Name: gpt-oss-120b API: OpenAI Open-Weight LLM
Brand: magicapi
SKU: api-magicapi-open-weight-language-model-openai-gpt-oss-120b
Availability: InStock
Author: API Market

Shashank Agarwal

# 💬 Open-Weight LLM API (OpenAI gpt-oss-120b)

## Overview
gpt-oss-120b is a large language model from OpenAI. OpenAI describes it as its 120b open-weight language model.

Send a text `prompt` and get a text answer. Control the answer length with `max_tokens` and sampling with `temperature` and `top_p`. The answer comes back as a list of text chunks; join them in order to get the full text.

## Features
- **Text in, text out**: one `prompt` field; there is no separate system prompt or image input, so put any instructions in the prompt itself.
- **Answer length**: `max_tokens` (default 2048) caps the output.
- **Sampling controls**: `temperature` (default 0.1) and `top_p` (default 1).
- **Repetition controls**: `presence_penalty` and `frequency_penalty` (both default 0).

## Use Cases
1. **Question answering**: short factual or explanatory answers, as in the example.
2. **Writing and summarizing**: drafts, rewrites and summaries of text you paste into the prompt.
3. **Code help**: explain, write or fix code described in the prompt.
4. **Open-weight baseline**: compare an OpenAI open-weight model with other models on your own prompts.

## Visual Examples
### Example Input
Prompt: "What does an API marketplace do? Answer in three sentences.", with `max_tokens` `1024` and `temperature` `0.1`.

```json
{
  "top_p": 1,
  "prompt": "What does an API marketplace do? Answer in three sentences.",
  "max_tokens": 1024,
  "temperature": 0.1,
  "presence_penalty": 0,
  "frequency_penalty": 0
}
```

### Example Output
The model returned 93 text chunks. Joined, they read:

~~~text
An API marketplace is a platform where providers list, monetize, and manage access to their APIs, while developers can discover, test, and purchase or subscribe to those APIs in one centralized catalog. It handles the entire transaction lifecycle - offering documentation, usage analytics, billing, authentication, and compliance tools - to simplify integration for both sides. By aggregating many APIs, the marketplace accelerates innovation, reduces integration friction, and creates a marketplace-style economy for API services.
~~~

## Input Parameters
| Field | Type | Required | Default | Allowed values | Description |
| --- | --- | --- | --- | --- | --- |
| `prompt` | string | No | `""` |  | The text prompt to send to the model |
| `max_tokens` | integer | No | `2048` |  | The maximum number of tokens the model should generate as output. |
| `temperature` | number | No | `0.1` |  | The value used to modulate the next token probabilities. |
| `presence_penalty` | number | No | `0` |  | Presence penalty |
| `frequency_penalty` | number | No | `0` |  | Frequency penalty |
| `top_p` | number | No | `1` |  | Top-p (nucleus) sampling |

## Pricing
- **Pay As You Go (PAYG) pricing**: you only pay for what you use.
- **Price**: $0.001 per API unit.
- **1 API unit = 1,000 weighted tokens**; an output token counts 1, an input token counts 0.25.
- That is **$0.25 per million input tokens** and **$1 per million output tokens**.
- Each request is rounded up to a whole API unit, so the minimum charge is 1 unit ($0.001).
- The input token count includes the model's prompt formatting, so it is higher than the length of your prompt alone (the 10-word example prompt counted as 79 input tokens).
- **Example cost**: the example request used 79 input tokens and 128 output tokens: 79 x 0.25 + 128 = 147.75 weighted tokens, rounded up to 1 API unit = **$0.001**.
- **Failed or cancelled predictions are not billed.**
- **GET requests to check prediction status** are free and don't count toward API unit usage.
- **No monthly subscription fee.**

## API Documentation

### Base URL
All API requests go to: `https://prod.api.market/api/v1/magicapi/open-weight-language-model-openai-gpt-oss-120b`

### Authentication
Send your api.market API key in the `x-api-market-key` header:
```
x-api-market-key: YOUR_API_MARKET_KEY
```

### Asynchronous Processing
**IMPORTANT**: This API is asynchronous. The flow is:
1. Send a POST request to `/predictions`. A successful request returns **201 Created** with a prediction `id` and a `status` of `starting` or `processing`.
2. Poll GET `/predictions/{id}` with that ID.
3. Keep polling while `status` is `starting` or `processing`.
4. Stop when `status` is `succeeded` (read `output`), `failed` or `canceled` (read `error`).

