Normalize blank tool-call content

[Yiivgeny]

Summary

Fix native tool-call request normalization for OpenAI-compatible clients that send assistant tool-call messages with blank content.

Current main contains a compatibility bug: when a client sends an assistant message with tool_calls and content as an empty or whitespace-only string, Gonka upstream can reject the request with:

{"error":{"message":"messages[1].content: must not be empty"}}

This request shape is produced by the latest version of opencode during tool/skill calls.

Problem

OpenAI-compatible tool-call turns may contain an assistant message like:

{
  "role": "assistant",
  "content": "",
  "tool_calls": [...]
}

or:

{
  "role": "assistant",
  "content": " ",
  "tool_calls": [...]
}

Gonka upstream accepts content: null for this case, but rejects blank string content, at least for Qwen.

Reproduction

Against a proxy running current main:

curl -sS -X POST http://127.0.0.1:8081/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{
    "model": "Qwen/Qwen3-235B-A22B-Instruct-2507-FP8",
    "stream": false,
    "max_tokens": 1,
    "messages": [
      {"role": "user", "content": "call test tool"},
      {
        "role": "assistant",
        "content": "",
        "tool_calls": [
          {
            "id": "call_1",
            "type": "function",
            "function": {
              "name": "test_tool",
              "arguments": "{}"
            }
          }
        ]
      },
      {
        "role": "tool",
        "tool_call_id": "call_1",
        "content": "ok"
      }
    ],
    "tools": [
      {
        "type": "function",
        "function": {
          "name": "test_tool",
          "description": "test",
          "parameters": {
            "type": "object",
            "properties": {}
          }
        }
      }
    ]
  }'

Expected: request is normalized and forwarded successfully.

Actual:

{"error":{"message":"messages[1].content: must not be empty"}}

The same issue is currently visible on the public proxy:

curl -sS -X POST https://proxy.gonka.gg/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Bearer <TOKEN>' \
  -d '{ ...same payload as above... }'

https://proxy.gonka.gg/v1 returns the same messages[1].content: must not be empty error for Qwen when content is "" or whitespace-only. The same request succeeds when content is changed to null.

Fix

In native tool-call mode, normalize assistant messages that have tool_calls and blank string content to:

"content": null

This preserves the OpenAI-compatible tool-call shape while avoiding Gonka upstream’s blank-content validation error.

Validation

• Added unit coverage for blank assistant tool-call content normalization.
• Verified locally with:
  • go test ./internal/api
  • go test ./...
• Verified smoke tests through local LiteLLM -> opengnk path:
  • Qwen with content: "" now returns 200
  • Kimi with content: " " now returns 200