Batch Contact Related

Sequences and/or lists for up to 100 contacts in ONE request. Writes nothing.

Use this instead of looping GET /contacts/{contact_id}/sequences or /lists per contact: the loop spends one rate-limit slot per contact and hits 429 on a large book; this spends one per 100.

Each contact gets exactly what its per-contact route returns, through the same access check. A contact that does not exist or is outside the key's scope for any requested include comes back found: false with no related records, rather than failing the batch. POST only because the ids are a body; it is tiered as a read.