diff --git a/server/api/members/update-contribution.post.js b/server/api/members/update-contribution.post.js
index fe47d7b..3e2c071 100644
--- a/server/api/members/update-contribution.post.js
+++ b/server/api/members/update-contribution.post.js
@@ -203,6 +203,10 @@ export default defineEventHandler(async (event) => {
       }
 
       const memberCadence = member.billingCadence || 'monthly';
+      // TODO: Cadence-switch UI on /member/account. Plain Helcim subscription
+      // updates can't change billing period — would need a sub-replacement flow
+      // (cancel current, create new at desired cadence). See
+      // docs/LAUNCH_READINESS.md "Known gotchas" → "Cadence switch rejected".
       if (body.cadence && body.cadence !== memberCadence) {
         throw createError({
           statusCode: 400,