LLMs work over a JSON API. You can read the JSON to see what's going on.

Using other people's harnesses I always got frustrated by not being able to see what the model sees. What are we actually sending it?

Context is pretty important when it comes to LLMs. Arguably, context is everything!

Usually the most straightforward way to see it, was to intercept the HTTP requests and dump them to a file.

This is usually trivial, just change the OPENAI_API_URL (or equivalent) to http://localhost:8080/ or whatever, which can be running a 20 line Python script, that just forwards the HTTP requests, and dumps them to a file.

It's pretty straightforward, although it really shouldn't be necessary in the first place.