Inject response bodies

Learn how to return a customized response body by using CEL expressions. The examples use request.path, request.method, request.headers[], request.body, json(), and string() to construct a dynamic response body string.

Before you begin

1. Set up an agentgateway proxy.
2. Install the httpbin sample app.

Inject request header fields into the response body

In this example, you set the response body to a JSON string built from request context variables. The gateway intercepts the upstream response and replaces the body with the CEL expression result before returning it to the client. The upstream never sees the change — only the client receives the modified body.

1. Create an AgentgatewayPolicy resource that sets the response body to a JSON object containing the request path, method, and x-request-id header value.

   kubectl apply -f- <<EOF
   apiVersion: agentgateway.dev/v1alpha1
   kind: AgentgatewayPolicy
   metadata:
     name: transformation
     namespace: httpbin
   spec:
     targetRefs:
     - group: gateway.networking.k8s.io
       kind: HTTPRoute
       name: httpbin
     traffic:
       transformation:
         response:
           body: '"{\"path\": \"" + request.path + "\", \"method\": \"" + request.method + "\", \"request-id\": \"" + request.headers["x-request-id"] + "\"}"'
   EOF

2. Send a request to the httpbin app and include an x-request-id request header. Verify that you get back a 200 HTTP response code and that the response body contains the transformed output with the request header value.

   Cloud Provider LoadBalancerPort-forward for local testing

   curl -vi http://$INGRESS_GW_ADDRESS:80/get \
    -H "host: www.example.com:80" \
    -H "x-request-id: user123"

   curl -vi localhost:8080/get \
   -H "host: www.example.com" \
   -H "x-request-id: user123"

   Example output:

   < HTTP/1.1 200 OK
   HTTP/1.1 200 OK
   < content-type: application/json
   content-type: application/json
   < content-length: 49
   content-length: 49
   
   {"path": "/get", "method": "GET", "request-id": "user123"}
   

Inject request body fields into a response body

In this example, you parse a JSON request body by using the json() function to extract a field and include it in the response body. Use request.body to access the raw incoming request body as a string.

1. Create an AgentgatewayPolicy resource that reads the name field from the JSON request body and echoes it back in the response.

   kubectl apply -f- <<EOF
   apiVersion: agentgateway.dev/v1alpha1
   kind: AgentgatewayPolicy
   metadata:
     name: transformation
     namespace: httpbin
   spec:
     targetRefs:
     - group: gateway.networking.k8s.io
       kind: HTTPRoute
       name: httpbin
     traffic:
       transformation:
         response:
           body: '"{\"hello\": \"" + string(json(request.body).name) + "\"}"'
   EOF

2. Send a POST request to the httpbin app with a JSON body. Verify that you get back a 200 HTTP response code and that the response body contains the name value from your request.

   Cloud Provider LoadBalancerPort-forward for local testing

   curl -vi http://$INGRESS_GW_ADDRESS:80/post \
    -H "host: www.example.com:80" \
    -H "content-type: application/json" \
    -d '{"name": "user123"}'

   curl -vi localhost:8080/post \
   -H "host: www.example.com" \
   -H "content-type: application/json" \
   -d '{"name": "user123"}'

   Example output:

   < HTTP/1.1 200 OK
   HTTP/1.1 200 OK
   < content-type: application/json
   content-type: application/json
   < content-length: 17
   content-length: 17
   
   {"hello": "user123"}
   

Cleanup

kubectl delete AgentgatewayPolicy transformation -n httpbin