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

# Detect a depeg in real time

> Subscribe to peg-deviation events and react inside one block.

You operate a stablecoin-heavy strategy and need to exit on the first credible signal that a peg is breaking. The risk-events feed delivers depeg starts as they happen, with peg health, deviation in basis points, and the underlying classification.

## What you build

A signed webhook receiver that listens for `peg.depeg.start` and triggers your existing kill-switch.

## 1. Subscribe to the event

<CodeGroup>
  ```python python theme={null}
  from alterscope import AlterscopeClient

  client = AlterscopeClient(api_key="sk_live_...")

  sub = client.webhooks.create(
      url="https://your.app/hooks/alterscope",
      events=["peg.depeg.start", "peg.depeg.end"],
  )
  # The signing secret is returned once on creation as sub.signing_secret — store it now.
  print(sub.id)
  ```

  ```bash curl theme={null}
  curl -H "Authorization: Bearer $ALTERSCOPE_API_KEY" \
       -H "Content-Type: application/json" \
       -X POST -d '{
         "url": "https://your.app/hooks/alterscope",
         "events": ["peg.depeg.start", "peg.depeg.end"]
       }' \
    "https://api.alterscope.org/v2/webhooks"
  ```
</CodeGroup>

## 2. Verify the signature

Every delivery sets `Alterscope-Signature: t=<unix>,v1=<hex>` (the `v1` MAC is HMAC-SHA256 of `<t>.<raw_body>`). Recompute, compare `v1` in constant time, and reject stale timestamps (\~5 min) or anything that doesn't match. A legacy `X-Alterscope-Signature` (raw-body hex) is dual-sent during the 60-day transition.

```python theme={null}
import hmac, hashlib

def verify(secret: bytes, body: bytes, header: str) -> bool:
    parts = dict(p.split("=", 1) for p in header.split(","))
    expected = hmac.new(secret, f"{parts['t']}.".encode() + body, hashlib.sha256).hexdigest()
    return hmac.compare_digest(parts["v1"], expected)
```

## 3. Sample payload

```json theme={null}
{
  "type": "peg.depeg.start",
  "data": {
    "asset_id": "0x6b17...USDC",
    "deviation_bps": 87,
    "peg_health_score": 31,
    "as_of_block": 22341123,
    "as_of": "2026-04-27T08:32:11Z",
    "classification": "credit_event_suspected"
  },
  "meta": { "_agentic": { "freshness": { "status": "fresh" } } }
}
```

## 4. Troubleshooting

* **Missed events**: webhooks retry with exponential backoff for 24 hours, then DLQ. Use the [replay recipe](/recipes/replay-missed-risk-events) to recover.
* **Duplicate deliveries**: receivers should be idempotent on `data.as_of_block + asset_id`.
* **Signature failures**: confirm body is the raw bytes, not the JSON-decoded object.