diff --git a/transport/grpc/headers.go b/transport/grpc/headers.go
index 4b1436f81..b4c646eb4 100644
--- a/transport/grpc/headers.go
+++ b/transport/grpc/headers.go
@@ -203,7 +203,7 @@ func metadataToApplicationErrorMeta(responseMD metadata.MD) *transport.Applicati
 // addApplicationHeaders adds the headers to md.
 func addApplicationHeaders(md metadata.MD, headers transport.Headers) error {
 	for header, value := range headers.Items() {
-		header = transport.CanonicalizeHeaderKey(header)
+		// Items() keys are already canonical (lowercased on insertion via With).
 		if isReserved(header) {
 			return yarpcerrors.InvalidArgumentErrorf("cannot use reserved header in application headers: %s", header)
 		}
diff --git a/transport/grpc/headers_test.go b/transport/grpc/headers_test.go
index 9fb3c6e3a..eb6e7e73b 100644
--- a/transport/grpc/headers_test.go
+++ b/transport/grpc/headers_test.go
@@ -404,3 +404,19 @@ func BenchmarkIsReserved(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkAddApplicationHeaders(b *testing.B) {
+	headers := transport.HeadersFromMap(map[string]string{
+		"x-uber-source": "service-a",
+		"x-request-id":  "abc-123",
+		"x-trace-id":    "trace-456",
+		"x-custom-1":    "val1",
+		"x-custom-2":    "val2",
+	})
+
+	b.ResetTimer()
+	for range b.N {
+		md := metadata.New(nil)
+		_ = addApplicationHeaders(md, headers)
+	}
+}