[staticman2]

Something like Sillytavern let's you edit the AI output which isn't an option in the web version.

Sillytavern also supports prefills which is an API feature not allowed on the web version.

Editing the system prompt is also not permitted in the web version but should be doable in any third party front-end.

I also use Poe sometimes which doesn't have all those features but at least allows custom system prompts when using Claude.