> If you define "hallucinations" to mean "any mistakes at all" then yes, a compiler won't catch them for you.

That's not quite my definition. If we're judging these tools by the same criteria we use to judge human programmers, then mistakes and bugs should be acceptable. I'm fine with this to a certain extent, even though these tools are being marketed as having superhuman abilities. But the problem is that LLMs create an entirely unique class of issues that most humans don't. Using nonexistent APIs is just one symptom of it. Like I mentioned in the comment below, they might hallucinate requirements that were never specified, or fixes for bugs that don't exist, all the while producing code that compiles and runs without errors.

But let's assume that we narrow down the definition of hallucination to usage of nonexistent APIs. Your proposed solution is to feed the error back to the LLM. Great, but can you guarantee that the proposed fix will also not contain hallucinations? As I also mentioned, in most occasions when I've done this the LLM simply produces more hallucinated code, and I get stuck in a neverending loop where the only solution is for me to dig into the code and fix the issue myself. So the LLM simply wastes my time in these cases.

> The new Phoenix.new coding agent actively tests the web applications it is writing using a headless browser

That's great, but can you trust that it will cover all real world usage scenarios, test edge cases and failure scenarios, and do so accurately? Tests are code as well, and it can have the same issues as application code.

I'm sure that we can continue to make these tools more useful by working around these issues and using better adjacent tooling as mitigation. But the fundamental problem of hallucinations still needs to be solved. Mainly because it affects tasks other than code generation, where it's much more difficult to deal with.

I think the more general/useful definition of "hallucination" is anytime the LLM predicts next word based on "least worst" (statistically) choice rather than based on any closely matching samples in the training data.

The LLM has to generate some word each time it is called, and unless it recognizes soon enough that "I don't know" is the best answer (in of itself problematic, since any such prediction would be based on the training data, not the LLM's own aggregate knowledge!), then it may back itself into a corner where it has no well-grounded continuation, but nonetheless has to spit out the statistically best prediction, even if that is a very bad ungrounded prediction such as a non-existent API, "fits the profile" concocted answer, or anything else ...

Of course the LLM's output builds on itself, so any ungrounded/hallucinated output doesn't need to be limited to a single word or API call, but may instead consist of a whole "just trying my best" sentence or chunk of code (better hope you have unit test code coverage to test/catch it).