diff --git a/docs/plan-web-fases.md b/docs/plan-web-fases.md
index 58d46cd..a2cae65 100644
--- a/docs/plan-web-fases.md
+++ b/docs/plan-web-fases.md
@@ -314,7 +314,7 @@ Resultado (implementado)
 - Idempotente: se ejecuta solo si la tabla tasks está vacía y con DEV_AUTOSEED_DB='true' en desarrollo; usa DEV_DEFAULT_USER numérico cuando está definido.
 - Documentación actualizada en docs/operations.md con instrucciones para activar y regenerar la base de datos de desarrollo.
 
-Fase 10 — Completar tarea sin responsable: auto-asignación al completador
+Fase 10 — Completar tarea sin responsable: auto-asignación al completador — Estado: Completada
 Objetivos
 - Resolver el edge case: al completar una tarea sin responsables, debe aparecer en “Completadas (24h)” del usuario y permitir “Deshacer”.
 - Mantener gating y trazabilidad coherentes.
@@ -341,6 +341,12 @@ Archivos a editar
 - apps/web/src/lib/ui/data/TaskItem.svelte (feedback UI)
 - tests/web/* (casos de integración)
 
+Resultado (implementado)
+- Auto-asignación atómica al completar tareas de grupo sin responsables; se registra completed_by.
+- Listado “Completadas (24 h)” incluye estas tareas gracias a la auto-asignación.
+- Uncomplete permitido dentro de la ventana configurada, manteniendo la asignación creada.
+- Tests de integración añadidos: complete-autoassign-recent, uncomplete-window y carrera de complete (con dos usuarios).
+
 Criterios de aceptación
 - Completar una tarea sin responsables la vincula al usuario y aparece inmediatamente en “Completadas (24h)”.
 - “Deshacer completar” funciona para ese caso dentro de la ventana configurada.
diff --git a/tests/web/api.tasks.complete-autoassign-recent.test.ts b/tests/web/api.tasks.complete-autoassign-recent.test.ts
new file mode 100644
index 0000000..3830fad
--- /dev/null
+++ b/tests/web/api.tasks.complete-autoassign-recent.test.ts
@@ -0,0 +1,86 @@
+import { describe, it, beforeAll, afterAll, expect } from 'bun:test';
+import { createTempDb } from './helpers/db';
+import { startWebServer, type RunningServer } from './helpers/server';
+
+describe('API Web - complete auto-assign y recientes', () => {
+  const USER = '34600123456';
+  const GROUP = 'g-1@g.us';
+  let dbHandle: any;
+  let cleanupDb: () => void;
+  let server: RunningServer;
+  let baseUrl: string;
+  let taskId: number;
+
+  beforeAll(async () => {
+    const tmp = createTempDb();
+    dbHandle = tmp.db;
+    cleanupDb = tmp.cleanup;
+
+    // Semilla mínima
+    dbHandle.prepare(`INSERT INTO users (id) VALUES (?) ON CONFLICT(id) DO NOTHING`).run(USER);
+    dbHandle
+      .prepare(`INSERT INTO groups (id, community_id, name, active) VALUES (?, 'comm-1', 'Test Group', 1)`)
+      .run(GROUP);
+    dbHandle.prepare(`INSERT INTO allowed_groups (group_id, status) VALUES (?, 'allowed')`).run(GROUP);
+    dbHandle
+      .prepare(
+        `INSERT INTO group_members (group_id, user_id, is_admin, is_active) VALUES (?, ?, 0, 1)
+      `
+      )
+      .run(GROUP, USER);
+
+    const ins = dbHandle
+      .prepare(`INSERT INTO tasks (description, group_id, created_by) VALUES ('Hacer algo', ?, ?)`)
+      .run(GROUP, USER);
+    taskId = Number(ins.lastInsertRowid);
+
+    server = await startWebServer({
+      port: 19101,
+      env: { DB_PATH: tmp.path, DEV_DEFAULT_USER: USER, TZ: 'UTC' }
+    });
+    baseUrl = server.baseUrl;
+  });
+
+  afterAll(async () => {
+    try {
+      await server.stop();
+    } catch {}
+    try {
+      cleanupDb?.();
+    } catch {}
+  });
+
+  it('completar una tarea sin responsables auto-asigna y aparece en recientes', async () => {
+    // Completar
+    const resComplete = await fetch(`${baseUrl}/api/tasks/${taskId}/complete`, { method: 'POST' });
+    expect(resComplete.status).toBe(200);
+    const jsonComplete = await resComplete.json();
