diff --git a/.planning/debug/phase-12-cloud-schema-cold-start.md b/.planning/debug/phase-12-cloud-schema-cold-start.md
new file mode 100644
index 0000000..2297da5
--- /dev/null
+++ b/.planning/debug/phase-12-cloud-schema-cold-start.md
@@ -0,0 +1,49 @@
+# Debug: Phase 12 Cloud Schema Cold Start
+
+**Status:** root cause found
+**Date:** 2026-06-19
+**UAT tests:** 1, 2
+
+## Symptoms
+
+- `GET /api/cloud/connections` raises `psycopg.errors.UndefinedColumn` for `cloud_connections.display_name_override`.
+- `POST /api/cloud/connections/webdav` reaches `_upsert_cloud_connection` and raises the same error, preventing Nextcloud account connection.
+
+## Root Cause
+
+The running application code and ORM are at Phase 12, but the live PostgreSQL schema remains at Alembic revision `0005`.
+
+Live evidence:
+
+```text
+$ docker compose run --rm backend alembic current
+0005
+```
+
+Migration `backend/migrations/versions/0006_cloud_resource_foundation.py` correctly adds `cloud_connections.display_name_override` and the Phase 12 cloud metadata tables. The ORM correctly maps that column. The defect is that `docker-compose.yml` has no migration service and the backend command starts Uvicorn directly. Backend, Celery worker, and Celery beat depend on PostgreSQL health, but none depends on `alembic upgrade head` completing. Therefore `docker compose up` can run new application code against an old persistent database.
+
+## Why Automated Tests Missed It
+
+- Most backend tests build schema directly from `Base.metadata.create_all`, which validates the final ORM shape but bypasses Alembic history and deployment ordering.
+- Existing Alembic tests target early migrations and do not exercise an existing PostgreSQL database upgraded from `0005` to `head`.
+- Phase verification checked migration file structure and model parity, not a real cold-start Compose upgrade path.
+
+## Files Involved
+
+- `docker-compose.yml` — no one-shot migration service; backend/workers start after DB health only.
+- `backend/migrations/versions/0006_cloud_resource_foundation.py` — contains the required column and tables but was not applied.
+- `backend/db/models.py` — queries `display_name_override`, exposing schema drift immediately.
+- `backend/tests/test_alembic.py` — lacks a 0005-to-head PostgreSQL upgrade regression.
+- `.planning/phases/12-cloud-resource-foundation/12-VALIDATION.md` — cold-start test expected migration completion, but execution evidence did not actually verify Compose migration orchestration.
+
+## Required Fix Direction
+
+1. Add a one-shot Compose migration service that runs `alembic upgrade head` with `DATABASE_MIGRATE_URL` after PostgreSQL becomes healthy.
+2. Make backend, Celery worker, and Celery beat wait for that migration service to complete successfully.
+3. Add a PostgreSQL/Compose regression proving an existing revision `0005` advances to `0006/head` before the API starts, and assert `display_name_override`, `cloud_items`, `cloud_item_topics`, and `cloud_folder_states` exist.
+4. Document the migration lifecycle and recovery command in README/RUNBOOK.
+5. For the currently running environment, apply `alembic upgrade head` and restart application processes before resuming UAT.
+
+## Scope
+
+Both UAT blockers share this root cause. No evidence currently indicates a separate Nextcloud credential or WebDAV defect.
diff --git a/.planning/phases/12-cloud-resource-foundation/12-UAT.md b/.planning/phases/12-cloud-resource-foundation/12-UAT.md
index df5d2c1..c8ff806 100644
--- a/.planning/phases/12-cloud-resource-foundation/12-UAT.md
+++ b/.planning/phases/12-cloud-resource-foundation/12-UAT.md
@@ -1,5 +1,5 @@
 ---
-status: partial
+status: diagnosed
 phase: 12-cloud-resource-foundation
 source:
   - 12-01-SUMMARY.md
@@ -68,16 +68,29 @@ blocked: 4
   reason: "User reported: GET /api/cloud/connections raises psycopg.errors.UndefinedColumn because cloud_connections.display_name_override does not exist in the live PostgreSQL schema."
   severity: blocker
   test: 1
-  root_cause: ""
-  artifacts: []
-  missing: []
-  debug_session: ""
+  root_cause: "The live database is at Alembic revision 0005 because docker-compose.yml starts backend/workers directly and has no migration service or dependency that runs alembic upgrade head. ORM metadata expects revision 0006."
+  artifacts:
+    - path: "docker-compose.yml"
+      issue: "No migration service; backend and Celery services depend only on PostgreSQL health."
+    - path: "backend/migrations/versions/0006_cloud_resource_foundation.py"
+      issue: "Required migration exists but is not applied during Compose startup."
+    - path: "backend/tests/test_alembic.py"
+      issue: "No PostgreSQL regression covering upgrade from revision 0005 to head."
+  missing:
+    - "Run alembic upgrade head before backend and worker startup."
+    - "Add cold-start migration regression and deployment documentation."
+  debug_session: ".planning/debug/phase-12-cloud-schema-cold-start.md"
 - truth: "A user can connect Nextcloud accounts and each connected account appears as an independently renameable cloud root."
   status: failed
   reason: "User reported: Cannot connect Nextcloud test users; POST /api/cloud/connections/webdav fails because cloud_connections.display_name_override does not exist."
   severity: blocker
   test: 2
-  root_cause: ""
-  artifacts: []
-  missing: []
-  debug_session: ""
+  root_cause: "Same schema-drift root cause as Test 1: Nextcloud connection creation queries the Phase 12 ORM while PostgreSQL remains at revision 0005."
+  artifacts:
+    - path: "docker-compose.yml"
+      issue: "No migration gate before the cloud connection API starts."
+    - path: "backend/api/cloud/connections.py"
+      issue: "Correctly queries CloudConnection, which exposes the unapplied schema immediately."
+  missing:
+    - "Apply revision 0006/head before accepting cloud connection requests."
+  debug_session: ".planning/debug/phase-12-cloud-schema-cold-start.md"