A polling interval of 1 second and an overall timeout of 600 seconds (10 minutes) work well.

### Endpoints

#### Create a Prediction
```bash
curl -X POST "https://prod.api.market/api/v1/magicapi/open-weight-language-model-openai-gpt-oss-120b/predictions" \
  -H "Content-Type: application/json" \
  -H "x-api-market-key: YOUR_API_MARKET_KEY" \
  -d '{
      "version": "openai/gpt-oss-120b",
      "input": {
        "top_p": 1,
        "prompt": "What does an API marketplace do? Answer in three sentences.",
        "max_tokens": 1024,
        "temperature": 0.1,
        "presence_penalty": 0,
        "frequency_penalty": 0
      }
    }'
```

Response (201 Created):
```json
{
  "id": "ys77m3qmahrmt0d1c71vdmjqd4",
  "version": "openai/gpt-oss-120b",
  "status": "starting",
  "created_at": "2026-10-22T03:59:54.638Z"
}
```

#### Check Prediction Status
```bash
curl -X GET "https://prod.api.market/api/v1/magicapi/open-weight-language-model-openai-gpt-oss-120b/predictions/{id}" \
  -H "x-api-market-key: YOUR_API_MARKET_KEY"
```

Response when the prediction has finished (200 OK):
```json
{
  "id": "ys77m3qmahrmt0d1c71vdmjqd4",
  "status": "succeeded",
  "output": [
    "An",
    " API",
    " marketplace",
    " is",
    " a",
    " platform",
    " where",
    " providers",
    " list",
    ",",
    " monetize",
    ","
  ],
  "error": null,
  "created_at": "2026-10-22T03:59:54.638Z",
  "started_at": "2026-10-22T03:59:54.648Z",
  "completed_at": "2026-10-22T03:59:58.956Z"
}
```

`output` is a list of text chunks. Join them in order to get the full answer. The response above is shortened to the first 12 of the 93 chunks; the joined text is shown under Example Output.

### Status Codes
- **201 Created**: prediction created (POST `/predictions`). This is the normal success response.
- **200 OK**: prediction details returned (GET `/predictions/{id}`).
- **400 Bad Request**: invalid input, for example a value of the wrong type, such as text in `max_tokens` or `temperature`.
- **401 Unauthorized**: missing or invalid API key.
- **404 Not Found**: no prediction with that ID.
- **429 Too Many Requests**: rate limit exceeded.
- **500 Internal Server Error** or **503 Service Unavailable**: temporary server problem; retry later.

## Rate Limits
All plans allow 10 requests per second. Requests above that return `429 Too Many Requests`; back off before retrying and honour any `Retry-After` header. Polling a prediction with GET is free but still counts toward the per-second limit, so poll every 1 to 5 seconds rather than in a tight loop.

## Error Handling
- Check the HTTP status code of every request and read the response body for the error message.
- A prediction can also end with `status` set to `failed`; the `error` field then explains why. Failed predictions are not billed.
- Retry 429, 500 and 503 responses with exponential backoff. Fix the input before retrying a 400.

