[Bug] Custom API fails with standard OpenAI JSON response: choices.0.message.content throws json.exception.type_error.302 (type must be string, but is null)

[soulmatelh]

Describe the Bug

When using the Custom translation provider with a local OpenAI-compatible endpoint (LM Studio), the plugin consistently fails to parse the standard chat completion response. Even though the backend successfully returns a valid JSON containing the translated text, the OBS log instantly throws a type mismatch exception.

Environment

• **OBS Studio Version:32.1.2
• **Plugin Version:obs-localvocal (latest build)
• LLM Backend: LM Studio (Running sakura-galtransl-7b-v3.7)
• API Endpoint: http://127.0.0.1:1234/v1/chat/completions
• Response JSON Path Specified: choices.0.message.content

Exact Evidence & Log Synchronization

Here is the exact verified response generated by the local LLM backend (LM Studio Developer Logs):

{
  "id": "chatcmpl-v4qesz0fj3dkzkfjj2pwk",
  "object": "chat.completion",
  "created": 1779590135,
  "model": "sakura-galtransl-7b-v3.7",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "现在就先保密吧",
        "reasoning_content": "",
        "tool_calls": []
      },
      "logprobs": null,
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 123,
    "completion_tokens": 6,
    "total_tokens": 129,
    "completion_tokens_details": {
      "reasoning_tokens": 0
    }
  },
  "stats": {},
  "system_fingerprint": "sakura-galtransl-7b-v3.7"
}

Immediately after this payload is sent back to the plugin, the OBS Log outputs the following parsing error:

10:36:24.016: [obs-localvocal] Translation error: JSON parsing error: JSON parsing error: [json.exception.type_error.302] type must be string, but is null

Technical Root Cause

The [json.exception.type_error.302] originates from the modern C++ JSON library (nlohmann/json). This specific error indicates that the plugin's C++ parsing logic expected to encounter a primitive string value at the targeted layer, but evaluated it as a null value instead.

Since the raw payload undeniably contains a valid string "现在就先保密吧" under choices[0].message.content, the type error proves that the plugin's internal path-walking evaluator fails to properly navigate multi-layered objects or array indices (choices.0), causing the pointer to drop out into null and crash the translation rendering pipeline.

Expected Behavior

The plugin should seamlessly parse standard nested OpenAI JSON structures when users supply a valid JSON Path like choices.0.message.content.

[Tabby]

Can you please share a screenshot of the custom API configuration settings you're using?

[soulmatelh]

[moorwp-jpg]

I’ve reproduced this issue and have a fix implemented and tested in my fork. The fix adds support for dotted/nested JSON response paths such as choices.0.message.content, including array indices and error handling for invalid paths. I’m happy to take care of this issue unless someone else is already working on it.