Using OpenAI libraries with Vertex AI

The Chat Completions API works as an Open AI-compatible endpoint, designed to make it easier to interface with Gemini on Vertex AI by using the OpenAI libraries for Python and REST. If you're already using the OpenAI libraries, you can use this API as a low-cost way to switch between calling OpenAI models and Vertex AI hosted models to compare output, cost, and scalability, without changing your existing code. If you aren't already using the OpenAI libraries, we recommend that you use the Google Gen AI SDK. To migrate your existing OpenAI SDK code to use the Google Gen AI SDK, see Migrate from OpenAI SDK to Google Gen AI SDK.

Supported models

The Chat Completions API supports both Gemini models and select self-deployed models from Model Garden.

Gemini models

The following models provide support for the Chat Completions API:

Self-deployed models from Model Garden

The Hugging Face Text Generation Interface (HF TGI) and Vertex AI Model Garden prebuilt vLLM containers support the Chat Completions API. However, not every model deployed to these containers supports the Chat Completions API. The following table includes the most popular supported models by container:

HF TGI

vLLM

Supported parameters

For Google models, the Chat Completions API supports the following OpenAI parameters. For a description of each parameter, see OpenAI's documentation on Creating chat completions. Parameter support for third-party models varies by model. To see which parameters are supported, consult the model's documentation.

messages
  • System message
  • User message: The text and image_url types are supported. The image_url type supports images stored in a Cloud Storage URI or a base64 encoding in the form "data:<MIME-TYPE>;base64,<BASE64-ENCODED-BYTES>". To learn how to create a Cloud Storage bucket and upload a file to it, see Discover object storage.
  • Assistant message
  • Tool message
  • Function message: This field is deprecated, but supported for backwards compatibility.
model
detail For models older than Gemini 3, the detail field must be consistent across all messages and contents (it is request-level). For Gemini 3 and onwards, this corresponds to a part-level `media_resolution`. For more information, see Media Resolution.
max_completion_tokens Alias for max_tokens.
modalities Supports audio, image, and text.
max_tokens
n
frequency_penalty
presence_penalty
reasoning_effort Configures how much time and how many tokens are used on a response.
  • low: 1024
  • medium: 8192
  • high: 24576
As no thoughts are included in the response, only one of reasoning_effort or extra_body.google.thinking_config may be specified.
response_format
  • json_object: Interpreted as passing "application/json" to the Gemini API.
  • json_schema. Fully recursive schemas are not supported. additional_properties is supported.
  • text: Interpreted as passing "text/plain" to the Gemini API.
  • Any other MIME type is passed as is to the model, such as passing "application/json" directly.
seed Corresponds to GenerationConfig.seed.
stop
stream
temperature
top_p
tools
  • type
  • function
    • name
    • description
    • parameters: Specify parameters by using the OpenAPI specification. This differs from the OpenAI parameters field, which is described as a JSON Schema object. To learn about keyword differences between OpenAPI and JSON Schema, see the OpenAPI guide.
tool_choice
  • none
  • auto
  • required: Corresponds to the mode ANY in the FunctionCallingConfig.
  • validated: Corresponds to the mode VALIDATED in the FunctionCallingConfig. This is Google-specific.
web_search_options Corresponds to the GoogleSearch tool. No sub-options are supported.
function_call This field is deprecated, but supported for backwards compatibility.
functions This field is deprecated, but supported for backwards compatibility.

If you pass any unsupported parameter, it is ignored.

Multimodal input parameters

The Chat Completions API supports select multimodal inputs.

input_audio
  • data: Any URI or valid blob format. We support all blob types, including image, audio, and video. Anything supported by GenerateContent is supported (HTTP, Cloud Storage, etc.).
  • format: OpenAI supports both wav (audio/wav) and mp3 (audio/mp3). Using Gemini, all valid MIME types are supported.
image_url
  • data: Like input_audio, any URI or valid blob format is supported.
    Note that image_url as a URL will default to the image/* MIME-type and image_url as blob data can be used as any multimodal input.
  • detail: Similar to media resolution, this determines the maximum tokens per image for the request. Note that while OpenAI's field is per-image, Gemini enforces the same detail across the request, and passing multiple detail types in one request will throw an error.

In general, the data parameter can be a URI or a combination of MIME type and base64 encoded bytes in the form "data:<MIME-TYPE>;base64,<BASE64-ENCODED-BYTES>". For a full list of MIME types, see GenerateContent. For more information on OpenAI's base64 encoding, see their documentation.

For usage, see our multimodal input examples.

Gemini-specific parameters

There are several features supported by Gemini that are not available in OpenAI models. These features can still be passed in as parameters, but must be contained within an extra_content or extra_body or they will be ignored.

extra_body features

Include a google field to contain any Gemini-specific extra_body features.

{
  ...,
  "extra_body": {
     "google": {
       ...,
       // Add extra_body features here.
     }
   }
}
safety_settings This corresponds to the Gemini SafetySetting.
cached_content This corresponds to the Gemini generateContent.cached_content field.
thinking_config This corresponds to the Gemini GenerationConfig.ThinkingConfig.
thought_tag_marker Used to separate a model's thoughts from its responses for models with Thinking available.
If not specified, no tags will be returned around the model's thoughts. If present, subsequent queries will strip the thought tags and mark the thoughts appropriately for context. This helps preserve the appropriate context for subsequent queries.
stream_function_call_arguments Streams function call arguments back as segments of JSON. For more information, see Streaming function call arguments.
tools Specify tools similar to `GenerateContent`. For more information, see Tool.
media_resolution Specify a request-level media resolution similar to `GenerateContent`. For more information, see MediaResolution.

extra_content features

extra_content lets you specify Gemini-specific content that shouldn't be ignored.

Include a google field to contain any Gemini-specific extra_content features.

{
  ...,
  "extra_content": {
     "google": {
       ...,
       // Add extra_content features here.
     }
   }
}
thought This field explicitly marks if a field is a thought and takes precedence over thought_tag_marker. It helps distinguish between different steps in a thought process, especially in tool use scenarios where intermediate steps might be mistaken for final answers. By tagging specific parts of the input as thoughts, you can guide the model to treat them as internal reasoning rather than user-facing responses.
thought_signature A bytes field that provides a thought signature to validate against thoughts returned by the model. This field is distinct from thought, which is a boolean field. For more information, see Thought signatures.
parts Specific to a Tool message to pass multi-modal function response parts back to the model. For more information, see FunctionResponsePart and Multimodal function response.

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