Validation doesn't work

According to clause No. 4.3 of RFC 7234:

4.3. Validation
When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; see Section 4.1), it can use the conditional request mechanism
...
This process is known as "validating" or "revalidating" the stored response.
4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache sends one or more precondition header fields containing validator metadata from its stored response(s), which is then compared by recipients to determine whether a stored response is equivalent to a current representation of the resource.
One such validator is the timestamp given in a Last-Modified header field...
Another validator is the entity-tag given in an ETag header field...

Thus, I expect caddy to cache the first response and subsequently validate cached data via a conditional request. However, caddy simply forwards a request to an upstream without any conditions.

I've tested it only with Last-Modified and no-cache, because of the bug I noticed with ETag. Additionally, I couldn't trigger revalidation using other directives like must-revalidate and stale-while-revalidate.

Reproduction steps:

{
    log
    cache {
        stale 60s
    }
}

http://localhost:80 {
    log
    cache
    reverse_proxy http://localhost:81
}

http://localhost:81 {
    log
    header Last-Modified "Mon, 15 Dec 2025 08:52:20 GMT"
    header Cache-Control "no-cache"
    respond "Hello world"
}

$ curl -ik localhost:80
HTTP/1.1 200 OK
Cache-Control: no-cache
Cache-Status: Souin; fwd=uri-miss; stored; key=GET-http-localhost-/
Last-Modified: Mon, 15 Dec 2025 08:52:20 GMT
...
$ curl -ik localhost:80
HTTP/1.1 200 OK
Cache-Control: no-cache
Cache-Status: Souin; fwd=request; fwd-status=200; key=GET-http-localhost-/; detail=REQUEST-REVALIDATION
Last-Modified: Mon, 15 Dec 2025 08:52:20 GMT
...

logs:

{"level":"info","ts":1766736377.4338582,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"127.0.0.1","remote_port":"48850","client_ip":"127.0.0.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"X-Forwarded-Proto":["http"],"Accept-Encoding":["gzip"],"Accept":["*/*"],"Date":["Fri, 26 Dec 2025 08:06:17 UTC"],"Via":["1.1 Caddy"],"X-Forwarded-Host":["localhost"],"User-Agent":["curl/8.7.1"],"X-Forwarded-For":["172.16.249.1"]}},"bytes_read":0,"user_id":"","duration":0.000199667,"size":11,"status":200,"resp_headers":{"Content-Type":["text/plain; charset=utf-8"],"Server":["Caddy"],"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Cache-Control":["no-cache"]}}
{"level":"info","ts":1766736377.4560177,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"172.16.249.1","remote_port":"57879","client_ip":"172.16.249.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"User-Agent":["curl/8.7.1"],"Accept":["*/*"]}},"bytes_read":0,"user_id":"","duration":0.023146958,"size":11,"status":200,"resp_headers":{"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Server":["Caddy"],"Content-Length":["11"],"Via":["1.1 Caddy"],"Date":["Fri, 26 Dec 2025 08:06:17 GMT"],"Cache-Control":["no-cache"],"Content-Type":["text/plain; charset=utf-8"],"Cache-Status":["Souin; fwd=uri-miss; stored; key=GET-http-localhost-/"]}}
{"level":"info","ts":1766736406.6892934,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"127.0.0.1","remote_port":"48850","client_ip":"127.0.0.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"X-Forwarded-For":["172.16.249.1"],"X-Forwarded-Host":["localhost"],"X-Forwarded-Proto":["http"],"Accept":["*/*"],"Date":["Fri, 26 Dec 2025 08:06:46 UTC"],"Via":["1.1 Caddy"],"Accept-Encoding":["gzip"],"User-Agent":["curl/8.7.1"]}},"bytes_read":0,"user_id":"","duration":0.000023417,"size":11,"status":200,"resp_headers":{"Server":["Caddy"],"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Cache-Control":["no-cache"],"Content-Type":["text/plain; charset=utf-8"]}}
{"level":"info","ts":1766736406.6932402,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"172.16.249.1","remote_port":"57892","client_ip":"172.16.249.1","proto":"HTTP/1.1","method":"GET","host":"localhost","uri":"/","headers":{"Accept":["*/*"],"User-Agent":["curl/8.7.1"]}},"bytes_read":0,"user_id":"","duration":0.019553791,"size":11,"status":200,"resp_headers":{"Content-Length":["11"],"Last-Modified":["Mon, 15 Dec 2025 08:52:20 GMT"],"Via":["1.1 Caddy"],"Cache-Control":["no-cache"],"Content-Type":["text/plain; charset=utf-8"],"Cache-Status":["Souin; fwd=request; fwd-status=200; key=GET-http-localhost-/; detail=REQUEST-REVALIDATION"],"Server":["Caddy"],"Date":["Fri, 26 Dec 2025 08:06:46 GMT"]}}

As we may see, the request made by caddy is not conditional.

Start by running the Caddyfile reproduction and comparing the upstream request made by the two curl commands. Trace the cache validation/revalidation entry point in the Go module and verify that stored Last-Modified metadata is sent as a conditional request; done means the upstream receives the condition and the response behavior matches HTTP validation.