Codex compatibility

Codex Hosted accepts OpenAI-compatible Chat Completions and Responses requests, then runs them through your ChatGPT/Codex subscription.

Works the same

Setting	Support
model	Supported. Stable tiers like flagship and mini resolve at request time.
messages / input	Supported.
instructions	Supported.
Streaming	Supported. ProxyLLM streams from Codex and returns OpenAI-compatible events.
Tools	Supported: function, web search, custom, and image generation tools.
tool_choice	Supported.
JSON schema / object	Supported.
Image and file input	Supported.
Prompt caching	Supported by Codex.

Codex controls

Control	Request field	Default
Thinking effort	reasoning.effort or reasoning_effort: none, low, medium, high, xhigh, max	low
Answer length	text.verbosity or verbosity: low, medium, high	None unless you set it
Priority speed	service_tier: priority	No priority tier unless you set it

You can set these on a Codex lane in the dashboard. A request can also override the lane for that call.

Not available on Codex

Field	What happens
temperature	Ignored on Codex.
top_p	Ignored on Codex.
seed	Ignored on Codex.
logprobs	Ignored on Codex.
stop	Ignored on Codex.
n	Ignored on Codex.
frequency_penalty	Ignored on Codex.
presence_penalty	Ignored on Codex.
logit_bias	Ignored on Codex.
max_output_tokens	Ignored on Codex.
max_tool_calls	Ignored on Codex.
truncation	Ignored on Codex.
metadata	Ignored on Codex.
user	Ignored on Codex.
previous_response_id	Ignored on Codex.
response_format	Ignored on Codex.

OpenAI’s subscription endpoint accepts a fixed set of parameters; the fields above are not in it. ProxyLLM returns the answer anyway and names ignored settings on the x-proxyllm-dropped-params header.

Every Codex response also includes x-proxyllm-applied-params, for example:

reasoning.effort=low,text.verbosity=medium

System messages

Chat Completions system messages are folded into instructions before the request is sent to Codex. User, assistant, and tool messages stay in the conversation.

Streaming and storage

Codex Hosted always streams upstream from Codex. ProxyLLM does not store Codex conversation state server-side.

store and previous_response_id do not apply on Codex. To continue a conversation, resend the message history.

See Connect Codex for setup and Chat completions for the request format.