gpt-5-codex through GitHub Copilot provider returns API call error #2890

My selected provider for opencode is GitHub Copilot. Whenever I try to select gpt-5-codex, I get this error:

This felt strange since gpt-5 non-codex works perfectly.

I figured out that gpt-5 supports the chat/completions and responses APIs, while gpt-5-codex does not.

Case in point:

• https://platform.openai.com/docs/guides/migrate-to-responses#migrating-from-chat-completions

• https://platform.openai.com/docs/models/gpt-5-codex#:~:text=It%27s%20available%20in,only

In opencode's provider implementation, only Azure/OpenAI models work with the responses endpoint. All others, including those from GitHub Copilot, are implemented using chat/completions. This is what's causing the problem with codex.

I will be creating a PR with a fix shortly. My fix will ensure that all gpt-5* models use the responses api rather than chat/completions. I also implemented a fallback, just in case.