opencode-openai-codex-headers

An opencode plugin for ChatGPT Plus/Pro users on the Codex backend. It smooths over the rough edges in opencode's OpenAI/Codex integration for the GPT-5.6 models (Sol, Terra, Luna) until native support lands upstream.

What it fixes

• Luna 404s. GPT-5.6 Luna models return 404 Model not found. The Codex backend gates them on the client identity, and opencode identifies as opencode rather than the Codex CLI. The plugin restores the genuine Codex signature so Luna is served, and keeps Terra in the Codex priority tier so it stops load-shedding with server_is_overloaded under contention.
• Missing session names. opencode's title agent runs on a Luna model, so the same 404 leaves every session stuck on its default New session - <timestamp> name. Recovering Luna fixes the titles.
• Reasoning-summary markers. GPT-5.6 ends each reasoning summary with an empty <!-- --> comment that opencode's TUI renders literally under the "Thought" headline. The plugin strips it on the wire so only the headline remains.

Install

Via the opencode CLI (-g writes it to your global config; drop it for the current project):

opencode plugin opencode-openai-codex-headers -g

Manual: add it to the plugin array in your opencode.json:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["opencode-openai-codex-headers"]
}

How it works

Identity. A chat.headers hook overwrites originator and User-Agent on the openai provider only, so requests present the genuine Codex CLI signature (the backend requires both). It loads after opencode's internal hooks and runs against the same shared output, so it wins; every other provider is untouched.

Reasoning cleanup. opencode exposes no hook to transform reasoning text, so the plugin rewrites the summary events on the wire, on both the HTTP/SSE path and opencode's experimental WebSocket transport. It touches only reasoning delta events on /responses endpoints (host-agnostic, so it also works through a proxy or custom baseURL); everything else passes through unchanged.

License

MIT