[Feature Request] Support traditional Chat Completions API for OpenAI provider

[boyan250422-code]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Description

The OpenAI provider in opencode uses the new Responses API format (SSE event stream) instead of the traditional Chat Completions API. Most OpenAI-compatible proxies only implement the Chat Completions API, causing compatibility issues.

Current Behavior

When configuring the OpenAI provider to point to an OpenAI-compatible proxy (e.g., Antigravity, LM Studio, Ollama, etc.), the proxy returns standard Chat Completions format, but opencode expects Responses API format, resulting in parse failures.

Error Message

AI_TypeValidationError: Type validation failed
Invalid input: expected "response.output_text.delta"

Reproduction Steps

1. Configure OpenAI provider to point to an OpenAI-compatible proxy:

{
  "provider": {
    "openai": {
      "options": {
        "apiKey": "your-api-key",
        "baseURL": "http://localhost:8045/v1"
      }
    }
  }
}

2. Run any opencode command
3. Parse error occurs

Environment

• OS: Windows 11
• opencode version: v1.0.150+
• oh-my-opencode version: v2.14.0

Root Cause

opencode's OpenAI provider uses OpenAI's new Responses API format instead of the widely-adopted Chat Completions API. Most OpenAI-compatible proxies only implement Chat Completions API.

Workaround

Use Anthropic provider instead: If your proxy supports Anthropic API format (/v1/messages):

{
  "provider": {
    "anthropic": {
      "options": {
        "apiKey": "your-api-key",
        "baseURL": "http://localhost:8045/v1"
      }
    }
  }
}

Expected Behavior

Ideally, opencode should support both:

1. Traditional Chat Completions API (for compatibility with most proxies)
2. New Responses API (for official OpenAI)

Or at least provide a configuration option to choose between them.

Proposed Solutions

1. Add a useResponsesApi or apiVersion option in OpenAI provider config
2. Fallback to Chat Completions if Responses API is not supported
3. Support both formats based on provider configuration

Additional Notes

This is a design limitation, not a bug per se. However, it significantly reduces compatibility with OpenAI-compatible proxies, which are widely used by developers.