diff --git a/.planning/ROADMAP.md b/.planning/ROADMAP.md
index 1727423..5da6c85 100644
--- a/.planning/ROADMAP.md
+++ b/.planning/ROADMAP.md
@@ -140,3 +140,14 @@ Note: Phase 4 depends only on Phase 1 and can begin as soon as Phase 1 is comple
 | 3. Event Write-Back + PWA Install | 0/? | Not started | - |
 | 4. Shared Lists + Live Sync | 0/? | Not started | - |
 | 5. Web Push Notifications | 0/? | Not started | - |
+
+## Backlog
+
+### Phase 999.1: Treat Fastmail as one calendar provider; framework supports adding more providers (BACKLOG)
+
+**Goal:** [Captured for future planning] Abstract the calendar backend behind a provider interface so Fastmail/CalDAV is one implementation among potentially many. Shipping with a single provider is fine, but the broker, sync, and event-expansion layers should be structured so additional providers (e.g. other CalDAV hosts, Google Calendar, generic ICS feeds) can be added without rework. Captures the "provider" seam as an explicit architectural concern.
+**Requirements:** TBD
+**Plans:** 0 plans
+
+Plans:
+- [ ] TBD (promote with /gsd-review-backlog when ready)
diff --git a/.planning/phases/999.1-treat-fastmail-as-a-provider-support-more-calendar-providers/.gitkeep b/.planning/phases/999.1-treat-fastmail-as-a-provider-support-more-calendar-providers/.gitkeep
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