Sonnet 4 and gpt5 are both good at SvelteKit. You might have to specify that you want to use Svelte 5 in the llm instructions for your editor.
Use Context7 mcp to get Svelte 5 docs if the agent messes up something you know should work.
The new experimental remote functions solved a big pain point with sveltekit, so I no longer have any reasons not to recommend svelte and sveltekit to people.