Real-time load injection

Add virtual users or extend the duration of a running load test without stopping and re-queuing it.

Real-time load injection lets you adjust a load test while it’s running. Add virtual users or extend the duration without stopping and re-queuing the test, so you can find a system’s breaking point or capture a failure window in one session instead of several throwaway runs.

It applies to both API load tests, which use k6 or JMeter, and browser load tests, which use Playwright or Selenium. The change takes a few clicks mid-run.

A virtual user (VU) is one simulated concurrent user that generates load. Your plan sets the maximum number of VUs you can run at once.

Prerequisites

• A load test that is currently running. Live Controls appear only on in-progress tests.
• Real-time load injection enabled for your account. If you don’t see the Live Controls panel on a running test, contact your BrowserStack account team to enable it.
• Enough VU headroom on your plan. You can add VUs only up to your plan’s total VU limit.
• Permission to create or edit tests in the project. This is the same permission used to start a test.

What you can change

The following changes are available based on your test’s load profile:

A few rules to know up front:

• You can add VUs once and extend the duration once per run.
• Iteration injection isn’t available. You can’t change the per-VU iteration count mid-run.
• Hybrid runs, which combine API and browser tests, aren’t eligible for real-time load injection.
• For Ramping VUs tests, changes apply only while the test is in a Hold stage with a steady VU count. The controls are disabled during ramp-up and ramp-down. This works the same way for API and browser load tests.

Add virtual users to a running test

+50 VUs (new total: 250 → 300)
Split across your regions:
  us-east-1     : +30
  eu-central-1  : +15
  ap-south-1    : +5
Estimated time to take effect: around 2 to 3 minutes

Extend a running test’s duration

How added VUs are distributed across regions

If your test runs across more than one region, the VUs you add are split across the same regions using the ratio you set when you created the test. You always see the exact split in the confirmation dialog before applying.

Keep these rules in mind when you add VUs or extend the duration on a multi-region test:

• Your plan’s VU limit applies to the total across all regions, not per region. For example, a 100-VU plan can run 80 VUs in us-east-1 and 20 VUs in eu-central-1.
• Extending the duration applies to every region uniformly. There is no per-region split for time.
• If a region is briefly unreachable when you apply a change, BrowserStack applies it to the reachable regions and tells you which region was skipped. See Common issues.

Limits and rules

Verify your change worked

• Live Controls panel: the changed control switches to a VUs added or Duration extended marker, and the VU count or time remaining updates.
• Run summary: hover over the VU count or duration field on the run’s summary to see the change that was applied, without opening the panel.
• Run report (PDF): completed runs include a Real-time load injection section that lists each change, with its time, type, before and after values, and status. The section is omitted if no changes were made.

Common issues

Next step

• API load testing: set up an API load test with k6, JMeter, Gatling, or Locust.
• Browser load testing: set up a browser-based load test with Selenium, Playwright, and more.
• Load zones: choose regions and distribute VUs when you create a test.
• Load profiles: understand Constant VUs, Ramping VUs, and Per VU iterations.