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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.