diff --git a/server/src/nest/database/database.service.ts b/server/src/nest/database/database.service.ts
index 0d18ea549..103f4c8e1 100644
--- a/server/src/nest/database/database.service.ts
+++ b/server/src/nest/database/database.service.ts
@@ -56,6 +56,25 @@ export class DatabaseService {
     return isOwner(tripId, userId);
   }
 
+  /**
+   * The user ids a trip may refer to: its members plus the owner. Guests count —
+   * a guest is a credential-less users row joined into trip_members, and #1362
+   * makes it assignable everywhere a real member is.
+   *
+   * This is canAccessTrip's question asked as a set rather than a predicate, so
+   * it lives beside it. Domains that accept user ids in a write body intersect
+   * against it: holding the permission says you may edit the trip, not that any
+   * id you name belongs to it. Runs on the connection rather than delegating to
+   * db/database, because it has no legacy export to stay parity with.
+   */
+  rosterUserIds(tripId: number | string): Set<number> {
+    const rows = this.all<{ user_id: number }>(
+      'SELECT user_id FROM trip_members WHERE trip_id = ? UNION SELECT user_id FROM trips WHERE id = ?',
+      tripId, tripId,
+    );
+    return new Set(rows.map(r => r.user_id));
+  }
+
   getPlaceWithTags(placeId: number | string): PlaceWithTags | null {
     return getPlaceWithTags(placeId);
   }
diff --git a/server/src/nest/packing/packing.service.ts b/server/src/nest/packing/packing.service.ts
index 1fef2f8a3..01c169ccb 100644
--- a/server/src/nest/packing/packing.service.ts
+++ b/server/src/nest/packing/packing.service.ts
@@ -231,7 +231,8 @@ export class PackingService {
       // "Shared with specific people" — record the recipients it covers.
       if (data.visibility === 'shared' && Array.isArray(data.recipient_ids)) {
         const ins = this.db.prepare('INSERT OR IGNORE INTO packing_item_recipients (item_id, user_id) VALUES (?, ?)');
-        for (const uid of data.recipient_ids) if (uid !== ownerId) ins.run(id, uid);
+        const roster = this.tripRosterIds(tripId);
+        for (const uid of data.recipient_ids) if (uid !== ownerId && roster.has(uid)) ins.run(id, uid);
       }
       return id;
     });
@@ -350,7 +351,8 @@ export class PackingService {
       if (visibility === 'shared') {
         const ins = this.db.prepare('INSERT OR IGNORE INTO packing_item_recipients (item_id, user_id) VALUES (?, ?)');
         const owner = item.owner_id ?? actingUserId;
-        for (const uid of recipientIds) if (uid !== owner) ins.run(id, uid);
+        const roster = this.tripRosterIds(tripId);
+        for (const uid of recipientIds) if (uid !== owner && roster.has(uid)) ins.run(id, uid);
       }
       // Leaving the Common tier drops any co-contributors (they only apply to Common).
       if (visibility !== 'common') this.db.run('DELETE FROM packing_item_contributors WHERE item_id = ?', id);
@@ -479,10 +481,14 @@ export class PackingService {
     }));
   }
 
-  /** Owner + collaborators of a trip — the only user ids that may be assigned to a bag. */
+  /**
+   * Owner + collaborators of a trip, guests included — the only user ids
+   * assignable anywhere on it, not just to a bag. The wording used to say
+   * "assigned to a bag", which is why the category and recipient writes below
+   * grew up without it.
+   */
   private tripRosterIds(tripId: string | number): Set<number> {
-    const rows = this.db.all<{ user_id: number }>('SELECT user_id FROM trip_members WHERE trip_id = ? UNION SELECT user_id FROM trips WHERE id = ?', tripId, tripId);
-    return new Set(rows.map(r => r.user_id));
+    return this.db.rosterUserIds(tripId);
   }
 
   setBagMembers(tripId: string | number, bagId: string | number, userIds: number[]) {
@@ -665,7 +671,9 @@ export class PackingService {
 
       if (Array.isArray(userIds) && userIds.length > 0) {
         const insert = this.db.prepare('INSERT OR IGNORE INTO packing_category_assignees (trip_id, category_name, user_id) VALUES (?, ?, ?)');
-        for (const uid of userIds) insert.run(tripId, categoryName, uid);
+        // Same rule as setBagMembers: only people on this trip may be assigned.
+        const roster = this.tripRosterIds(tripId);
+        for (const uid of userIds) if (roster.has(uid)) insert.run(tripId, categoryName, uid);
       }
     });
 
diff --git a/server/src/nest/todo/todo.service.ts b/server/src/nest/todo/todo.service.ts
index 8d2025ba0..31bf80953 100644
--- a/server/src/nest/todo/todo.service.ts
+++ b/server/src/nest/todo/todo.service.ts
@@ -136,7 +136,12 @@ export class TodoService {
 
