> [The] way to do is to register a tool / function to load and extend the base prompt and presto - you have implemented your own version of skills.

So are they basically just function tool calls whose return value is a constant string? Do we know if that’s how they’re implemented, or is the string inserted into the new input context as something other than a function_call_output?

No. You basically call a function to temporarily or permanently extend the base prompt. But of course you can think of other patterns to do more interesting things depending on your use-case. The prompt selection is a RAG.