[Bug]: Volcengine Ark (Kimi-K3) 400 MissingParameter input.content.text on assistant message with tool_calls

[hooliy-01]

Client or integration

Codex App

Area

Provider adapter

Summary

When routing to Volcengine Ark endpoints (e.g. huoshan-kimi-k3 via doubao/ark), multi-turn agent workflows that involve tool calls fail with a 400 error once an assistant message with tool_calls is present in history.

Expected: the request should succeed, matching the behavior of OpenAI/DeepSeek/xAI providers, which accept empty-string content on tool-call assistant messages.

Reproduction

1. ocx start --port 10100
2. Send a chat-completions request where the history contains an assistant message with tool_calls and empty content:

curl -X POST http://127.0.0.1:10100/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "9router/huoshan-kimi-k3",
    "messages": [
      {"role": "user", "content": "list dir"},
      {"role": "assistant", "content": "", "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "shell", "arguments": "{\"cmd\":\"ls\"}"}}]},
      {"role": "tool", "tool_call_id": "call_1", "content": "file1.txt"}
    ],
    "stream": false
  }'

3. Observe:

Provider error 400: {"error":{"code":"MissingParameter","message":"The request failed because it is missing `input.content.text` parameter.","param":"input.content.text","type":"BadRequest"}}

A single-turn request (no tool history) works fine. The failure requires an assistant message with tool_calls + empty content.

Root cause

In src/adapters/openai-chat.ts, opencodex sets content = "" (empty string) in three places when an assistant message has tool_calls but no text:

• Line ~209: if (!chatMsg.content) chatMsg.content = "";
• Line ~212: chatMsg.content = ""; (reasoning-only path)
• Line ~243: content: "" (orphan tool-call synthesis)

Volcengine Ark endpoints (Kimi-K3) treat empty-string content as missing input.content.text and return 400.

assistant content	Volcengine Ark result
"" (empty string)	400 MissingParameter
null	400 MissingParameter
" " (single space)	200 OK
"any text"	200 OK

Suggested fix

Change the empty-string fallback to a single space in all three locations:

- if (!chatMsg.content) chatMsg.content = "";
+ if (!chatMsg.content) chatMsg.content = " ";

A single space satisfies both xAI's "at least one content element" validator and Volcengine Ark's non-empty content.text requirement, without altering model semantics.

Version

2.7.43-preview.20260728

Operating system

macOS (arm64, Darwin)

[github-actions]

_{Automated translation bookkeeping — detected language: unknown.}

[lidge-jun]

A fix is staged on codex/260731-pr-merge-round (2eebd9268), but it needs your confirmation before it can be called done — I have no Ark credentials, so nothing here has touched a live endpoint.

What the code does now. For Ark hostnames only, an assistant history entry that carries just tool calls (or just reasoning) is serialized with content: [{"type":"text","text":""}] instead of content: "". Every other provider keeps "".

Why not simply drop content, or send a space. Both were considered and rejected:

• "" cannot be changed globally. xAI's validator rejects the opposite way — "Each message must have at least one content element" — so the two provider contracts are in direct conflict. That is why the fix is host-gated rather than a general change.
• A single space was my first attempt. There is no evidence Ark accepts it, and a validator that trims whitespace would reject it exactly as it rejects "". It would have passed every local test and still 400'd against your endpoint.

The reasoning behind the array, and its limit. Your error names input.content.text — a nested path. No bare string exposes a content.text field at all, which points at the structured content form. But Ark's published examples only show array content for multimodal user input; nothing in the reference covers an assistant history entry. So this is inferred from the error message, not confirmed. The code and tests both say so rather than implying certainty.

What would settle it. If you can re-run your exact reproduction against the branch, two things are worth knowing: whether the array form is accepted, and whether omitting content entirely also works (which would be cleaner if Ark allows it). Also useful: which Ark regional hostname you use. The gate currently covers ark.cn-beijing.volces.com and ark.ap-southeast.volces.com.

Staying open until that lands. The branch merges into dev and ships once the optimization work in flight there completes.