# Kong post function : change code based on body content

**URL:** <https://discuss.konghq.com/t/kong-post-function-change-code-based-on-body-content/13426>\
**Category:** Questions\
**Tags:** plugins, kong-gateway\
**Created:** [March 14, 2025, 4:55pm UTC](https://discuss.konghq.com/t/kong-post-function-change-code-based-on-body-content/13426 "2025-03-14T16:55:29Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![yroffin](https://avatars.discourse-cdn.com/v4/letter/y/9dc877/32.png) [@yroffin](https://discuss.konghq.com/u/yroffin)\
**Post date:** [March 14, 2025, 4:55pm UTC](https://discuss.konghq.com/t/kong-post-function-change-code-based-on-body-content/13426/1 "2025-03-14T16:55:29Z")

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hi,

i’m trying to change with a post function http code, and i only succed to change it in header filter

did someone try chnage ttp code on body based content ?

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<div class="post-metadata">

**Author:** ![rick](https://yyz2.discourse-cdn.com/flex036/user_avatar/discuss.konghq.com/rick/32/4446_2.png) [@rick](https://discuss.konghq.com/u/rick)\
**Post date:** [March 14, 2025, 5:35pm UTC](https://discuss.konghq.com/t/kong-post-function-change-code-based-on-body-content/13426/2 "2025-03-14T17:35:55Z")

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@yroffin If you’re using Kong Gateway Enterprise, there is a plugin to enable this, the Exit Transformer.

> **[Exit Transformer - Plugin | Kong Docs](https://docs.konghq.com/hub/kong-inc/exit-transformer/)**
>
> Documentation for Kong, the Cloud Connectivity Company for APIs and Microservices.

If you’re not an Enterprise user, I believe you’ll have to write a custom plugin to accomplish this (if I understand correctly).

> **[Introduction - Kong Gateway | Kong Docs](https://docs.konghq.com/gateway/latest/plugin-development/)**
>
> Documentation for Kong, the Cloud Connectivity Company for APIs and Microservices.

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<div class="post-metadata">

**Author:** ![JackGPT](https://avatars.discourse-cdn.com/v4/letter/j/c68b51/32.png) [@JackGPT](https://discuss.konghq.com/u/JackGPT)\
**Post date:** [March 14, 2025, 6:03pm UTC](https://discuss.konghq.com/t/kong-post-function-change-code-based-on-body-content/13426/3 "2025-03-14T18:03:29Z")

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Hey there, yes it can be done!

It’s a little complicated and requires buffering the whole response into RAM, but try this:

```yaml
_format_version: "3.0"
services:
  - name: "echo"
    url: "https://httpbin.org:443/anything"

    routes:
      - name: "echo"
        paths:
          - "~/echo$"
        request_buffering: true
        response_buffering: true

        plugins:
          - name: post-function
            config:
              access:
                - |
                  kong.service.request.enable_buffering()
              header_filter:
                - |
                  local cjson = require("cjson.safe")
                  local body_t, err = cjson.decode(kong.service.response.get_raw_body())

                  if err then
                    kong.response.set_status(500)
                    kong.response.clear_header("Content-Length") -- convert to chunked
                    kong.ctx.plugin.replacement_body = '{"failure": true, "message": "response is not json"}'

                    return
                  end

                  -- now analyse the response!
                  if not body_t
                      or not body_t.json
                      or not body_t.json.is_dog
                  then
                    kong.response.set_status(400)
                    kong.response.clear_header("Content-Length") -- convert to chunked
                    kong.ctx.plugin.replacement_body = '{"failure": true, "message": "response object is not a dog"}'

                    return
                  end

              body_filter:
                - |
                  if kong.ctx.plugin.replacement_body then
                    kong.response.set_raw_body(kong.ctx.plugin.replacement_body)
                  end

```

Explanation:

1. The `access` function forces Kong into response buffering mode, where you can access headers and body separately
2. The `header_filter` decodes the JSON body, checks for errors, then analyses the body (`if not body_t or not body_t.is_dog`)
3. If there is a problem, the `header_filter` must set the response code and header, but it **cannot set the body!!**
4. This is why we use ngx ctx to pass the replacement body JSON (or string or whatever) into the `body_filter` function at the end, who replaces the body if there has been some issue during `header_filter`.

⚠ Note: you also have to allow `cjson.safe` package in the post-function sandbox, by setting this Kong environment variable:

`KONG_UNTRUSTED_LUA_SANDBOX_REQUIRES=cjson.safe`