OK

Bad Request. Each cause returns its own message: ruleIds missing or empty, any entry not a valid rule id, or more than 100 ruleIds in one call. The batch size is checked before per-id validity, so an over-cap request reports the cap even if it also contains a malformed id.

Forbidden

Not Found. Returned when groupId does not match an existing group and is not the literal "UNASSIGNED".

Internal Server Error. On the partial-assign message this is a PARTIAL result: some rules in the batch were already assigned and their history events already published. That body carries updated/notFound/failed/groupId alongside the message, so the partial state is reported rather than left to be discovered - failed is exactly the set to retry. The writes are idempotent, so retrying the whole batch is also safe and is the intended recovery. The group-validation message is message-only: nothing was written.