Kong Gateway: Preventing the Response Transformer Advanced plugin from running on large response bodies

The documentation for the Response Transformer Advanced plugin notes potential performance issues when large bodies are received. Specifically,

Note on transforming bodies: Be aware of the performance of transformations on the response body. In order to parse and modify a JSON body, the plugin needs to retain it in memory, which might cause pressure on the worker's Lua VM when dealing with large bodies (several MBs). Because of Nginx's internals, the Content-Length header will not be set when transforming a response body.

We want to prevent the plugin from executing when the response body meets a certain threshold.

Kong Gateway supports feature flags that allow for configuring this. To enable this you will need to follow the below:

1. Create a conf file containing these settings (i.e.: /home/gruber/resp.conf)

   response_transformation_enable_limit_body=on
   response_transformation_limit_body_size=16384

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   Where the limit_body_size is the max response body size you would like to operate on

2. Update your kong.conf (or environment variables depending on deployment type)

   feature_conf_path=/home/gruber/resp.conf

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3. If Kong is already running this will require a restart

Once this is in place you can attempt to use the plugin when proxying a request that results in a large payload.

For example, let’s set the limit very low for testing purposes:

response_transformation_limit_body_size=1

When we try to proxy a request exceeding this, the original body will be returned along with this error message in the log:

2026/08/27 07:09:59 [error] 2914#0: *3224 [lua] limit_body.lua:67: header_filter(): [response-transformer-advanced] { "message": "response body size limit exceeded", "allowed" : 1, "current" : 387 }