@mauve Object IDs can't be relative. The spec requires them to be globally unique identifiers / publicly dereferencable URIs:
https://www.w3.org/TR/activitypub/#obj-id
I would use url property to provide a secondary ID which can be relative. See this FEP for examples: https://codeberg.org/fediverse/fep/src/branch/main/fep/fffd/fep-fffd.md