diff --git a/prd.json b/prd.json
index 429b4ac..4f9ad08 100644
--- a/prd.json
+++ b/prd.json
@@ -232,7 +232,7 @@
         "Typecheck passes"
       ],
       "priority": 13,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {
diff --git a/progress.txt b/progress.txt
index a9bc7d7..2fdeb52 100644
--- a/progress.txt
+++ b/progress.txt
@@ -43,6 +43,8 @@
 - `ChoiceOption` type includes optional `condition?: Condition`. When counting variable usage, check variable nodes + edge conditions + choice option conditions.
 - React Compiler lint forbids `setState` in effects and reading `useRef().current` during render. Use `useState(() => computeValue())` lazy initializer pattern for one-time initialization logic.
 - For detecting legacy data shape (pre-migration), pass a flag from the server component (page.tsx) to the client component, since only the server reads raw DB data.
+- Collaboration tables: `project_collaborators` (roles), `collaboration_sessions` (presence), `audit_trail` (history) — all with RLS scoped by project ownership or collaborator membership
+- RLS pattern for shared resources: check `projects.user_id = auth.uid()` OR `project_collaborators.user_id = auth.uid()` to cover both owners and collaborators
 
 ---
 
@@ -198,3 +200,17 @@
   - Pre-existing lint issues in `ConditionEditor.tsx` and `OptionConditionEditor.tsx` (React Compiler `preserve-manual-memoization` errors) are from prior stories and not related to this change.
   - No browser testing tools are available; manual verification is needed.
 ---
+
+## 2026-01-23 - US-043
+- What was implemented: Database migration adding project_collaborators, collaboration_sessions, and audit_trail tables with RLS policies and indexes
+- Files changed:
+  - `supabase/migrations/20260123100000_add_collaboration_and_audit_trail.sql` - New migration with three tables, RLS policies, indexes, and updated projects RLS for collaborator access
+- **Learnings for future iterations:**
+  - `project_collaborators` has a UNIQUE constraint on (project_id, user_id) to prevent duplicate invitations
+  - RLS policies for collaboration tables use subqueries to check either project ownership (via `projects.user_id`) or collaboration membership (via `project_collaborators.user_id`)
+  - The audit_trail insert policy requires both `auth.uid() = user_id` AND project access (owner or editor role) to prevent unauthorized audit writes
+  - New RLS policies were added to the existing `projects` table to allow collaborators to SELECT and UPDATE (editors/owners only) shared projects
+  - The audit_trail index uses `created_at DESC` for efficient reverse-chronological pagination in the history sidebar
+  - `collaboration_sessions.cursor_position` is JSONB to store flexible coordinate data (x, y, and potentially viewport info)
+  - `collaboration_sessions.selected_node_id` is nullable text since a user may not have any node selected
+---