> For a complete page index, fetch https://docs.synthflow.ai/llms.txt. For full documentation content, fetch https://docs.synthflow.ai/llms-full.txt.

# Concurrency

> Understand Synthflow concurrency and CAPS limits, how they differ, and what happens when your workspace exceeds them.

Every workspace has two call limits that protect system stability and call quality. They matter most when you plan outbound volume or troubleshoot `429` responses.

| Limit                               | What it controls                                                                                                                                                                                                                                                                     | Example                                                                                            |
| ----------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------------------------------- |
| **Concurrency**                     | How many calls are active at the same time.                                                                                                                                                                                                                                          | With a limit of 5, you can have 5 live calls. When one ends, another can start.                    |
| **CAPS** (call attempts per second) | How many outbound calls your account can start per second, including calls from the [Make a Phone Call](/api-reference/platform-api/calls/voice-call) API, [campaigns](/batch-calling), and [warm transfers](/call-transfers#transfer-modes). It applies to every phone number type. | With a limit of 2, you can start 2 calls per second. Sending 10 requests in one second exceeds it. |

## Workspace limits

To see your workspace's concurrency limit, select **Admin** → **Workspace Settings** → **Plan & Billing** and check the **Add-Ons** section.

| Account setup                            | Concurrency                                        | CAPS                        |
| ---------------------------------------- | -------------------------------------------------- | --------------------------- |
| Enterprise or account-managed deployment | Scoped with Synthflow, shown in **Plan & Billing** | Scoped with Synthflow       |
| Legacy workspace                         | Fixed by your plan, shown in **Plan & Billing**    | Shown in your account terms |

Legacy workspaces are on the earlier Pro, Growth, and Agency plans, which came with a fixed concurrency limit and a default CAPS limit. These plans are no longer sold, and existing workspaces keep their limits.

Subaccounts do not get their own limits. All subaccounts share the main account's total. Subaccounts have no **Plan & Billing** page, so check the limit from the main account.

## Warm transfers and CAPS

Every warm transfer attempt starts a new outbound call to the transfer destination, so it counts against CAPS. During a campaign, those transfer calls compete with the campaign's own calls for the same capacity. When many campaign calls end in a warm transfer, new campaign calls wait for free capacity, which can interrupt the campaign.

Cold transfers hand the call to the destination with SIP REFER instead of starting a new call, so they do not count against CAPS.

If your outbound calls often end in a warm transfer, we recommend a CAPS limit of at least 2. To raise it, see [Custom limits](#custom-limits).

## Over-limit behavior

When you reach a limit, extra calls either return an HTTP `429` error or wait briefly in a queue, depending on your account setup.

For an Enterprise or account-managed deployment, agree on expected volume, queueing behavior, and limits with Synthflow before launch.

## Custom limits

Enterprise and account-managed customers can request higher concurrency or CAPS limits. [Contact our sales team](https://synthflow.ai/talk-to-sales) with your expected volume.

## FAQ

#### How do I avoid hitting the CAPS limit when calling a large list?

Use [Campaigns](/batch-calling). A campaign dials within your concurrency limit instead of sending requests to the API, so it is the safest way to work through a long list. Set [`reserved_concurrency`](/batch-calling#control-dialing-speed) to keep slots free for inbound calls and one-off calls.

If you start calls yourself from GoHighLevel, HubSpot, Zapier, or another automation tool, add a 1-second delay between calls. That keeps you within the default CAPS limit.

#### Can I monitor my current usage?

Yes. Your dashboard [analytics](/collect-analytics) show active call counts. If you call the API, track `429` responses, which mean you hit a limit.