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model_server_rest_api_chat.md

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OpenAI API chat/completions endpoint {#ovms_docs_rest_api_chat}

Note: This endpoint works only with LLM graphs.

API Reference

OpenVINO Model Server includes now the chat/completions endpoint using OpenAI API. Please see the OpenAI API Reference for more information on the API. The endpoint is exposed via a path:

http://server_name:port/v3/chat/completions

Example request

curl http://localhost/v3/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "llama3",
    "messages": [
      {
        "role": "system",
        "content": "You are a helpful assistant."
      },
      {
        "role": "user",
        "content": "hello"
      }
    ],
    stream: false
  }'

Example response

{
  "choices": [
    {
      "finish_reason": "stop",
      "index": 0,
      "logprobs": null,
      "message": {
        "content": "\n\nHow can I help you?",
        "role": "assistant"
      }
    }
  ],
  "created": 1716825108,
  "model": "llama3",
  "object": "chat.completion",
  "usage": {
        "completion_tokens": 38,
        "prompt_tokens": 22,
        "total_tokens": 60
  }
}

Request

Generic

Param OpenVINO Model Server OpenAI /chat/completions API vLLM Serving Sampling Params Type Description
model ✅ ✅ ✅ string (required) Name of the model to use. From administrator point of view it is the name assigned to a MediaPipe graph configured to schedule generation using desired model.
stop ✅ ✅ ✅ string/array of strings (optional) Up to 4 sequences where the API will stop generating further tokens. If stream is set to false matched stop string is not included in the output by default. If stream is set to true matched stop string is included in the output by default. It can be changed with include_stop_str_in_output parameter, but for stream=true setting include_stop_str_in_output=false is invalid.
stream ✅ ✅ ✅ bool (optional, default: false) If set to true, partial message deltas will be sent to the client. The generation chunks will be sent as data-only server-sent events as they become available, with the stream terminated by a data: [DONE] message. Example Python code
stream_options ✅ ✅ ✅ object (optional) Options for streaming response. Only set this when you set stream: true
stream_options.include_usage ✅ ✅ ✅ bool (optional) Streaming option. If set, an additional chunk will be streamed before the data: [DONE] message. The usage field in this chunk shows the token usage statistics for the entire request, and the choices field will always be an empty array. All other chunks will also include a usage field, but with a null value.
messages ✅ ✅ ✅ array (required) A list of messages comprising the conversation so far. Each object in the list should contain role and content - both of string type. Example Python code
max_tokens ✅ ✅ ✅ integer The maximum number of tokens that can be generated. If not set, the generation will stop once EOS token is generated.
ignore_eos ✅ ❌ ✅ bool (default: false) Whether to ignore the EOS token and continue generating tokens after the EOS token is generated. If set to true, the maximum allowed max_tokens value is 4000.
include_stop_str_in_output ✅ ❌ ✅ bool (default: false if stream=false, true if stream=true) Whether to include matched stop string in output. Setting it to false when stream=true is invalid configuration and will result in error.
logprobs ⚠️ ✅ ✅ bool (default: false) Include the log probabilities on the logprob of the returned output token. _ in stream mode logprobs are not returned. Only info about selected tokens is returned _

Beam search sampling specific

Param OpenVINO Model Server OpenAI /chat/completions API vLLM Serving Sampling Params Type Description
n ✅ ✅ ✅ integer (default: 1) Number of output sequences to return for the given prompt. This value must be between 1 <= N <= BEST_OF.
best_of ✅ ❌ ✅ integer (default: 1) Number of output sequences that are generated from the prompt. From these best_of sequences, the top n sequences are returned. best_of must be greater than or equal to n. This is treated as the beam width for beam search sampling.
diversity_penalty ✅ ❌ ❌ float (default: 1.0) This value is subtracted from a beam's score if it generates the same token as any beam from other group at a particular time. See arXiv 1909.05858.
length_penalty ✅ ❌ ✅ float (default: 1.0) Exponential penalty to the length that is used with beam-based generation. It is applied as an exponent to the sequence length, which in turn is used to divide the score of the sequence. Since the score is the log likelihood of the sequence (i.e. negative), length_penalty > 0.0 promotes longer sequences, while length_penalty < 0.0 encourages shorter sequences.

