diff --git a/services/gateway/app.py b/services/gateway/app.py
index 2f7f1a4..ea0d520 100644
--- a/services/gateway/app.py
+++ b/services/gateway/app.py
@@ -155,7 +155,11 @@ async def submit_transaction(body: TransactionRequest):
     finally:
         transaction_duration.record(time.perf_counter() - start, {"kind": body.kind})
         currency = body.currency if body.currency in KNOWN_CURRENCIES else "other"
-        transactions_total.add(1, {"kind": body.kind, "status": status, "currency": currency})
+        # Phase 5 step 4: deliberate cardinality explosion. Revert after the experiment.
+        transactions_total.add(
+            1,
+            {"kind": body.kind, "status": status, "currency": currency, "account_id": body.account_id},
+        )
     return JSONResponse(status_code=201, content={**result, "request_id": request_id})