API Reference – Westtree

Chat Completions

OpenAI-compatible conversation completions for text and reasoning models.

POST /v1/chat/completions Bearer

Generate a chat completion for any supported model.

Parameters

Parameter	Type	Required	Description
`model`	string	Yes	Model ID from the model catalog
`messages`	array	Yes	Conversation messages, OpenAI format
`max_tokens`	integer	No	Maximum output tokens
`temperature`	number	No	Sampling temperature 0-2
`stream`	boolean	No	Stream the response incrementally

Examples

curl https://api.onefast.ai/v1/chat/completions 
  -H "Content-Type: application/json" 
  -H "Authorization: Bearer sk-your-api-key" 
  -d '{
    "model": "gpt-4o",
    "messages": [{"role": "user", "content": "Hello!"}],
    "max_tokens": 200
  }'

{
  "id": "chatcmpl-xxx",
  "object": "chat.completion",
  "model": "gpt-4o",
  "choices": [
    { "index": 0, "message": { "role": "assistant", "content": "Hi there!" }, "finish_reason": "stop" }
  ],
  "usage": { "prompt_tokens": 5, "completion_tokens": 20, "total_tokens": 25 }
}

Responses API

Newer Responses endpoint for agentic and reasoning workloads.

POST /v1/responses Bearer

Create a response (Responses API).

Parameters

Parameter	Type	Required	Description
`model`	string	Yes	Model ID (openai-response compatible)
`input`	array	Yes	Input messages or items

Examples

curl https://api.onefast.ai/v1/responses 
  -H "Content-Type: application/json" 
  -H "Authorization: Bearer sk-your-api-key" 
  -d '{
    "model": "gpt-5.4",
    "input": "Tell me a short joke"
  }'

{
  "id": "resp_xxx",
  "object": "response",
  "model": "gpt-5.4",
  "output": [
    { "type": "message", "role": "assistant", "content": [{ "type": "output_text", "text": "Why did the ..." }] }
  ]
}

Models & Usage

List available models and check usage / balance.

GET /v1/models Bearer

List all available models.

Examples

curl https://api.onefast.ai/v1/models 
  -H "Authorization: Bearer sk-your-api-key"

{
  "object": "list",
  "data": [
    { "id": "gpt-4o", "object": "model", "owned_by": "openai" }
  ]
}

GET /v1/usage Bearer

Query usage and balance.

Examples

curl https://api.onefast.ai/v1/usage 
  -H "Authorization: Bearer sk-your-api-key"

{
  "balance": 12.34,
  "total_usage": 88.20
}

Image Generation

Generate images with image-capable models (per-call billing).

POST /v1/images/generations Bearer

Generate an image from a prompt.

Parameters

Parameter	Type	Required	Description
`model`	string	Yes	Image model ID
`prompt`	string	Yes	Image description
`size`	string	No	e.g. 1024x1024

Examples

curl https://api.onefast.ai/v1/images/generations 
  -H "Content-Type: application/json" 
  -H "Authorization: Bearer sk-your-api-key" 
  -d '{
    "model": "gpt-image-2",
    "prompt": "A red panda surfing in space",
    "size": "1024x1024"
  }'

{
  "created": 1730000000,
  "data": [
    { "url": "https://.../generated-image.png" }
  ]
}