This event is dispatched before final TypoScript 'config' is written to cache, and not when a page can be successfully retrieved from cache, which is typically the case in 'page is fully cached' scenarios.

This incoming $configTree has already been merged with the determined PAGE "page.config" TypoScript of the requested 'type' / 'typeNum' and the global TypoScript setup 'config'.

The result of this event is available as Request attribute: $request->getAttribute('frontend.typoscript')->getConfigTree(), and its array variant $request->getAttribute('frontend.typoscript')->getConfigArray().

Registered listener can set a modified setup config AST. Note the TypoScript AST structure is still marked @internal within v13 core and may change later, using the event to write different 'config' data is thus still a bit risky.