diff --git a/server/api/lists.get.ts b/server/api/lists.get.ts
index 59c2d1b..da6fc4c 100644
--- a/server/api/lists.get.ts
+++ b/server/api/lists.get.ts
@@ -1,4 +1,5 @@
-// GET /api/lists (web) — all lists.
-export default defineEventHandler(async () => {
-  return getLists(getSql());
+// GET /api/lists (web) — the caller's lists (plus legacy unowned).
+export default defineEventHandler(async (event) => {
+  const rows = await getLists(getSql(), ownerOf(event));
+  return rows.map((r) => ({ id: r.id, name: r.name, ownerId: r.owner_id }));
 });
diff --git a/server/api/lists.put.ts b/server/api/lists.put.ts
index 7da538c..fa7be69 100644
--- a/server/api/lists.put.ts
+++ b/server/api/lists.put.ts
@@ -1,4 +1,6 @@
-// PUT /api/lists (desktop) — full-replace catalog. Upsert all supplied; delete the rest.
+// PUT /api/lists (desktop) — full-replace of the caller's catalog. Upsert all supplied;
+// delete the caller's lists not present. Any client-supplied ownerId is ignored — the
+// server stamps ownership from the verified token.
 export default defineEventHandler(async (event) => {
   const body = await readBody(event);
   if (
@@ -9,6 +11,7 @@ export default defineEventHandler(async (event) => {
   }
   await replaceLists(
     getSql(),
+    ownerOf(event),
     body.map((l) => ({ id: l.id, name: l.name })),
   );
   return { ok: true };
diff --git a/server/api/lists/[id]/tasks.get.ts b/server/api/lists/[id]/tasks.get.ts
index 7255355..2686ca2 100644
--- a/server/api/lists/[id]/tasks.get.ts
+++ b/server/api/lists/[id]/tasks.get.ts
@@ -1,10 +1,11 @@
-// GET /api/lists/:id/tasks (web) — Idle tasks for a list. 404 if the list is unknown.
+// GET /api/lists/:id/tasks (web) — the caller's Idle tasks for a list. 404 if the list is unknown (to the caller).
 export default defineEventHandler(async (event) => {
   const id = getRouterParam(event, "id")!;
+  const ownerId = ownerOf(event);
   const sql = getSql();
-  if (!(await listExists(sql, id))) {
+  if (!(await listExists(sql, ownerId, id))) {
     throw createError({ statusCode: 404, statusMessage: "list not found" });
   }
-  const rows = await getTasksForList(sql, id);
+  const rows = await getTasksForList(sql, ownerId, id);
   return rows.map(toTaskDto);
 });
diff --git a/server/api/tasks.get.ts b/server/api/tasks.get.ts
index d237ff5..5f4b1bb 100644
--- a/server/api/tasks.get.ts
+++ b/server/api/tasks.get.ts
@@ -1,11 +1,12 @@
-// GET /api/tasks?consumed=false (desktop) — web-created tasks not yet imported.
-export default defineEventHandler(async () => {
-  const rows = await getUnconsumed(getSql());
+// GET /api/tasks?consumed=false (desktop) — the caller's web-created tasks not yet imported.
+export default defineEventHandler(async (event) => {
+  const rows = await getUnconsumed(getSql(), ownerOf(event));
   return rows.map((r) => ({
     id: r.id,
     listId: r.list_id,
     title: r.title,
     description: r.description,
+    ownerId: r.owner_id,
     createdAt: new Date(r.created_at).toISOString(),
   }));
 });
diff --git a/server/api/tasks.post.ts b/server/api/tasks.post.ts
index 523a875..b695850 100644
--- a/server/api/tasks.post.ts
+++ b/server/api/tasks.post.ts
@@ -1,4 +1,4 @@
-// POST /api/tasks (web) — create an Idle task with a server-generated GUID.
+// POST /api/tasks (web) — create an Idle task with a server-generated GUID, owned by the caller.
 export default defineEventHandler(async (event) => {
   const body = await readBody(event);
   const title = typeof body?.title === "string" ? body.title.trim() : "";
@@ -9,12 +9,13 @@ export default defineEventHandler(async (event) => {
   const description =
     typeof body?.description === "string" && body.description.trim() ? body.description : null;
 
+  const ownerId = ownerOf(event);
   const sql = getSql();
-  if (!(await listExists(sql, listId))) {
+  if (!(await listExists(sql, ownerId, listId))) {
     throw createError({ statusCode: 404, statusMessage: "list not found" });
   }
 
