Add documentation related to phase

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Sameer Kankute 2026-02-24 17:50:38 +05:30
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---
slug: gpt_5_3_codex
title: "Day 0 Support: GPT-5.3-Codex"
date: 2026-02-24T10:00:00
authors:
- name: Sameer Kankute
title: SWE @ LiteLLM (LLM Translation)
url: https://www.linkedin.com/in/sameer-kankute/
image_url: https://pbs.twimg.com/profile_images/2001352686994907136/ONgNuSk5_400x400.jpg
- name: Krrish Dholakia
title: "CEO, LiteLLM"
url: https://www.linkedin.com/in/krish-d/
image_url: https://pbs.twimg.com/profile_images/1298587542745358340/DZv3Oj-h_400x400.jpg
- name: Ishaan Jaff
title: "CTO, LiteLLM"
url: https://www.linkedin.com/in/reffajnaahsi/
image_url: https://pbs.twimg.com/profile_images/1613813310264340481/lz54oEiB_400x400.jpg
description: "Day 0 support for GPT-5.3-Codex on LiteLLM, including phase parameter handling for Responses API."
tags: [openai, gpt-5.3-codex, codex, day 0 support]
hide_table_of_contents: false
---
import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';
LiteLLM now supports GPT-5.3-Codex on Day 0, including support for the new assistant `phase` metadata on Responses API output items.
## Why `phase` matters for GPT-5.3-Codex
`phase` appears on assistant output items and helps distinguish preamble/commentary turns from final closeout responses.
Reference: [Phase parameter docs](https://developers-site-git-alphas-venusaur-api-openai.vercel.app/alphas/venusaur-api/phase-parameter)
Supported values:
- `null`
- `"commentary"`
- `"final_answer"`
Important:
- Persist assistant output items with `phase` exactly as returned.
- Send those assistant items back on the next turn.
- Do **not** add `phase` to user messages.
## Docker Image
```bash
docker pull ghcr.io/berriai/litellm:v1.81.3-stable.sonnet-4-6
```
## Usage
<Tabs>
<TabItem value="proxy" label="LiteLLM Proxy">
**1. Setup config.yaml**
```yaml
model_list:
- model_name: gpt-5.3-codex
litellm_params:
model: openai/gpt-5.3-codex
```
**2. Start the proxy**
```bash
docker run -d \
-p 4000:4000 \
-e ANTHROPIC_API_KEY=$ANTHROPIC_API_KEY \
-v $(pwd)/config.yaml:/app/config.yaml \
ghcr.io/berriai/litellm:v1.81.3-stable.sonnet-4-6 \
--config /app/config.yaml
```
**3. Test it**
```bash
curl -X POST "http://0.0.0.0:4000/v1/responses" \
-H "Content-Type: application/json" \
-H "Authorization: Bearer $LITELLM_KEY" \
-d '{
"model": "gpt-5.3-codex",
"input": "Write a Python script that checks if a number is prime."
}'
```
</TabItem>
</Tabs>
## Python Example: Persist `phase` with OpenAI Client + LiteLLM Base URL
```python
from openai import OpenAI
client = OpenAI(
base_url="http://0.0.0.0:4000/v1", # LiteLLM Proxy
api_key="your-litellm-api-key",
)
items = [] # Persist this per conversation/thread
def _item_get(item, key, default=None):
if isinstance(item, dict):
return item.get(key, default)
return getattr(item, key, default)
def run_turn(user_text: str):
global items
# User message: no phase field
items.append(
{
"type": "message",
"role": "user",
"content": [{"type": "input_text", "text": user_text}],
}
)
resp = client.responses.create(
model="gpt-5.3-codex",
input=items,
)
# Persist assistant output items verbatim, including phase
for out_item in (resp.output or []):
items.append(out_item)
# Optional: inspect latest phase for UI/telemetry routing
latest_phase = None
for out_item in reversed(resp.output or []):
if _item_get(out_item, "type") == "output_item.done" and _item_get(out_item, "phase") is not None:
latest_phase = _item_get(out_item, "phase")
break
return resp, latest_phase
```
## Notes
- Use `/v1/responses` for GPT Codex models.
- Preserve full assistant output history for best multi-turn behavior.
- If `phase` metadata is dropped during history reconstruction, output quality can degrade on long-running tasks.