diff --git a/docs/impl/plans/INITIAL.md b/docs/impl/plans/INITIAL.md
index 6a561bd..b15b0ce 100644
--- a/docs/impl/plans/INITIAL.md
+++ b/docs/impl/plans/INITIAL.md
@@ -45,3 +45,12 @@ the recording, on the same clock as the CAN frames, turns the Phase 1 manual eve
 log from a notebook-and-wristwatch exercise into structured data. It is what makes
 a raw capture decodable later, and it is a Phase 1 capability that happens to live
 in this repo.
+
+## Mock data must look like mock data
+
+`svc-gw` can synthesise vehicle state without any real readings, which is how this
+app is developed with no Jetson present. The stream carries a flag when it does.
+
+Rendering synthetic readings indistinguishable from real ones, on a screen in a
+moving car, is a safety problem. The indication belongs on the display surface
+itself, not in a settings page or a startup log.