diff --git a/openspec/specs/architecture.md b/openspec/specs/architecture.md
index ff71f5e..677e3b3 100644
--- a/openspec/specs/architecture.md
+++ b/openspec/specs/architecture.md
@@ -33,3 +33,11 @@ Implementation notes
 
 Decisions
 - <record cross‑cutting decisions here>
+
+Client platform decisions
+- Mobile: React Native + Expo (TypeScript).
+  Rationale: strong a11y ecosystem, mature tooling, and DX.
+  Notes: Expo modules; E2E via Detox; push via Expo/FCM/APNs.
+
+Decisions
+- 2025-11-17 — Mobile stack: React Native + Expo.