diff --git a/CLAUDE.md b/CLAUDE.md
index 6e456f3b..a37424d1 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -232,3 +232,41 @@ catches undefined names instantly.
   open the systray helpdesk dialog. The Mine/All toggle appears for the owner; "All" shows
   all 50 ENTECH tickets, "Mine" shows the count matching the owner's profile email.
   Tracebacks live in `/var/log/odoo/odoo-server.log` on entech (LXC 111 / pve-worker5).
+
+## Fusion Centralized Billing (`fusion_centralize_billing`) — engine + test harness
+
+Odoo (`odoo-nexa`, live DB `nexamain`) is being made the single billing brain for every
+NexaSystems app (NexaCloud, NexaDesk/Fusion-Chat, NexaMaps), **superseding Lago**. The
+module adds only the metering + integration layer (service registry, identity links,
+metric/charge catalog, aggregate-push usage engine, inbound Lago-shaped REST API at
+`/api/billing/v1/*`, outbound HMAC webhooks, dual-run reconciliation); all financial
+behaviour is native Odoo **Enterprise** (`sale_subscription` + `payment_stripe` +
+`account_accountant`). Design + rollout live in `docs/superpowers/specs/`
+(`2026-05-27-nexa-billing-centralized-design.md` = architecture;
+`2026-06-02-nexacloud-odoo-billing-cutover-design.md` = NexaCloud pilot: build → import →
+dual-run → gated flip) and `docs/superpowers/plans/`.
+
+**Testing it — NOT on local `odoo-modsdev` (community) and NEVER `-u` against live `nexamain`.**
+It needs Enterprise deps, so tests run on `odoo-nexa` in an **isolated throwaway container**
+against a **fresh** DB with the Canadian localization:
+```
+ssh odoo-nexa
+# fresh DB (inside odoo-nexa-db): dropdb --if-exists fcb_test; createdb fcb_test
+cp -a /opt/odoo/custom-addons /opt/odoo/custom-addons-staging   # edit/sync HERE, never the live module dir
+docker run --rm --network odoo_odoo-network \
+  -v /opt/odoo/custom-addons-staging:/mnt/extra-addons:ro -v /opt/odoo/enterprise-addons:/mnt/enterprise-addons:ro \
+  -v /opt/odoo/odoo.conf:/etc/odoo/odoo.conf:ro -v /opt/odoo/staging-data:/var/lib/odoo \
+  odoo-nexa:19 -c /etc/odoo/odoo.conf -d fcb_test --db_host=db --db_user=odoo \
+  --addons-path=/usr/lib/python3/dist-packages/odoo/addons,/mnt/extra-addons,/mnt/enterprise-addons \
+  --without-demo=all --test-enable --test-tags /fusion_centralize_billing \
+  -i l10n_ca,fusion_centralize_billing --stop-after-init --no-http
+```
+Iterate with `-u fusion_centralize_billing` (reuse fcb_test). Gotchas that cost hours:
+- **`l10n_ca` is required** — the ledger tests need a Canadian CoA + active CAD + 13% HST.
+- A **prod clone is the wrong base** — its existing rows collide with fixed-code test fixtures
+  (`nexacloud` service / `cpu_seconds` metric) across 5 test files.
+- odoo.conf sets `log_level=warn`, so **passing tests log nothing** — exit 0 alone does NOT
+  prove tests ran (a tag matching zero tests is also exit 0). Confirm execution with
+  `--log-handler=odoo.addons.fusion_centralize_billing.tests:INFO` (look for `Starting
+  <Class>.<method>`). The **exit code is authoritative** (1 on any failure).
+- Do **NOT** pass `--workers=0` (blanks captured stdout) or `--logfile=/dev/stdout` (errors out).