# Vercel Log Drains Webhooks

Vercel Log Drains forward deployment logs to any HTTPS endpoint you configure.
These are HTTP log-drain deliveries (not "Vercel webhooks" and not Standard
Webhooks): Vercel POSTs batches of log entries and signs the raw body with
HMAC-SHA1.

## When to Use This Skill

* How do I receive Vercel Log Drains?
* How do I verify the `x-vercel-signature` header?
* How do I complete the `x-vercel-verify` endpoint handshake?
* Why is my Vercel log drain signature verification failing?
* How do I parse batched log entries (JSON array or NDJSON)?
* How do I handle logs from `lambda`, `edge`, `build`, or `firewall` sources?

## Verification (core)

Vercel signs the raw request body with HMAC-SHA1 keyed on your drain's
signature secret and sends the hex digest in the `x-vercel-signature`
header (the raw digest — no `sha1=` prefix). Use the raw body (do not
re-serialize), and compare timing-safe.

Node:

```javascript
const crypto = require('crypto');

function verify(rawBody, signatureHeader, secret) {
  if (!signatureHeader) return false;
  const expected = crypto.createHmac('sha1', secret).update(rawBody).digest('hex');
  try {
    return crypto.timingSafeEqual(
      Buffer.from(signatureHeader, 'hex'),
      Buffer.from(expected, 'hex')
    );
  } catch {
    return false; // wrong length / not hex
  }
}

```

Python:

```python
import hmac, hashlib

def verify(raw_body: bytes, signature_header: str, secret: str) -> bool:
    if not signature_header:
        return False
    expected = hmac.new(secret.encode(), raw_body, hashlib.sha1).hexdigest()
    return hmac.compare_digest(signature_header, expected)

```

### Endpoint handshake (`x-vercel-verify`)

When a drain is created or tested, Vercel sends an unsigned probe request.
Your endpoint must respond `200 OK` with an `x-vercel-verify` response header
echoing the verification token shown in the dashboard. Because the probe is
unsigned, treat a request with no `x-vercel-signature` as the handshake: return
200 with the verify header and do not process logs. Signed deliveries then
carry `x-vercel-signature`; reject those with an invalid signature (`403`).

> For complete handlers with route wiring, log parsing, and tests, see:
> 
> * [examples/express/](https://github.com/hookdeck/webhook-skills/tree/main/skills/vercel-log-drains-webhooks/examples/express/)
> * [examples/nextjs/](https://github.com/hookdeck/webhook-skills/tree/main/skills/vercel-log-drains-webhooks/examples/nextjs/)
> * [examples/fastapi/](https://github.com/hookdeck/webhook-skills/tree/main/skills/vercel-log-drains-webhooks/examples/fastapi/)

## Log Sources (the "event" dimension)

Log drains do not have named event types. Each log entry carries a `source`
field — dispatch on it the way you would on an event type.

| `source` | Emitted by |
| --- | --- |
| `static` | Requests to static assets (HTML, CSS, images) |
| `lambda` | Vercel Functions (Node.js API routes) |
| `edge` | Vercel Functions using the Edge runtime |
| `build` | The build step |
| `external` | External rewrites to another domain |
| `firewall` | Requests denied by Vercel Firewall rules |
| `redirect` | Requests handled by redirect rules |

Each entry also has a `level` (`info`, `warning`, `error`, `fatal`). A
`statusCode` of `-1` means the lambda crashed with no response.

> For the full log schema, see [Vercel Log Drains Reference](https://vercel.com/docs/drains/reference/logs).

## Delivery Formats

A single request contains a batch of log entries in one of two encodings
(set per drain):

* JSON — a JSON array of log objects: `[{…},{…}]`
* NDJSON — one JSON object per line (newline-delimited)

Handlers should support both (optionally gzip-compressed). Message fields may be
truncated when they exceed 256 KB.

## Important Headers

| Header | Description |
| --- | --- |
| `x-vercel-signature` | HMAC-SHA1 hex digest of the raw body (only on signed deliveries) |
| `x-vercel-verify` | Sent on the setup probe; echo it back as a response header |

## Environment Variables

```bash
VERCEL_LOG_DRAIN_SECRET=your_drain_signature_secret   # Team Settings > Drains > Edit
VERCEL_VERIFY=your_verification_token                  # shown when creating the drain

```

## Local Development

```bash
# Start tunnel (no account needed)
npx hookdeck-cli listen 3000 vercel-log-drains --path /webhooks/vercel-log-drains

```

## Reference Materials

* [references/overview.md](https://github.com/hookdeck/webhook-skills/blob/main/skills/vercel-log-drains-webhooks/references/overview.md) - What log drains are, sources, log schema
* [references/setup.md](https://github.com/hookdeck/webhook-skills/blob/main/skills/vercel-log-drains-webhooks/references/setup.md) - Dashboard and REST API configuration
* [references/verification.md](https://github.com/hookdeck/webhook-skills/blob/main/skills/vercel-log-drains-webhooks/references/verification.md) - Signature verification and the verify handshake