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

# Rate limits

> API request limits and concurrency caps

Nixflex applies rate limits to protect the platform and ensure fair use. Most applications stay well under these limits, but if you're building high-volume integrations, plan around them.

## API request limits

Per API key:

| Endpoint group                                      | Limit                   |
| --------------------------------------------------- | ----------------------- |
| Read operations (`GET`)                             | 600 requests per minute |
| Write operations (`POST`, `PUT`, `PATCH`, `DELETE`) | 120 requests per minute |
| Authentication endpoints (`POST /v1/keys`)          | 10 requests per hour    |

Headers on every response tell you where you stand:

```
X-RateLimit-Limit: 600
X-RateLimit-Remaining: 587
X-RateLimit-Reset: 1715900060
```

`X-RateLimit-Reset` is a Unix timestamp when the window resets.

## When you hit the limit

You get a 429 response:

```json theme={null}
{
  "error": {
    "type": "rate_limit_error",
    "code": "rate_limit_exceeded",
    "message": "Rate limit exceeded. Try again in 12 seconds.",
    "details": {
      "retry_after": 12
    }
  }
}
```

`retry_after` tells you how long to wait. Honour it.

## Concurrent call limits

The number of calls running simultaneously per account:

| Tier          | Concurrent calls |
| ------------- | ---------------- |
| Pay-as-you-go | 10               |
| Starter       | 25               |
| Scale         | 100              |
| Enterprise    | Custom           |

If you trigger more outbound calls than your concurrency limit allows, they queue automatically. Inbound calls beyond the limit hear a busy signal.

To raise your concurrent call limit, contact [support@nixflex.com](mailto:support@nixflex.com).

## Batch campaign concurrency

Within a batch campaign, the engine launches calls in waves up to your account's concurrent call limit. A 1,000-recipient campaign on a 25-concurrent plan takes roughly the average call duration × (1,000 / 25) to complete.

You can also throttle a campaign deliberately:

```json theme={null}
{
  "agent_id": "agent_xxx",
  "recipients": [...],
  "max_concurrent": 5
}
```

Useful when you want to keep some headroom for inbound calls.

## SMS rate limits

SMS is rate-limited by Twilio, not Nixflex. Default: 1 message per second per sending number.

For high-volume SMS:

* Add multiple sending numbers (the engine round-robins them)
* Request a higher Twilio MPS limit from Twilio support
* Use SMS campaigns, which respect Twilio's pacing automatically

## Webhook delivery limits

Webhooks fire as fast as your endpoint can accept them, with no Nixflex-side throttle. If your server is slow:

* Receive the webhook quickly (return 2xx immediately)
* Process the data in a background queue
* Don't do heavy work inside the webhook handler

## When you need more

If you're running into limits regularly, that's a signal you've outgrown the default tier. Email [support@nixflex.com](mailto:support@nixflex.com) and we'll raise your limits or move you to a custom plan.