When enabling debug_cacheability_headers, the X-Drupal-Cache-Tags header contains the cache tags used in the page, but it does not contain the cache tags of elements rendered by big_pipe.
This can be confusing because a developer can think some cache tag is missing from the page but it is not (as it is not always easy to remember which parts of the page are rendered by big_pipe).
Our specific use case is that we use big_pipe_sessionless in production and we want to make sure the non-big_pipe response cached by Varnish will have the appropriate cache tags without having to disable big_pipe on our development environment to check this.
big_pipe happens too late to add an HTTP header, but a solution could be to add an HTML comment at the end of the page containing this information.
Active
9.5
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Pipeline appears to have failures.
Probably will need test coverage
Removing tests tag as it can be seen right here https://git.drupalcode.org/issue/drupal-3281955/-/jobs/4588575
Not sure if this warrants a CR but will see but can verify the debug info appears.
Automatically closed - issue fixed for 2 weeks with no activity.