YMMV but upgrading Astro 5 to Astro 6 has some breaking changes (that the @astro team has documented for you! https://docs.astro.build/en/guides/upgrade-to/v6/ )

for my relatively simple site with just a few content collections, the fixes were minimal. after Claude Code and i fixed one of my sites, i asked CC to document the changes for ... themself, in my other sites. so they created this, i copied into my other projects to help CC fix the same things there.

(it's a weird time we live in)

https://github.com/mikesusz/mikesusz.dev/blob/main/ASTRO6_UPGRADE.md