> ## Documentation Index
> Fetch the complete documentation index at: https://docs.beliefsystems.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Data API

> The keyed REST surface for Belief index levels and Belief Volatility series – base URL, the response envelope, and what each endpoint returns.

<Card horizontal icon="server" title="https://api.beliefsystems.xyz">
  Read-only. Every endpoint is a `GET`, authenticated with a free API key.
</Card>

Belief indices are rules-based benchmarks of how prediction markets price a defined set of events. Belief Volatility measures how much those probabilities are expected to move. This API serves both: current levels, trailing history, the catalogs, and the composition-change record.

<CardGroup cols={2}>
  <Card title="Make your first request" icon="play" href="/api-reference/quickstart">
    A key and a working call in about two minutes.
  </Card>

  <Card title="Authentication" icon="key" href="/api-reference/authentication">
    Key format, both accepted headers, and where a key belongs.
  </Card>

  <Card title="Conventions" icon="ruler" href="/api-reference/conventions">
    The envelope, decimal strings, timestamps, and history windows.
  </Card>

  <Card title="Limits and errors" icon="gauge" href="/api-reference/limits">
    Rate limits, the header set, and every error code.
  </Card>
</CardGroup>

## The envelope

Every successful response is `{ data, meta }`. `data` is the payload. `meta` carries the attribution line, the license and terms URLs, and when the response was generated. History endpoints add the window they actually served.

```json theme={null}
{
  "data": {
    "ticker": "CONFLICT",
    "indexLevel": "36.11697608",
    "baseIndexLevel": "100.00000000",
    "computedAt": "2026-08-31T13:44:44.963697Z",
    "stale": false
  },
  "meta": {
    "attribution": "Source: Belief Systems (beliefsystems.xyz)",
    "license": "https://beliefsystems.xyz/license",
    "termsUrl": "https://beliefsystems.xyz/terms",
    "generatedAt": "2026-08-31T14:01:28.178537Z"
  }
}
```

Failures replace both with a single `error` object carrying a stable `code`, a message, a deep link to the documentation for that code, and a request id.

```json theme={null}
{
  "error": {
    "code": "rate_limited",
    "message": "Rate limit exceeded: 60 requests per minute per key.",
    "docsUrl": "https://docs.beliefsystems.xyz/api-reference/limits#rate-limited",
    "requestId": "9f2c1ad4e6b74c0f8a3d5e1b7c904a26"
  }
}
```

Match on `code`. The message is written for a person and may be reworded.

## Endpoints

### Indices

| Endpoint                                   | Returns                                                 |
| ------------------------------------------ | ------------------------------------------------------- |
| `GET /v1/indices`                          | The published catalog: ticker, name, status, staleness  |
| `GET /v1/indices/{ticker}/latest`          | Current level, undelayed, with input-quality counts     |
| `GET /v1/indices/{ticker}/history`         | Level history at `1h` or `1d` over the trailing 90 days |
| `GET /v1/indices/{ticker}/reconstitutions` | Completed composition changes, newest first             |

### Belief Volatility

| Endpoint                              | Returns                                        |
| ------------------------------------- | ---------------------------------------------- |
| `GET /v1/volatility`                  | The Belief Volatility catalog                  |
| `GET /v1/volatility/{ticker}/latest`  | Latest computation, with every published tenor |
| `GET /v1/volatility/{ticker}/history` | Volatility history, newest first               |

### Service

| Endpoint            | Returns                                                              |
| ------------------- | -------------------------------------------------------------------- |
| `GET /v1`           | Every endpoint, the free-tier limits, and your key's remaining quota |
| `GET /v1/downloads` | Where the full-depth snapshot bundles live                           |

Tickers are case-insensitive. `GET /v1/indices` and `GET /v1/volatility` are the catalogs.

## What the API does not carry

Constituent compositions, the per-market panel, raw NAV, and sub-hourly granularity are not on this surface. Full history since inception, composition membership, and target weights are published as free point-in-time [snapshot bundles](/data-access/downloads). [Data boundaries](/api-reference/boundaries) states each line and where the rest lives.

## Attribution

`Source: Belief Systems (beliefsystems.xyz)` belongs on anything you publish from this data. It travels in `meta.attribution` on every response. Production, redistribution, and commercial use require a license – [Index Services](https://beliefsystems.xyz/license).