Yes, there are two kinds of evidence. Firstly, there is behavioral evidence. This is, to me, the less compelling kind. But it's important to understand. You ar

Yes, there are two kinds of evidence. Firstly, there is behavioral evidence. This is, to me, the less compelling kind. But it's important to understand. You are of course correct that, once Cluade has said "An", it will be inclined to say something starting with a vowel. But the mystery is really why, in setups like these, Claude is much more likely to say "An" than "A" in the first place. Regardless of what the underlying mechanism is -- and you could maybe imagine ways in which it could just "pattern match" without planning here -- it is preferred because in situations like this, you need to say "An" so that "astronomer" can follow. But now we also have mechanistic evidence. If you make an attribution graph, you can literally see an astronomer feature fire, and that cause it to say "An". We didn't publish this example, but you can see a more sophisticated version of this in the poetry planning section - https://transformer-circuits.pub/2025/attribution-graphs/biology.html#dives-poems https://transformer-circuits.pub/2025/attribution-graphs/bio...