litellm/docs/my-website/docs/completion/prefix.md

import Tabs from '@theme/Tabs'; import TabItem from '@theme/TabItem';

Pre-fix Assistant Messages

Supported by:

• Deepseek
• Mistral
• Anthropic

{
  "role": "assistant", 
  "content": "..", 
  ...
  "prefix": true # 👈 KEY CHANGE
}

Quick Start

from litellm import completion
import os 

os.environ["DEEPSEEK_API_KEY"] = ""

response = completion(
  model="deepseek/deepseek-chat",
  messages=[
    {"role": "user", "content": "Who won the world cup in 2022?"},
    {"role": "assistant", "content": "Argentina", "prefix": True}
  ]
)
print(response.choices[0].message.content)

curl http://0.0.0.0:4000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $LITELLM_KEY" \
  -d '{
    "model": "deepseek/deepseek-chat",
    "messages": [
      {
        "role": "user",
        "content": "Who won the world cup in 2022?"
      },
      {
        "role": "assistant", 
        "content": "Argentina", "prefix": true
      }
    ]
}'

Expected Response

{
    "id": "3b66124d79a708e10c603496b363574c",
    "choices": [
        {
            "finish_reason": "stop",
            "index": 0,
            "message": {
                "content": " won the FIFA World Cup in 2022.",
                "role": "assistant",
                "tool_calls": null,
                "function_call": null
            }
        }
    ],
    "created": 1723323084,
    "model": "deepseek/deepseek-chat",
    "object": "chat.completion",
    "system_fingerprint": "fp_7e0991cad4",
    "usage": {
        "completion_tokens": 12,
        "prompt_tokens": 16,
        "total_tokens": 28,
    },
    "service_tier": null
}

Check Model Support

Call litellm.get_model_info to check if a model/provider supports prefix.

from litellm import get_model_info

params = get_model_info(model="deepseek/deepseek-chat")

assert params["supports_assistant_prefill"] is True

Call the /model/info endpoint to get a list of models + their supported params.

curl -X GET 'http://0.0.0.0:4000/v1/model/info' \
-H 'Authorization: Bearer $LITELLM_KEY' \