+    expect(['updated', 'already']).toContain(jsonComplete?.status);
+
+    // Recientes (24h)
+    const resRecent = await fetch(`${baseUrl}/api/me/tasks?status=recent&limit=50`);
+    expect(resRecent.status).toBe(200);
+    const recentBody = await resRecent.json();
+    const items = Array.isArray(recentBody?.items) ? recentBody.items : [];
+    const found = items.find((it: any) => Number(it?.id) === taskId);
+    expect(found).toBeTruthy();
+    expect(found.completed).toBe(true);
+    expect(typeof found.completed_at === 'string' && found.completed_at.length > 0).toBe(true);
+    expect(Array.isArray(found.assignees)).toBe(true);
+    expect(found.assignees.map(String)).toContain(USER);
+
+    // Uncomplete inmediato permitido
+    const resUncomplete = await fetch(`${baseUrl}/api/tasks/${taskId}/uncomplete`, { method: 'POST' });
+    expect(resUncomplete.status).toBe(200);
+    const jsonUnc = await resUncomplete.json();
+    expect(jsonUnc?.status).toBe('updated');
+
+    // Ya no aparece en recientes tras reabrir
+    const resRecent2 = await fetch(`${baseUrl}/api/me/tasks?status=recent&limit=50`);
+    expect(resRecent2.status).toBe(200);
+    const recentBody2 = await resRecent2.json();
+    const items2 = Array.isArray(recentBody2?.items) ? recentBody2.items : [];
+    const found2 = items2.find((it: any) => Number(it?.id) === taskId);
+    expect(found2).toBeFalsy();
+  });
+});
diff --git a/tests/web/api.tasks.complete-concurrency.test.ts b/tests/web/api.tasks.complete-concurrency.test.ts
new file mode 100644
index 0000000..16b04aa
--- /dev/null
+++ b/tests/web/api.tasks.complete-concurrency.test.ts
@@ -0,0 +1,91 @@
+import { describe, it, beforeAll, afterAll, expect } from 'bun:test';
+import { createTempDb } from './helpers/db';
+import { startWebServer, type RunningServer } from './helpers/server';
+
+describe('API Web - carreras en complete con auto-asignación', () => {
+  const U1 = '34600123456';
+  const U2 = '34600123457';
+  const GROUP = 'g-3@g.us';
+
+  let dbHandle: any;
+  let cleanupDb: () => void;
+  let serverA: RunningServer;
+  let serverB: RunningServer;
+  let baseA: string;
+  let baseB: string;
+  let taskId: number;
+
+  beforeAll(async () => {
+    const tmp = createTempDb();
+    dbHandle = tmp.db;
+    cleanupDb = tmp.cleanup;
+
+    // Semilla mínima
+    dbHandle.prepare(`INSERT INTO users (id) VALUES (?) ON CONFLICT(id) DO NOTHING`).run(U1);
+    dbHandle.prepare(`INSERT INTO users (id) VALUES (?) ON CONFLICT(id) DO NOTHING`).run(U2);
+    dbHandle
+      .prepare(`INSERT INTO groups (id, community_id, name, active) VALUES (?, 'comm-1', 'G3', 1)`)
+      .run(GROUP);
+    dbHandle.prepare(`INSERT INTO allowed_groups (group_id, status) VALUES (?, 'allowed')`).run(GROUP);
+    dbHandle
+      .prepare(`INSERT INTO group_members (group_id, user_id, is_admin, is_active) VALUES (?, ?, 0, 1)`)
+      .run(GROUP, U1);
+    dbHandle
+      .prepare(`INSERT INTO group_members (group_id, user_id, is_admin, is_active) VALUES (?, ?, 0, 1)`)
+      .run(GROUP, U2);
+
+    const ins = dbHandle
+      .prepare(`INSERT INTO tasks (description, group_id, created_by) VALUES ('Carrera complete', ?, ?)`)
+      .run(GROUP, U1);
+    taskId = Number(ins.lastInsertRowid);
+
+    // Iniciar dos servidores contra la misma DB
+    serverA = await startWebServer({
+      port: 19111,
+      env: { DB_PATH: tmp.path, DEV_DEFAULT_USER: U1, TZ: 'UTC' }
+    });
+    baseA = serverA.baseUrl;
+    serverB = await startWebServer({
+      port: 19112,
+      env: { DB_PATH: tmp.path, DEV_DEFAULT_USER: U2, TZ: 'UTC' }
+    });
+    baseB = serverB.baseUrl;
+  });
+
+  afterAll(async () => {
+    try {
+      await serverA.stop();
+    } catch {}
+    try {
+      await serverB.stop();
+    } catch {}
+    try {
+      cleanupDb?.();
+    } catch {}
