Basic Chat Completions

Overview

Basic chat completions allow you to send a message and receive a response from the model. This is the simplest way to interact with the LLM Gateway.

Getting started

Send a message and receive a response:

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import requests
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headers = {
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  "authorization": "<YOUR_API_KEY>"
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}
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response = requests.post(
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    "https://llm-gateway.assemblyai.com/v1/chat/completions",
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    headers = headers,
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    json = {
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        "model": "claude-sonnet-4-5-20250929",
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        "messages": [
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            {"role": "user", "content": "What is the capital of France?"}
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        ],
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        "max_tokens": 1000
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    }
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)
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result = response.json()
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print(result["choices"][0]["message"]["content"])

API reference

Request

The LLM Gateway accepts POST requests to https://llm-gateway.assemblyai.com/v1/chat/completions with the following parameters:

$
curl -X POST \
>
  "https://llm-gateway.assemblyai.com/v1/chat/completions" \
>
  -H "Authorization: YOUR_API_KEY" \
>
  -H "Content-Type: application/json" \
>
  -d '{
>
    "model": "claude-sonnet-4-5-20250929",
>
    "messages": [
>
      {
>
        "role": "user",
>
        "content": "What is the capital of France?"
>
      }
>
    ],
>
    "max_tokens": 1000
>
  }'

Request parameters

Message object

Content part object

Response

The API returns a JSON response with the model’s completion:

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{
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  "request_id": "abc123",
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  "choices": [
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    {
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      "message": {
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        "role": "assistant",
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        "content": "The capital of France is Paris."
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      },
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      "finish_reason": "stop"
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    }
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  ],
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  "request": {
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    "model": "claude-sonnet-4-5-20250929",
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    "max_tokens": 1000
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  },
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  "usage": {
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    "prompt_tokens": 15,
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    "completion_tokens": 8,
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    "total_tokens": 23
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  }
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}

Response fields

Error response

If an error occurs, the API returns an error response:

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{
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  "error": {
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    "code": 400,
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    "message": "Invalid request: missing required field 'model'",
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    "metadata": {}
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  }
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}

Common error codes