       if (Array.isArray(userIds) && userIds.length > 0) {
         const insert = this.db.prepare('INSERT OR IGNORE INTO todo_category_assignees (trip_id, category_name, user_id) VALUES (?, ?, ?)');
-        for (const uid of userIds) insert.run(tripId, categoryName, uid);
+        // Only people on this trip may be assigned, the way packing filters bag
+        // members and reservations filter travellers. Dropped rather than
+        // rejected: a copied trip carries assignee ids across before its members
+        // exist, and a 400 would make the picker unusable there.
+        const roster = this.db.rosterUserIds(tripId);
+        for (const uid of userIds) if (roster.has(uid)) insert.run(tripId, categoryName, uid);
       }
     });
 
diff --git a/server/tests/unit/nest/packing.service.test.ts b/server/tests/unit/nest/packing.service.test.ts
index 44571e593..649db5838 100644
--- a/server/tests/unit/nest/packing.service.test.ts
+++ b/server/tests/unit/nest/packing.service.test.ts
@@ -515,6 +515,7 @@ describe('three-tier packing sharing (#858)', () => {
     const { user: friend } = createUser(testDb);
     const { user: stranger } = createUser(testDb);
     const trip = createTrip(testDb, owner.id);
+    addTripMember(testDb, trip.id, friend.id);
 
     const item = svc.createItem(trip.id, { name: 'Power bank', visibility: 'shared', recipient_ids: [friend.id] }, owner.id) as any;
     expect(item.is_private).toBe(1);
@@ -539,6 +540,7 @@ describe('three-tier packing sharing (#858)', () => {
     const { user: owner } = createUser(testDb);
     const { user: friend } = createUser(testDb);
     const trip = createTrip(testDb, owner.id);
+    addTripMember(testDb, trip.id, friend.id);
     const item = svc.createItem(trip.id, { name: 'First aid', visibility: 'personal' }, owner.id) as any;
 
     // A non-owner who cannot see the item at all gets the missing-item answer, so
@@ -1007,6 +1009,8 @@ describe('packing item object-level authorization', () => {
     const { user: intruder } = createUser(testDb);
     const { user: friend } = createUser(testDb);
     const trip = createTrip(testDb, owner.id);
+    // The recipient is a fellow traveller; the intruder stays off the trip on purpose.
+    addTripMember(testDb, trip.id, friend.id);
     const personal = svc.createItem(trip.id, { name: 'Diary', visibility: 'personal' }, owner.id) as any;
     const shared = svc.createItem(trip.id, { name: 'Power bank', visibility: 'shared', recipient_ids: [friend.id] }, owner.id) as any;
     const common = svc.createItem(trip.id, { name: 'Tent', visibility: 'common' }, owner.id) as any;
diff --git a/server/tests/unit/nest/todo.service.test.ts b/server/tests/unit/nest/todo.service.test.ts
index b52e7931a..b3fcc351e 100644
--- a/server/tests/unit/nest/todo.service.test.ts
+++ b/server/tests/unit/nest/todo.service.test.ts
@@ -270,6 +270,35 @@ describe('getCategoryAssignees / updateCategoryAssignees', () => {
     expect(assignees['Logistics']).toHaveLength(1);
   });
 
+  // Holding todo_edit says you may edit this trip's list, not that any user id
+  // you name belongs to it. The read-back joins users and hands the caller a
+  // display name and an avatar, so an unfiltered id turns the endpoint into a
+  // lookup for every account on the instance.
+  it('TODO-SVC-017a: drops a user who is not on the trip, and says nothing about them', () => {
+    const { user: owner } = createUser(testDb);
+    const { user: member } = createUser(testDb);
+    const { user: stranger } = createUser(testDb);
+    const trip = createTrip(testDb, owner.id);
+    addTripMember(testDb, trip.id, member.id);
+
+    const rows = svc.updateCategoryAssignees(trip.id, 'Packing', [owner.id, stranger.id, member.id]) as { user_id: number }[];
+
+    expect(rows.map(r => r.user_id).sort()).toEqual([owner.id, member.id].sort());
+    expect(JSON.stringify(rows)).not.toContain(stranger.username);
+    expect(testDb.prepare('SELECT COUNT(*) AS n FROM todo_category_assignees WHERE user_id = ?').get(stranger.id)).toEqual({ n: 0 });
+  });
+
+  it('TODO-SVC-017b: keeps a guest, who is a trip member like any other', () => {
+    const { user: owner } = createUser(testDb);
+    const { user: guest } = createUser(testDb);
+    testDb.prepare('UPDATE users SET is_guest = 1 WHERE id = ?').run(guest.id);
+    const trip = createTrip(testDb, owner.id);
+    addTripMember(testDb, trip.id, guest.id);
+
+    const rows = svc.updateCategoryAssignees(trip.id, 'Packing', [guest.id]) as { user_id: number }[];
+    expect(rows.map(r => r.user_id)).toEqual([guest.id]);
+  });
+
   it('TODO-SVC-020: updateCategoryAssignees replaces existing assignees (not append)', () => {
     const { user: owner } = createUser(testDb);
     const { user: member } = createUser(testDb);