Understanding OnWebPagePrerender and Cached Output

OnWebPagePrerender is often described as running just before cached output is written, which makes it sound like any change there should become part of the cached page permanently. In practice, developers sometimes see the opposite.

the underlying example

$out = &$modx->documentOutput;
$out = str_replace('<title>', '<title>' . date('H:i:s'), $out);

If the time in the title changes on every refresh, it means the code path is still affecting output after the cache assumptions you expected.

How to reason about it

• check whether the page is really being served from full document cache
• check whether an uncached snippet or plugin changes output afterward
• confirm whether your test page is excluded from cache for another reason

Why this matters

Cache debugging in Evolution CMS is rarely about one event in isolation. It is about the full rendering chain: cached document output, uncached snippet calls, plugins, and any later output transformations.