Chat Completions

Create Chat Completion

post

Creates a chat completion for the provided messages. Supports streaming, function calling, vision, multimodal inputs, and reasoning tokens for supported models (OpenAI o-series, Grok, Gemini thinking, Anthropic). Compatible with OpenAI SDK.

Authorizations

bearerAuth

AuthorizationstringRequired

FastRouter API Key. Get yours at https://fastrouter.ai

Format: Authorization: Bearer YOUR_API_KEY

Body

application/json

modelstringRequired

Model ID in format 'provider/model'. Examples: openai/gpt-5.1, google/gemini-3-pro-preview, anthropic/claude-4.5-sonnet

Example: anthropic/claude-4.5-sonnet

temperaturenumber · max: 2Optional

Controls randomness in responses. Lower values (0-0.7) make output more focused and deterministic. Higher values (0.8-2) make output more creative and random.

Default: 1Example: 0.7

max_tokensinteger · min: 1Optional

Maximum number of tokens to generate in the completion. Limits the length of the response.

Example: 1000

top_pnumber · max: 1Optional

Nucleus sampling parameter. Alternative to temperature. Lower values make output more focused.

Default: 1Example: 0.9

frequency_penaltynumber · min: -2 · max: 2Optional

Penalizes repeated tokens based on frequency. Positive values reduce repetition.

Default: 0Example: 0

presence_penaltynumber · min: -2 · max: 2Optional

Penalizes tokens that have appeared. Positive values encourage new topics.

Default: 0Example: 0

streambooleanOptional

Enable streaming responses for real-time output. When true, responses are sent as Server-Sent Events (SSE).

Default: falseExample: true

stopone ofOptional

Stop sequences where the model will stop generating. Can be a string or array of strings.

Example: ["\\n","END"]

stringOptional

or

string[]Optional

toolsarrayOptional

Array of tool/function definitions

aspectRatiostring · enumOptional

Image aspect ratio for Nano Banana (google/gemini-2.5-flash-image). Supported ratios: 1:1, 2:3, 3:2, 3:4, 4:3, 4:5, 5:4, 9:16, 16:9, 21:9

Example: 3:2Possible values:

promptstringOptional

Natural-language description for audio generation (ace-step/prompt-to-audio). Describe the audio, music, or ambient sound to generate.

Example: A lofi hiphop song with a chill vibe about a sunny day on the boardwalk.

durationinteger · min: 1Optional

Duration of the audio clip in seconds for text-to-audio generation (ace-step/prompt-to-audio). Optional parameter.

Example: 5

Responses

200

Successful chat completion response

application/json

400

Bad Request - Invalid parameters

401

Invalid Credentials - Your API key is invalid, disabled, or your OAuth session has expired. Check your credentials.

application/json

429

Rate Limited - You have exceeded your request limits (TPM/RPM). Slow down or increase your limits.

application/json

500

Internal Server Error

post

/chat/completions

HTTPcURLJavaScriptPython

POST /api/v1/chat/completions HTTP/1.1
Host: api.fastrouter.ai
Authorization: Bearer YOUR_SECRET_TOKEN
Content-Type: application/json
Accept: */*
Content-Length: 96

{
  "model": "openai/gpt-5.1",
  "messages": [
    {
      "role": "user",
      "content": "What is the meaning of life?"
    }
  ]
}

OpenAI GPT-5.1Anthropic ClaudeGoogle Gemini with StreamingMulti-turn ConversationWith Temperature ControlForce Specific Provider (Only)Provider Fallback (Order)Image Generation (Nano Banana)Image Edit (Nano Banana via Chat)Text-to-Audio Generation (Async)Ambient Audio GenerationReasoning Tokens (High Effort)Reasoning with Max Tokens (Anthropic)Reasoning Excluded (Internal Only)

{
  "id": "chatcmpl-abc123",
  "object": "chat.completion",
  "created": 1677652288,
  "model": "anthropic/claude-4.5-sonnet",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "Hello! How can I assist you today?"
      },
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 10,
    "completion_tokens": 20,
    "total_tokens": 30,
    "cost": 0.00015
  }
}