Updated 2026-06-29
DeepSeek Chat Prefix Completion (Beta): follow the assistant-prefix contract exactly
DeepSeek's official Chat Prefix Completion (Beta) page documents a narrow but useful pattern: you can start an assistant response yourself and ask DeepSeek to complete the rest. The key is that this is not ordinary prompting. The last message must already be an assistant message, that message must set `prefix: true`, and the request must go to the Beta base URL. Those constraints make this a strong long-tail support page for developers who want code-only or format-controlled output without guessing.
1. The last message has to be the assistant prefix
DeepSeek's first rule is structural, not stylistic. The last message in `messages` must be an `assistant` message, because you are giving the model the beginning of its own reply.
That same last message also needs `prefix: true`. If you end the request on a user message or forget the prefix flag, you are no longer using the documented Beta feature correctly.
messages = [
{"role": "user", "content": "Please write quick sort code"},
{"role": "assistant", "content": "```python\n", "prefix": True},
]Sources checked
- DeepSeek official Chat Prefix Completion (Beta) guide - Primary source for the assistant-role requirement, prefix flag, beta base URL, and sample code.
2. This feature only works on DeepSeek's Beta route
The official docs are explicit that Chat Prefix Completion is a Beta feature. To enable it, the client must call `https://api.deepseek.com/beta` instead of the ordinary stable base URL.
That means many failed experiments are routing mistakes rather than model problems. If the request shape looks right but the feature still behaves like normal chat completion, check the base URL before changing anything else.
client = OpenAI(
api_key="<your api key>",
base_url="https://api.deepseek.com/beta",
)3. Prefix completion is useful when you want the answer to stay inside a format boundary
DeepSeek's own example starts the assistant reply with a Python code fence. That is a practical way to bias the continuation toward code instead of prose.
The same sample also uses `stop=["\`\`\`"]` so the model does not keep talking after the code block closes. This is a simple but important operational detail for teams that want code-only completions.
| Check | What to verify | Why it matters |
|---|---|---|
| 1 | Last message role is `assistant` | The feature completes an assistant prefix, not a user message |
| 2 | Last message sets `prefix: true` | Without the flag, the request is not the documented Beta pattern |
| 3 | Base URL is `https://api.deepseek.com/beta` | The feature is Beta-only on the official docs |
| 4 | Use `stop` when you need clean format boundaries | It helps keep code-only or fenced output from spilling into explanation |
5. Keep the product boundary disciplined
Chat Prefix Completion is a documented API capability, not a new stocked plan. It should not trigger any pricing-card, stock, or subscription-plan changes.
Use `/pricing` only when you need an in-stock DeepSeek key route the site actually sells. The Beta completion feature itself does not imply new inventory.
FAQ
What does DeepSeek require for Chat Prefix Completion (Beta)?
The last message must be an `assistant` message, that message must set `prefix: true`, and the request must use `https://api.deepseek.com/beta`.
Can I use a user message as the prefix?
No. DeepSeek's official Beta guide says the last message must be an assistant message.
Why use the `stop` parameter here?
DeepSeek's own example uses `stop` to keep the response inside the intended format boundary, such as a fenced Python block.
Is Chat Prefix Completion a stable default endpoint feature?
No. The official docs mark it as Beta and require the Beta base URL.
Does this page mean DeepSeek sells a special prefix-completion product on `/pricing`?
No. This page documents an API feature only. Purchasable plans still depend on actual stocked inventory.
The practical DeepSeek prefix-completion rule is simple: end on an assistant message, mark it with `prefix: true`, call the Beta base URL, and use `stop` when you need the output to stay inside a clean format boundary.
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