---
title: VCL memory overflows
summary: null
url: https://www.fastly.com/documentation/reference/vcl/memory-overflows
---

During VCL execution, exceeding some limits will cause the VCL program to terminate immediately, without invoking `vcl_error`. In these cases a `503` error is always generated and emitted to the client, and cannot be modified by your service configuration. The most common types of overflows are:

- **Workspace exhaustion**: VCL programs allocate memory greedily and free it when the request ends. Use `workspace.bytes_free` to understand how much workspace is available.
- **Header overflows**: VCL programs may only define around 90 HTTP headers in total. The exact number available to your program depends on the number of headers used by Fastly during your request.