ai-proxy upstream returns error json

[Arjen10]

Is there an existing issue for this?

• I have searched the existing issues

Kong version ($ kong version)

kong 3.10

Current Behavior

when the ai-proxy plugin is configured with route_type = preserve, if the upstream returns an invalid json res, the request is closed unexpectedly and the client receives an empty response

Expected Behavior

kong gateway is proxy gateway, client should not receive a empty response, so return the original upstream response body to the client

Steps To Reproduce

1. configure a fake upstream API

fake upstream returns an incomplete json response

location /fake {
    content_by_lua_block {
        ngx.status = 200
        ngx.header["Content-Type"] = "application/json"
        ngx.print("{")
    }
}

fake upstream URL:

http://172.0.18.100:6443/fake

2. Configure the ai-proxy plugin

{
  "name": "ai-proxy",
  "config": {
    "route_type": "preserve",
    "llm_format": "openai",
    "model": {
      "provider": "openai",
      "options": {
        "upstream_url": "http://172.0.18.100:6443/fake"
      }
    },
    "auth": {
      "header_name": "Authorization",
      "header_value": "Bearer dummy"
    },
    "logging": {
      "log_statistics": true
    }
  }
}

3. Send a request through Kong gateway

curl --location 'http://172.18.0.21:8100/fake' \
  --header 'Content-Type: application/json' \
  --data '{
    "model": "gpt-4",
    "messages": [
      {
        "role": "user",
        "content": "hello"
      }
    ]
  }'

4. actual behavior

client receives an empty response:

curl: (52) Empty reply from server

kong logs the following error:

failed to run body_filter_by_lua*: .../kong/llm/plugin/shared-filters/serialize-analytics.lua:53:
Expected object key string but found T_END at character 2

full error log:

2026/06/09 21:18:15 [error] 2631#0: *5185 failed to run body_filter_by_lua*: ...1/kong/llm/plugin/shared-filters/serialize-analytics.lua:53: Expected object key string but found T_END at character 2
stack traceback:
        [C]: in function 'decode'
        ...1/kong/llm/plugin/shared-filters/serialize-analytics.lua:53: in function 'run'
        /usr/local/share/lua/5.1/kong/llm/plugin/base.lua:63: in function 'run_stage'
        /usr/local/share/lua/5.1/kong/llm/plugin/base.lua:135: in function </usr/local/share/lua/5.1/kong/llm/plugin/base.lua:122>
        /usr/local/share/lua/5.1/kong/init.lua:509: in function 'execute_collected_plugins_iterator'
        /usr/local/share/lua/5.1/kong/init.lua:1997: in function 'body_filter'
        body_filter_by_lua(nginx-kong.conf:142):2: in main chunk while sending to client, client: 172.18.0.21, server: kong, request: "POST /fake HTTP/1.1", host: "172.18.0.21:8100", request_id: "d82f3e71d7ca4846e0636e33d09b3f1a"