codex-shim

codex-shim is a local proxy that lets Codex Desktop talk to models it was never built to support. OpenAI's Codex app normally locks you into a short list of models. This shim sits between the app and whatever provider you want, translating Codex's Responses API format into calls to Anthropic, DeepSeek, or any generic chat-completion endpoint, with an optional ChatGPT passthrough that reuses your existing subscription. MIT licensed and completely free.

Running it means Python 3.11 or newer, a pip install, and editing a JSON config to point Codex at localhost. On macOS you may also patch the app so your custom models show up in the picker. None of this is hard for someone comfortable in a terminal, but it is fiddly, and it can break when Codex ships an update that changes its internal API.

This is a workaround, not a product. Solo developers who want Claude or DeepSeek inside Codex without paying for a second tool get real value here. Small teams might standardize on it to route through a cheaper model. Larger teams should think hard before depending on an unofficial shim that lives or dies by one maintainer.

The catch is fragility. You are patching around a closed app's limits, so every Codex release is a chance for this to stop working. Use it knowing that.