diff --git a/docs/superpowers/specs/2026-05-27-nexacloud-billing-importer-design.md b/docs/superpowers/specs/2026-05-27-nexacloud-billing-importer-design.md
index 8eb222d0..1c171004 100644
--- a/docs/superpowers/specs/2026-05-27-nexacloud-billing-importer-design.md
+++ b/docs/superpowers/specs/2026-05-27-nexacloud-billing-importer-design.md
@@ -183,6 +183,13 @@ until this is granted.
 - Importing historical NexaCloud invoices / `usage_records` (2d reads NexaCloud actuals).
 - Add-ons (`deployment_addons`) as recurring lines — revisit if material.
 
+> **Flip-day note (carry into 2b):** the inbound `/usage` API resolves a subscription by
+> its **Odoo integer id** (`int(subscription_external_id)`), but imported shadow subs are
+> keyed by NexaCloud's UUID in `x_fc_nexacloud_subscription_id`. Before NexaCloud can push
+> usage (2b), decide how it learns the Odoo id (return the mapping from the importer, or
+> extend the usage API to also resolve by `x_fc_nexacloud_subscription_id`). Not a 2a bug
+> (2a is read-only), but it must be resolved before the flip.
+
 ## 11. Verify at implementation (do NOT code from memory — CLAUDE rule #1)
 
 Confirm on odoo-trial Enterprise before relying on them: