ai-proxy Bedrock: `bedrock.additionalModelRequestFields` is sent as a JSON array (400) and `bedrock.toolConfig.toolChoice` is ignored

Thanks for adding the bedrock passthrough fields to the Converse driver in #13354. They are how llm/v1/chat clients reach Converse options such as reasoning effort, top_k or toolChoice, since the driver maps only max_tokens, stop, temperature and top_p (to_bedrock_generation_config). Both currently reach Bedrock wrapped in an array. Found while setting reasoning effort for the OpenAI GPT-6 models.

Is there an existing issue for this?

• I have searched the existing issues

Kong version ($ kong version)

master ba2c74a (3.10.0-dev); same code in 3.9.3. No Kong node: kong/llm/drivers/bedrock.lua ran under LuaJIT 2.1 + lua-cjson with its Kong/nginx imports stubbed, and the body from to_format went to live Bedrock Converse (us-east-1).

Current Behavior

kong/llm/drivers/bedrock.lua L56-68 wrap the client's object in a new table:

local function to_additional_request_fields(request_table)
  return {
    request_table.bedrock.additionalModelRequestFields
  }
end
-- ...
local function to_tool_config(request_table)
  return {
    request_table.bedrock.toolConfig
  }
end

The 400 does not depend on the model: GPT-6 Sol/Luna/Astra (us. and global.), GPT-5.6 Sol, gpt-oss-120b (reasoning_effort) and Claude Haiku 4.5 (top_k) all return it.

Expected Behavior

Both fields go out as objects, the way to_bedrock_guardrail_config (L50-52) already passes guardrailConfig. Live results with that change:

The high replies above start with a reasoning block, and Kong still drops their text (#15006, L429, fixed separately in #15014).

Steps To Reproduce

1. bedrock.to_format({ messages = {...}, bedrock = { additionalModelRequestFields = { reasoning = { effort = "high" } } } }, model_info, "llm/v1/chat")
2. POST the encoded body to /model/us.openai.gpt-6-sol/converse.
3. 400 as above.

Anything else?

 local function to_additional_request_fields(request_table)
-  return {
-    request_table.bedrock.additionalModelRequestFields
-  }
+  return request_table.bedrock.additionalModelRequestFields
 end
@@
 local function to_tool_config(request_table)
-  return {
-    request_table.bedrock.toolConfig
-  }
+  return request_table.bedrock.toolConfig
 end

A new it in the bedrock tools block of spec/03-plugins/38-ai-proxy/01-unit_spec.lua calls _to_bedrock_chat_openai with both fields plus tools and asserts objects come out. Run through a LuaJIT shim that loads that block as is (busted is not set up here):

master:  FAIL bedrock tools passes bedrock additionalModelRequestFields and toolConfig through as objects
           spec/03-plugins/38-ai-proxy/01-unit_spec.lua:1385: Expected objects to be the same.
           Passed in:
           [{"reasoning":{"effort":"high"}}]
           Expected:
           {"reasoning":{"effort":"high"}}
         5 successes / 1 failures
patched: 6 successes / 0 failures
luacheck: 0 warnings / 0 errors

The change, a changelog entry and that spec case are in #15016.