## Implementation Guide
### Example Code

#### JavaScript
```javascript
const apiKey = "YOUR_API_MARKET_KEY";
const baseUrl = "https://prod.api.market/api/v1/magicapi/open-weight-language-model-openai-gpt-oss-120b";

async function createAndWaitForPrediction(input) {
  const response = await fetch(`${baseUrl}/predictions`, {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "x-api-market-key": apiKey
    },
    body: JSON.stringify({ version: "openai/gpt-oss-120b", input })
  });

  if (response.status !== 201) {
    throw new Error(`API error: ${response.status} ${await response.text()}`);
  }

  const prediction = await response.json();
  return pollForResults(prediction.id);
}

const delay = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

async function pollForResults(predictionId) {
  const maxTimeout = 600000;
  const pollInterval = 1000;
  const startTime = Date.now();

  while (Date.now() - startTime < maxTimeout) {
    const response = await fetch(`${baseUrl}/predictions/${predictionId}`, {
      headers: { "x-api-market-key": apiKey }
    });

    if (!response.ok) {
      throw new Error(`API error: ${response.status} ${await response.text()}`);
    }

    const prediction = await response.json();

    if (prediction.status === "succeeded") {
      return prediction.output;
    }

    if (prediction.status === "failed" || prediction.status === "canceled") {
      throw new Error(`Prediction ${prediction.status}: ${prediction.error || "Unknown error"}`);
    }

    await delay(pollInterval);
  }

  throw new Error("Prediction timed out after 10 minutes");
}

createAndWaitForPrediction({
    "top_p": 1,
    "prompt": "What does an API marketplace do? Answer in three sentences.",
    "max_tokens": 1024,
    "temperature": 0.1,
    "presence_penalty": 0,
    "frequency_penalty": 0
  }).then((output) => console.log(output.join("")));
```

#### Python
```python
import time
import requests

API_KEY = "YOUR_API_MARKET_KEY"
BASE_URL = "https://prod.api.market/api/v1/magicapi/open-weight-language-model-openai-gpt-oss-120b"


def create_and_wait_for_prediction(input_data):
    headers = {"Content-Type": "application/json", "x-api-market-key": API_KEY}
    response = requests.post(
        f"{BASE_URL}/predictions",
        headers=headers,
        json={"version": "openai/gpt-oss-120b", "input": input_data},
    )

    if response.status_code != 201:
        raise Exception(f"API error: {response.status_code} {response.text}")

    return poll_for_results(response.json()["id"])


def poll_for_results(prediction_id):
    max_timeout = 600
    poll_interval = 1
    headers = {"x-api-market-key": API_KEY}
    start_time = time.time()

    while time.time() - start_time < max_timeout:
        response = requests.get(f"{BASE_URL}/predictions/{prediction_id}", headers=headers)

        if response.status_code != 200:
            raise Exception(f"API error: {response.status_code} {response.text}")

        prediction = response.json()

        if prediction["status"] == "succeeded":
            return prediction["output"]

        if prediction["status"] in ("failed", "canceled"):
            raise Exception(f"Prediction {prediction['status']}: {prediction.get('error') or 'Unknown error'}")

        time.sleep(poll_interval)

    raise Exception("Prediction timed out after 10 minutes")


output = create_and_wait_for_prediction({
    "top_p": 1,
    "prompt": "What does an API marketplace do? Answer in three sentences.",
    "max_tokens": 1024,
    "temperature": 0.1,
    "presence_penalty": 0,
    "frequency_penalty": 0
})
print("".join(output))
```

## FAQ
- **How do I get the full answer text?**
  - Join the chunks in `output` in order, for example `output.join("")` in JavaScript or `"".join(output)` in Python.
- **Is there a system prompt?**
  - No. The API has a single `prompt` field. Put instructions such as the role, tone or format at the start of the prompt.
- **Why is the default temperature so low?**
  - `temperature` defaults to 0.1, which keeps answers consistent between runs. Raise it for more varied wording.
- **How long can an answer be?**
  - `max_tokens` sets the limit and defaults to 2048. Lower it to cap the length and cost of each answer.
- **How much does a request cost?**
  - Input tokens cost $0.25 per million and output tokens $1 per million, billed in units of 1,000 weighted tokens at $0.001 each and rounded up per request. The example above cost $0.001.
- **Is the 201 response from POST an error?**
  - No. 201 Created means the prediction was accepted. Use the returned `id` to poll GET `/predictions/{id}` until `status` is `succeeded`, `failed` or `canceled`.

## Contact Us
If you have any questions, reach out to us through the chat on the bottom right of the page. You can also email our support team at hello@magicapi.com.


Run OpenAI's gpt-oss-120b open-weight language model on text prompts with temperature, top-p and penalty controls. Pay as you go, no monthly fee.