Open System One

An open specification for System One model APIs: what Open Responses is for LLMs, for System One models.

Status: early draft. The 2026-11-07 release is a first snapshot, implemented by one gateway against three models. Expect breaking changes in the next releases. A dated release never changes, so pin one if you depend on it.

System One models answer typed questions about an input: the probability that something is true, a choice, a score, or a value read from a document. They return structured answers with probabilities instead of generated text, and software reads those answers directly.

Why

LLM APIs began with chat completions: text in, text out. Each provider shipped its own variant, structured output was added later, and applications collected an adapter per provider until Open Responses gave them a shared contract.

System One APIs are at that same starting point. Three providers already ship three formats for the same idea:

This matters more than it did for chat, because code reads every answer. A company can call a System One model from hundreds of places across its apps. If each call depends on one provider’s format, changing models becomes a migration project instead of a configuration change.

What it defines

• One contract, at any URL. A request {model, state, questions} and a response {model, answers, usage}. Switching implementers changes the endpoint URL and the model, nothing else.
• TypeSafe’s format as the base. Any valid TypeSafe request is a valid Open System One request.
• Decision and extraction questions. noul, choice, score, and box judge the input. text, number, date, time, datetime, duration, list, estimate, and forecast read values from it.
• Answer guarantees that code can rely on, such as when a value is null, what a span points at, and that estimates stay within bounds.
• Standard errors for a question type or parameter a model does not support.
• Dated releases that never change, and owner:type extensions that can become core.

Example

POST https://api.example.com/v1/systemone

{
  "model": "your-model",
  "state": "Invoice INV-2291 from Northwind Supply. Total due: $1,080.00 by 2026-10-02.",
  "questions": {
    "is_invoice": { "type": "noul", "instructions": "Is this an invoice?" },
    "total": { "type": "number", "decimals": 2, "unit": "USD" }
  }
}

{
  "model": "your-model",
  "answers": {
    "is_invoice": { "type": "noul", "noul": 0.99 },
    "total": {
      "type": "number",
      "value": 1080,
      "p_null": 0.001,
      "confidence": 0.99,
      "guess": 1080,
      "span": [52, 60]
    }
  },
  "usage": { "input_tokens": 64, "output_tokens": 8 }
}

Get involved

Open System One is developed in the open on GitHub. Providers, gateways, and client authors are welcome: open an issue or a pull request with what your API needs, or tell us when you implement it.