maxHeadersCount in BackendConfigPolicy not taken into account

[jgkirschbaum]

kgateway version

v2.3.5

Kubernetes Version

v1.34.8

Describe the bug

The maxHeadersCount key in a BackendConfigPolicy is not taken into account, when doing a http request.

apiVersion: gateway.kgateway.dev/v1alpha1
kind: BackendConfigPolicy
metadata:
  name: mypolicy
  namespace: myapp
spec:
  targetRefs:
    - name: myservice
      group: ""
      kind: Service
  commonHttpProtocolOptions:
    maxHeadersCount: 200
  connectTimeout: 5s

The following curl request receives a HTTP/1.1 431 Request Header Fields Too Large.

#!/bin/bash

set -eEo pipefail

COUNT=150
HEADERS=()
for ((i=0; i<COUNT; i++)); do
    HEADERS+=(-H "x-h${i}: ${i}")
done

echo "Additional headers: $COUNT"
echo "Bytes in headers: $(printf "%s" "${HEADERS[@]}" | wc -c)"
curl -isSI --http1.1 https://gateway.example.com/path/ "${HEADERS[@]}"

Expected Behavior

The request is answered with a HTTP/1.1 200 OK.

Steps to reproduce the bug

See description.

Proxy pod logs

The following logentry is generated for every request.

{
    "authority": null,
    "backendCluster": null,
    "backendHost": null,
    "bytes_received": 0,
    "bytes_sent": 31,
    "method": null,
    "path": null,
    "protocol": "HTTP/1.1",
    "req_x_forwarded_for": null,
    "request_id": null,
    "resp_backend_service_time": null,
    "response_code": 431,
    "response_flags": "DPE",
    "start_time": "2026-07-09T08:51:01.005Z",
    "total_duration": 0,
    "user_agent": null
}