Virtual service request header addition size limit

I’m trying to raise the following virtual service

apiVersion: networking.istio.io/v1beta1
kind: VirtualService
metadata:
  name: example-virtual-service
  namespace: example
spec:
  gateways:
  - istio-system/istio-gateway
  hosts:
  - '*'
  http:
  - match:
    - uri:
        prefix: /app/
    rewrite:
      uri: /
    route:
    - destination:
        host: example-host
        port:
          number: 8000
      headers:
        request:
          add:
            X-SSL-Client-Certificate: '-----BEGIN%20CERTIFICATE-----%0AMIIC1..........%0A-----END%20CERTIFICATE-----%0A'

Without the header I can easily curl to my application but with the certificate as an header it fails.
If I change the value from the encoded certificate to example for example I get the value successfully in my logs Request has env of HTTP_X: HTTP_X_SSL_CLIENT_CERTIFICATE with value: example. I also get this warning that I bet it is related but it is an warning to an error: warning envoy config gRPC config for type.googleapis.com/envoy.config.route.v3.RouteConfiguration rejected: field '20CERTIFICATE-----' not supported as custom header.

I need to pass the certificate as an header. Anyway to do so?