+  });
+
+  it('solo una actualización de completed; la asignación incluye al menos al completador', async () => {
+    const [r1, r2] = await Promise.all([
+      fetch(`${baseA}/api/tasks/${taskId}/complete`, { method: 'POST' }),
+      fetch(`${baseB}/api/tasks/${taskId}/complete`, { method: 'POST' })
+    ]);
+    expect([200, 200]).toContain(r1.status);
+    expect([200, 200]).toContain(r2.status);
+
+    const row = dbHandle
+      .prepare(`SELECT COALESCE(completed,0) as completed, completed_by FROM tasks WHERE id = ?`)
+      .get(taskId) as any;
+    expect(Number(row?.completed || 0)).toBe(1);
+    const completedBy = String(row?.completed_by || '');
+
+    const assigns = dbHandle
+      .prepare(`SELECT user_id FROM task_assignments WHERE task_id = ? ORDER BY assigned_at ASC`)
+      .all(taskId) as any[];
+
+    // Debe existir al menos una asignación y contener al que completó
+    expect(assigns.length >= 1 && assigns.length <= 2).toBe(true);
+    const assignedUsers = assigns.map((r) => String(r.user_id));
+    expect([U1, U2]).toContain(completedBy);
+    expect(assignedUsers).toContain(completedBy);
+  });
+});
diff --git a/tests/web/api.tasks.uncomplete-window.test.ts b/tests/web/api.tasks.uncomplete-window.test.ts
new file mode 100644
index 0000000..d024448
--- /dev/null
+++ b/tests/web/api.tasks.uncomplete-window.test.ts
@@ -0,0 +1,67 @@
+import { describe, it, beforeAll, afterAll, expect } from 'bun:test';
+import { createTempDb } from './helpers/db';
+import { startWebServer, type RunningServer } from './helpers/server';
+
+describe('API Web - uncomplete dentro de ventana', () => {
+  const USER = '34600123456';
+  const GROUP = 'g-2@g.us';
+  let dbHandle: any;
+  let cleanupDb: () => void;
+  let server: RunningServer;
+  let baseUrl: string;
+  let taskId: number;
+
+  beforeAll(async () => {
+    const tmp = createTempDb();
+    dbHandle = tmp.db;
+    cleanupDb = tmp.cleanup;
+
+    // Semilla mínima
+    dbHandle.prepare(`INSERT INTO users (id) VALUES (?) ON CONFLICT(id) DO NOTHING`).run(USER);
+    dbHandle
+      .prepare(`INSERT INTO groups (id, community_id, name, active) VALUES (?, 'comm-1', 'G2', 1)`)
+      .run(GROUP);
+    dbHandle.prepare(`INSERT INTO allowed_groups (group_id, status) VALUES (?, 'allowed')`).run(GROUP);
+    dbHandle
+      .prepare(`INSERT INTO group_members (group_id, user_id, is_admin, is_active) VALUES (?, ?, 0, 1)`)
+      .run(GROUP, USER);
+
+    const ins = dbHandle
+      .prepare(`INSERT INTO tasks (description, group_id, created_by) VALUES ('Probar ventana', ?, ?)`)
+      .run(GROUP, USER);
+    taskId = Number(ins.lastInsertRowid);
+
+    server = await startWebServer({
+      port: 19102,
+      env: { DB_PATH: tmp.path, DEV_DEFAULT_USER: USER, TZ: 'UTC' }
+    });
+    baseUrl = server.baseUrl;
+  });
+
+  afterAll(async () => {
+    try {
+      await server.stop();
+    } catch {}
+    try {
+      cleanupDb?.();
+    } catch {}
+  });
+
+  it('uncomplete permitido inmediatamente después de completar y es idempotente', async () => {
+    // Completar
+    const resComplete = await fetch(`${baseUrl}/api/tasks/${taskId}/complete`, { method: 'POST' });
+    expect(resComplete.status).toBe(200);
+
+    // Uncomplete (dentro de ventana)
+    const resUncomplete = await fetch(`${baseUrl}/api/tasks/${taskId}/uncomplete`, { method: 'POST' });
+    expect(resUncomplete.status).toBe(200);
+    const jsonUnc = await resUncomplete.json();
+    expect(jsonUnc?.status).toBe('updated');
+
+    // Uncomplete de nuevo (idempotente → already)
+    const resUncomplete2 = await fetch(`${baseUrl}/api/tasks/${taskId}/uncomplete`, { method: 'POST' });
+    expect(resUncomplete2.status).toBe(200);
+    const jsonUnc2 = await resUncomplete2.json();
+    expect(jsonUnc2?.status).toBe('already');
+  });
+});