---
title: h3.alt_svc
summary: null
url: >-
  https://www.fastly.com/documentation/reference/vcl/functions/tls-and-http/h3-alt-svc
---

```
VOID h3.alt_svc()
```

**Available in:** recv, hash, hit, miss, pass, fetch, deliver, error

When a client connection is using a version of HTTP older than HTTP/3, invoking this function triggers Fastly to advertise HTTP/3 support, on the eventual client response, allowing the client to switch to HTTP/3 for future requests.

When the connection is over HTTP/3 already, this function has no effect.

The addition of the response header happens after the header block has been finalised in `vcl_deliver`.  As a result, using `restart` or `error` within your VCL code (which will destroy the current response and create a new one), will not affect the addition of the `Alt-Svc` header.  It is also not possible to read or unset the value of the header in VCL, since it has not yet been added to the response.

If `resp.http.alt-svc` is already set, using this function will cause it to be overwritten.