The concurrency cap is a comment above the handler

The partner API your integration calls allows five simultaneous requests, and your webhook traffic arrives in bursts of forty. One line above the handler: @concurrent 5. The runtime caps parallel execution of that script at five, and the burst stops being an outage.

The cap is per-file and travels with the code: it works in worker mode and serverless mode alike, and anyone who reads the script can see it.

Concurrency control used to be a console knob three screens deep, or a queue you stood up just to slow yourself down.