-  const row = await createWebTask(sql, { listId, title, description });
+  const row = await createWebTask(sql, ownerId, { listId, title, description });
   setResponseStatus(event, 201);
   return toTaskDto(row);
 });
diff --git a/server/api/tasks/[id].delete.ts b/server/api/tasks/[id].delete.ts
index d4da7e7..afc48c6 100644
--- a/server/api/tasks/[id].delete.ts
+++ b/server/api/tasks/[id].delete.ts
@@ -1,6 +1,6 @@
-// DELETE /api/tasks/:id (desktop) — task left Idle on desktop. Idempotent.
+// DELETE /api/tasks/:id (desktop) — task left Idle on desktop. Idempotent, scoped to the caller's rows.
 export default defineEventHandler(async (event) => {
-  await deleteTask(getSql(), getRouterParam(event, "id")!);
+  await deleteTask(getSql(), ownerOf(event), getRouterParam(event, "id")!);
   setResponseStatus(event, 204);
   return null;
 });
diff --git a/server/api/tasks/[id].put.ts b/server/api/tasks/[id].put.ts
index cebdfef..c151940 100644
--- a/server/api/tasks/[id].put.ts
+++ b/server/api/tasks/[id].put.ts
@@ -1,4 +1,4 @@
-// PUT /api/tasks/:id (desktop) — idempotent upsert mirroring a desktop Idle task.
+// PUT /api/tasks/:id (desktop) — idempotent upsert mirroring a desktop Idle task, owned by the caller.
 export default defineEventHandler(async (event) => {
   const id = getRouterParam(event, "id")!;
   const body = await readBody(event);
@@ -9,12 +9,13 @@ export default defineEventHandler(async (event) => {
   }
   const description = typeof body?.description === "string" ? body.description : null;
 
+  const ownerId = ownerOf(event);
   const sql = getSql();
-  if (!(await listExists(sql, listId))) {
+  if (!(await listExists(sql, ownerId, listId))) {
     throw createError({ statusCode: 404, statusMessage: "list not found" });
   }
 
-  const { created } = await upsertDesktopTask(sql, id, { listId, title, description });
+  const { created } = await upsertDesktopTask(sql, ownerId, id, { listId, title, description });
   setResponseStatus(event, created ? 201 : 200);
   return { id };
 });
diff --git a/server/api/tasks/[id]/consume.post.ts b/server/api/tasks/[id]/consume.post.ts
index a64fe0c..7125dec 100644
--- a/server/api/tasks/[id]/consume.post.ts
+++ b/server/api/tasks/[id]/consume.post.ts
@@ -1,6 +1,6 @@
-// POST /api/tasks/:id/consume (desktop) — mark a web task imported. Idempotent.
+// POST /api/tasks/:id/consume (desktop) — mark the caller's web task imported. Idempotent.
 export default defineEventHandler(async (event) => {
-  const ok = await consume(getSql(), getRouterParam(event, "id")!);
+  const ok = await consume(getSql(), ownerOf(event), getRouterParam(event, "id")!);
   if (!ok) {
     throw createError({ statusCode: 404, statusMessage: "task not found" });
   }
diff --git a/server/api/tasks/mirror.put.ts b/server/api/tasks/mirror.put.ts
index 47750af..3254b8d 100644
--- a/server/api/tasks/mirror.put.ts
+++ b/server/api/tasks/mirror.put.ts
@@ -1,7 +1,8 @@
-// PUT /api/tasks/mirror (desktop) — full-replace of the desktop's current Idle backlog.
+// PUT /api/tasks/mirror (desktop) — full-replace of the caller's desktop Idle backlog.
 // Body: [{ id, listId, title, description? }, ...] (camelCase). An empty array is valid and
-// clears the desktop-owned partition. Mirrors PUT /lists. Web-created tasks awaiting pull
-// (consumed=false) are never touched here.
+// clears the caller's desktop-owned partition. Mirrors PUT /lists. Web-created tasks awaiting
+// pull (consumed=false) and other users' rows are never touched. Any client-supplied ownerId
+// on items is ignored — ownership comes from the verified token.
 export default defineEventHandler(async (event) => {
   const body = await readBody(event);
   if (!Array.isArray(body)) {
@@ -28,16 +29,17 @@ export default defineEventHandler(async (event) => {
     });
   }
 
+  const ownerId = ownerOf(event);
   const sql = getSql();
 
-  // Every referenced list must exist (lists are full-replaced before tasks are mirrored).
+  // Every referenced list must exist for the caller (lists are full-replaced before tasks are mirrored).
   const listIds = [...new Set(items.map((t) => t.listId))];
   for (const id of listIds) {
-    if (!(await listExists(sql, id))) {
+    if (!(await listExists(sql, ownerId, id))) {
       throw createError({ statusCode: 400, statusMessage: `unknown listId: ${id}` });
     }
   }
 
-  await mirrorDesktopTasks(sql, items);
+  await mirrorDesktopTasks(sql, ownerId, items);
   return { ok: true, count: items.length };
 });