Reverse engineering Codex CLI rollout traces

I spent a weekend building a DeepSeek proxy for Codex CLI so I could generate real tracing data. The proxying was straightforward. What I found when I looked at the actual trace files was not.

The documented format and the real format don't match.

This is the story of that mismatch: what I expected, what I found, and why it matters if you're building runtime tooling for coding agents.

The setup: why a proxy?

Codex CLI uses OpenAI's Responses API (via their SDK). DeepSeek only supports Chat Completions. To use DeepSeek as the backend, I needed a translation proxy — intercept Responses API calls and translate them to Chat Completions.

The proxy ( tools/codex_deepseek_proxy.py ) was straightforward:

Accept POST requests at /responses (or /v1/responses )

Translate the input field (Responses API format) to Chat Completions messages

Translate tool definitions from {"name": "bash", "parameters": {...}} to {"type": "function", "function": {"name": "bash",…