto b/envoy/config/core/v4alpha/protocol.proto
index 1f0af4d12..7cc09a5fb 100644
--- a/envoy/config/core/v4alpha/protocol.proto
+++ b/envoy/config/core/v4alpha/protocol.proto
@@ -2,6 +2,7 @@ syntax = "proto3";
 
 package envoy.config.core.v4alpha;
 
+import "envoy/config/core/v4alpha/extension.proto";
 import "envoy/type/v3/percent.proto";
 
 import "google/protobuf/duration.proto";
@@ -121,6 +122,7 @@ message Http1ProtocolOptions {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.core.v3.Http1ProtocolOptions";
 
+  // [#next-free-field: 9]
   message HeaderKeyFormat {
     option (udpa.annotations.versioning).previous_message_type =
         "envoy.config.core.v3.Http1ProtocolOptions.HeaderKeyFormat";
@@ -139,6 +141,11 @@ message Http1ProtocolOptions {
       // Note that while this results in most headers following conventional casing, certain headers
       // are not covered. For example, the "TE" header will be formatted as "Te".
       ProperCaseWords proper_case_words = 1;
+
+      // Configuration for stateful formatter extensions that allow using received headers to
+      // affect the output of encoding headers. E.g., preserving case during proxying.
+      // [#extension-category: envoy.http.stateful_header_formatters]
+      TypedExtensionConfig stateful_formatter = 8;
     }
   }
 
diff --git a/envoy/extensions/http/header_formatters/preserve_case/v3/BUILD b/envoy/extensions/http/header_formatters/preserve_case/v3/BUILD
new file mode 100644
index 000000000..ee92fb652
--- /dev/null
+++ b/envoy/extensions/http/header_formatters/preserve_case/v3/BUILD
@@ -0,0 +1,9 @@
+# DO NOT EDIT. This file is generated by tools/proto_format/proto_sync.py.
+
+load("@envoy_api//bazel:api_build_system.bzl", "api_proto_package")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_package(
+    deps = ["@com_github_cncf_udpa//udpa/annotations:pkg"],
+)
diff --git a/envoy/extensions/http/header_formatters/preserve_case/v3/preserve_case.proto b/envoy/extensions/http/header_formatters/preserve_case/v3/preserve_case.proto
new file mode 100644
index 000000000..64bdd497e
--- /dev/null
+++ b/envoy/extensions/http/header_formatters/preserve_case/v3/preserve_case.proto
@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+package envoy.extensions.http.header_formatters.preserve_case.v3;
+
+import "udpa/annotations/status.proto";
+
+option java_package = "io.envoyproxy.envoy.extensions.http.header_formatters.preserve_case.v3";
+option java_outer_classname = "PreserveCaseProto";
+option java_multiple_files = true;
+option (udpa.annotations.file_status).package_version_status = ACTIVE;
+
+// [#protodoc-title: Preserve case header formatter]
+// [#extension: envoy.http.stateful_header_formatters.preserve_case]
+
+// Configuration for the preserve case header formatter.
+// See the :ref:`header casing <config_http_conn_man_header_casing>` configuration guide for more
+// information.
+message PreserveCaseFormatterConfig {
+}
diff --git a/versioning/BUILD b/versioning/BUILD
index 83e85ed89..daadfd39e 100644
--- a/versioning/BUILD
+++ b/versioning/BUILD
@@ -126,6 +126,7 @@ proto_library(
         "//envoy/extensions/filters/udp/dns_filter/v3alpha:pkg",
         "//envoy/extensions/filters/udp/udp_proxy/v3:pkg",
         "//envoy/extensions/health_checkers/redis/v3:pkg",
+        "//envoy/extensions/http/header_formatters/preserve_case/v3:pkg",
         "//envoy/extensions/internal_redirect/allow_listed_routes/v3:pkg",
         "//envoy/extensions/internal_redirect/previous_routes/v3:pkg",
         "//envoy/extensions/internal_redirect/safe_cross_scheme/v3:pkg",