That's a fun variation. The one I test out models with is usually a vegetarian wolf and a carnivorous goat, but the variation to no other objects is an interesting one too.
By the way, here's Claude 3 Opus's answer:
The solution is quite simple:
The man gets into the boat and rows himself and the goat across the river to the other side.
Once they reach the other side, both the man and the goat get out of the boat.
And that's it! Since there are no additional constraints or complications mentioned in the problem, the man and the goat can directly cross the river together using the boat.
I got your exact same answer, but then I tried again prefacing the question with:
Can you help me with a puzzle?
This changed the answer drastically:
Great! Let's solve this river crossing puzzle step by step. The key is to find a sequence of actions that gets the man and the goat safely across the river without leaving the goat unattended with something it might eat.
Given:
A man and a goat are on one side of a river.
They have a boat.
Step-by-step solution:
The man takes the goat in the boat and crosses the river.
The man leaves the goat on the other side of the river and returns alone in the boat.
The man gets out of the boat on the original side of the river.
The man gets back into the boat alone and crosses the river again.
The man and the goat are now together on the other side of the river.
Therefore, the man and the goat have successfully crossed the river using the boat, without leaving the goat unattended on either side of the river.