Why even offer the edit tool in that case? Also, what kind of editing could they possible do what wouldn't be possible with POSIX ed?

You can chain a lot more commands together with this technique than with a single Edit tool call.

The funny thing is that... POSIX ed is composable :-)

You can do a gazillion edits with it in one shot.

Of course, LLM edit tools are probably small bits of their custom code, I just find it funny. I wonder if it's a desire for certain technical characteristics that require custom code or just a lack of info on basic tools. Heck, if it's about platform availability, using an LLM to port ed to Windows (for example) should be trivial[1].

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[1] And there are probably a million existing ports. Also, sed, ex, vi, whatever.