diff --git a/docs/PRD.md b/docs/PRD.md
index 02d8e89..d4e60bf 100644
--- a/docs/PRD.md
+++ b/docs/PRD.md
@@ -83,7 +83,7 @@ lanes:
 watchdog:
   interval: 60
 openbrain:
-  url: https://brain.woltje.com
+  url: ${OPENBRAIN_URL}  # required — no default
   autoCapture: false
 ```