---
title: Rust SDK 0.13.0
summary: null
url: https://www.fastly.com/documentation/reference/changes/2026/06/rust-sdk-0.13.0
---

- Make HTTP guest caching compatible with `send_async`.
- Add `PendingRequest::send_to_client` method for handing off responsibility to
  the host for sending eventual responses to in-flight requests back to the client.
- Use `LazyLock`/`OnceLock` instead of `lazy_static`
- Support stale-if-error in the HTTP cache when using guest caching (default).
  See Fastly's documentation on stale-if-error for more details.

The HTTP cache will now store and serve stale-if-error responses. The
duration is configurable via `Request::set_stale_if_error` or
`CandidateResponse::set_stale_if_error`. If you wish to disable this behavior,
you can use `Request::set_stale_if_error(Duration::ZERO)` to avoid storing any
cache entries with a stale-if-error period.

If the HTTP cache has a response in its stale-if-error period and if
`Request::send` or `Request::send_async` would otherwise return a `SendError`
instead of a `Response`, `Request::send` and `Request::send_async` will instead
provide the cached response. For the request collapse leader,
`Response::masked_error` returns the error that would otherwise have been
returned.

Note that 5xx responses from a server are not, by default, replaced with a
stale-if-error response. You can override this behavior by returning an error
from the `after_send` cache hook.