***

title: Concurrency
subtitle: null
slug: cuncurrency-calling-limits
description: null
-----------------

Synthflow enforces concurrency and rate limits to maintain system stability and call quality. This page explains how concurrency and CAPS work, how they differ, and what happens when you exceed your plan's limits.

## Concurrent Active Calls

**Concurrent Active Calls** define how many live calls you can have active simultaneously.

| Plan          | Concurrency Limit                   |
| ------------- | ----------------------------------- |
| Pay As You Go | 5 units (expandable at US\$20/unit) |
| Enterprise    | Unlimited                           |

For example, if your concurrency limit is 5, you can have up to 5 active calls at the same time. When one call ends, another can begin. On PAYG, you can purchase additional concurrency units in **Settings > Add-Ons**.

<Note>
  **Legacy plans**: Pro, Growth, and Agency plans have different concurrency limits (25, 50, and 100 respectively). These plans are no longer available for new subscriptions but remain supported for existing customers.
</Note>

## CAPS — Call Attempts Per Second

**CAPS (Call Attempts Per Second)** defines how frequently you can trigger the [Make a Phone Call](/api-reference/platform-api/calls/voice-call) API. It limits how often you can start calls, not how many calls can be active at once.

| Plan          | CAPS Limit                |
| ------------- | ------------------------- |
| Pay As You Go | 1 call attempt per second |
| Enterprise    | Custom limits available   |

For example, if you have a CAPS limit of 2, you can initiate 2 new outbound calls per second. Sending 10 call requests in 1 second would exceed the limit and some would be rejected or queued.

Legacy plans (Pro, Growth, Agency) also have a default CAPS limit of 1 call attempt per second.

## Understanding the difference

* **Concurrency = how many calls you have active** — controls the number of simultaneous live calls
* **CAPS = how fast you start calls** — controls the rate at which you can initiate new calls

Both limits work together to ensure system stability and call quality.

## Behavior when limits are exceeded

* **Pay As You Go (and legacy plans):** Exceeding the concurrency or CAPS limit returns a `429` HTTP error. Excess calls are not processed.
* **Enterprise:** A priority queueing system processes calls exceeding the limit with a brief delay, providing flexibility and scalability.

## Custom limits

Enterprise customers can request custom concurrency or CAPS limits. Contact [our sales team](https://synthflow.ai/talk-to-sales) to discuss your requirements.

## FAQs

<AccordionGroup>
  <Accordion title="What's the difference between concurrency and CAPS?">
    Concurrency limits control how many calls can be active at the same time. CAPS limits control how quickly you can start new calls. Both limits work together to ensure system stability and call quality.
  </Accordion>

  <Accordion title="What happens when I exceed my concurrency limit?">
    On Pay As You Go and legacy plans, you'll receive a `429` HTTP error and excess calls will not be processed. On Enterprise, a priority queueing system handles calls exceeding the limit with a brief delay.
  </Accordion>

  <Accordion title="Can I increase my limits?">
    Yes. On Pay As You Go, you can purchase additional concurrency units at US\$20/month each in **Settings > Add-Ons**. Enterprise customers can request custom concurrency or CAPS limits—contact our sales team to discuss your requirements.
  </Accordion>

  <Accordion title="How do subaccounts share limits?">
    Concurrency is set by your plan, and your accounts and subaccounts share the main account's total. [Contact our sales team](https://synthflow.ai/talk-to-sales) if you need more capacity.
  </Accordion>

  <Accordion title="How do I avoid hitting CAPS limits when batch calling?">
    If you use workflows with GoHighLevel, HubSpot, Zapier, or other automation tools, implement a delay or throttle mechanism when batching calls. For example, add a 1-second delay between each call initiation to stay within the default CAPS limit.
  </Accordion>

  <Accordion title="Can I monitor my current usage?">
    You can view active call counts in your dashboard analytics. For API-based monitoring, track `429` HTTP error responses which indicate you've hit a limit.
  </Accordion>
</AccordionGroup>