Multinomial sampling specific

Param OpenVINO Model Server OpenAI /chat/completions API vLLM Serving Sampling Params Type Description
temperature ✅ ✅ ✅ float (default: 1.0) The value is used to modulate token probabilities for multinomial sampling. It enables multinomial sampling when set to > 0.0.
top_p ✅ ✅ ✅ float (default: 1.0) Controls the cumulative probability of the top tokens to consider. Must be in (0, 1]. Set to 1 to consider all tokens.
top_k ✅ ❌ ✅ int (default: all tokens) Controls the number of top tokens to consider. Set to empty or -1 to consider all tokens.
repetition_penalty ✅ ❌ ✅ float (default: 1.0) Penalizes new tokens based on whether they appear in the prompt and the generated text so far. Values > 1.0 encourage the model to use new tokens, while values < 1.0 encourage the model to repeat tokens. 1.0 means no penalty.
frequency_penalty ✅ ✅ ✅ float (default: 0.0) Number between -2.0 and 2.0. Positive values penalize new tokens based on their existing frequency in the text so far, decreasing the model's likelihood to repeat the same line verbatim.
presence_penalty ✅ ✅ ✅ float (default: 0.0) Number between -2.0 and 2.0. Positive values penalize new tokens based on whether they appear in the text so far, increasing the model's likelihood to talk about new topics.
seed ✅ ✅ ✅ integer (default: 0) Random seed to use for the generation.

Speculative decoding specific

Note that below parameters are valid only for speculative pipeline. See speculative decoding demo for details on how to prepare and serve such pipeline.

Param OpenVINO Model Server OpenAI /completions API vLLM Serving Sampling Params Type Description
num_assistant_tokens ✅ ❌ ⚠️ int This value defines how many tokens should a draft model generate before main model validates them. Equivalent of num_speculative_tokens in vLLM. Cannot be used with assistant_confidence_threshold.
assistant_confidence_threshold ✅ ❌ ❌ float This parameter determines confidence level for continuing generation. If draft model generates token with confidence below that threshold, it stops generation for the current cycle and main model starts validation. Cannot be used with num_assistant_tokens.

Unsupported params from OpenAI service:

  • logit_bias
  • top_logprobs
  • response_format
  • tools
  • tool_choice
  • user
  • function_call
  • functions

Unsupported params from vLLM:

  • min_p
  • use_beam_search (In OpenVINO Model Server just simply increase best_of param to enable beam search)
  • early_stopping
  • stop_token_ids
  • min_tokens
  • prompt_logprobs
  • detokenize
  • skip_special_tokens
  • spaces_between_special_tokens
  • logits_processors
  • truncate_prompt_tokens

Response

Param OpenVINO Model Server OpenAI /chat/completions API Type Description
choices ✅ ✅ array A list of chat completion choices. Can be more than one if n is greater than 1 (beam search or multinomial samplings).
choices.index ✅ ✅ integer The index of the choice in the list of choices.
choices.message ✅ ✅ object A chat completion message generated by the model. When streaming, the field name is delta instead of message.
choices.message.role ⚠️ ✅ string The role of the author of this message. Currently hardcoded as assistant
choices.message.content ✅ ✅ string The contents of the message.
choices.finish_reason ✅ ✅ string or null The reason the model stopped generating tokens. This will be stop if the model hit a natural stop point or a provided stop sequence, length if the maximum number of tokens specified in the request was reached, or null when generation continues (streaming).
choices.logprobs ⚠️ ✅ object or null Log probability information for the choice. In current version, only one logprob per token can be returned.
created ✅ ✅ string The Unix timestamp (in seconds) of when the chat completion was created.
model ✅ ✅ string The model used for the chat completion.
object ✅ ✅ string chat.completion for unary requests and chat.completion.chunk for streaming responses
usage ✅ ✅ object Usage statistics for the completion request. Consists of three integer fields: completion_tokens, prompt_tokens and total_tokens that inform how many tokens have been generated in a completion, number of tokens in a prompt and the sum of both

Unsupported params from OpenAI service:

  • id
  • system_fingerprint
  • usage
  • choices.message.tool_calls
  • choices.message.function_call
  • choices.logprobs.content

NOTE: OpenAI python client supports a limited list of parameters. Those native to OpenVINO Model Server, can be passed inside a generic container parameter extra_body. Below is an example how to encapsulated top_k value.

response = client.completions.create(
    model=model,
    messages=[{"role": "user", "content": "hello"}],
    max_tokens=100,
    extra_body={"top_k" : 1},
    stream=False
)

References

LLM quick start guide

End to end demo with LLM model serving over OpenAI API

Code snippets

LLM calculator