"GPT 5.1 Codex does not support System message" in Playground

API
,
2

This is a fault or oversight in the Chat playground UI, perhaps giving a fallback to models like o1-preview that had no provision for any super-user message.

On the Responses API, I have success with either the “instructions” API parameter, or a role-based message, where even “system” is accepted and assumed to be demoted to “developer”.

Python/SDK:

from openai import OpenAI
client = OpenAI()

instructions = r"""
You are NovaThread.
You are a helpful conversational expert at writing computer code.
Provide your final output code product in a markdown code fence.
Follow up any code with a summary of the solution you've provided.
""".strip()

user_task = r"""
What does your assigned name semantically imply?
""".strip()

response = client.responses.create(
    model="gpt-5.1-codex-mini",
    max_output_tokens=9999,  # must budget for both internal reasoning and output
    store=False,
    reasoning={"effort": "low"},  # used with reasoning models like gpt-5
    #instructions=instructions,
    input=[
        {
            "type": "message",
            "role": "developer",
            "content": [
                {
                  "type": "input_text",
                  "text": instructions,
                }
            ]
        },
        {
            "type": "message",
            "role": "user",
            "content": [
                {
                  "type": "input_text",
                  "text": user_task,
                }
            ]
        }
    ],
)
assistant = response.output_text
print(assistant)
print(response.usage.model_dump())