diff --git a/src/tasks/service.ts b/src/tasks/service.ts
index 9b8f2cb..19d86f1 100644
--- a/src/tasks/service.ts
+++ b/src/tasks/service.ts
@@ -1,3 +1,4 @@
+import type { Database } from 'better-sqlite3';
 import { db } from '../db';
 
 type CreateTaskInput = {
@@ -13,9 +14,11 @@ type CreateAssignmentInput = {
 };
 
 export class TaskService {
+  static dbInstance: Database = db;
+
   static createTask(task: CreateTaskInput, assignments: CreateAssignmentInput[] = []): number {
-    const runTx = db.transaction(() => {
-      const insertTask = db.prepare(`
+    const runTx = this.dbInstance.transaction(() => {
+      const insertTask = this.dbInstance.prepare(`
         INSERT INTO tasks (description, due_date, group_id, created_by)
         VALUES (?, ?, ?, ?)
         RETURNING id
@@ -29,7 +32,7 @@ export class TaskService {
       ) as { id: number };
 
       if (assignments.length > 0) {
-        const insertAssignment = db.prepare(`
+        const insertAssignment = this.dbInstance.prepare(`
           INSERT INTO task_assignments (task_id, user_id, assigned_by)
           VALUES (?, ?, ?)
         `);
diff --git a/tests/unit/tasks/service.test.ts b/tests/unit/tasks/service.test.ts
new file mode 100644
index 0000000..fbe279e
--- /dev/null
+++ b/tests/unit/tasks/service.test.ts
@@ -0,0 +1,170 @@
+import { describe, it, expect, beforeEach, afterEach } from 'bun:test';
+import Database from 'better-sqlite3';
+import { initializeDatabase, ensureUserExists } from '../../../src/db';
+import { TaskService } from '../../../src/tasks/service';
+
+let memDb: Database;
+
+beforeEach(() => {
+  memDb = new Database(':memory:');
+  initializeDatabase(memDb);
+  TaskService.dbInstance = memDb;
+});
+
+afterEach(() => {
+  try {
+    memDb.close();
+  } catch {}
+});
+
+describe('TaskService.createTask', () => {
+  it('crea una tarea mínima con created_by y sin due_date ni group_id', () => {
+    const creatorRaw = '555111222@s.whatsapp.net';
+    const createdBy = ensureUserExists(creatorRaw, memDb);
+    expect(createdBy).toBeTruthy();
+
+    const id = TaskService.createTask(
+      {
+        description: 'Comprar agua',
+        created_by: createdBy!,
+        due_date: null,
+        group_id: null,
+      },
+      [] // sin asignaciones
+    );
+
+    expect(typeof id).toBe('number');
+
+    const task = memDb
+      .prepare(
+        `SELECT id, description, due_date, group_id, created_by, completed 
+         FROM tasks WHERE id = ?`
+      )
+      .get(id) as any;
+
+    expect(task).toBeTruthy();
+    expect(task.description).toBe('Comprar agua');
+    expect(task.due_date).toBeNull();
+    expect(task.group_id).toBeNull();
+    expect(task.created_by).toBe(createdBy);
+    expect(task.completed).toBe(0); // BOOLEAN en SQLite como 0/1
+  });
+
+  it('guarda due_date como cadena YYYY-MM-DD y group_id como JID completo', () => {
+    const creatorRaw = '555333444@s.whatsapp.net';
+    const createdBy = ensureUserExists(creatorRaw, memDb)!;
+
+    const due = '2025-09-10';
+    const groupId = '12345@g.us';
+
+    const id = TaskService.createTask(
+      {
+        description: 'Pagar servicios',
+        created_by: createdBy,
+        due_date: due,
+        group_id: groupId,
+      },
+      []
+    );
+
+    const row = memDb
+      .prepare(
+        `SELECT description, due_date, group_id, created_by 
+         FROM tasks WHERE id = ?`
+      )
+      .get(id) as any;
+
+    expect(row.due_date).toBe(due);
+    expect(row.group_id).toBe(groupId);
+    expect(row.created_by).toBe(createdBy);
+  });
+
+  it('inserta asignaciones y evita duplicados por usuario', () => {
+    const creator = ensureUserExists('555000001@s.whatsapp.net', memDb)!;
+    const assigneeA = ensureUserExists('555000002@s.whatsapp.net', memDb)!;
+    const assigneeB = ensureUserExists('555000003@s.whatsapp.net', memDb)!;
+
+    const id = TaskService.createTask(
+      {
+        description: 'Organizar reunión',
+        created_by: creator,
+        due_date: null,
+        group_id: null,
+      },
+      [
+        { user_id: assigneeA, assigned_by: creator },
+        { user_id: assigneeA, assigned_by: creator }, // duplicado
+        { user_id: assigneeB, assigned_by: creator },
+      ]
+    );
+
+    const count = memDb
+      .prepare(
+        `SELECT COUNT(*) AS c FROM task_assignments WHERE task_id = ?`
+      )
+      .get(id) as any;
+
+    expect(count.c).toBe(2);
+
+    const users = memDb
+      .prepare(
+        `SELECT user_id FROM task_assignments WHERE task_id = ? ORDER BY user_id`
+      )
+      .all(id) as any[];
+
+    expect(users.map(u => u.user_id)).toEqual([assigneeA, assigneeB].sort());
+  });
+
+  it('hace rollback si una asignación viola FK (usuario inexistente)', () => {
+    const creator = ensureUserExists('555010101@s.whatsapp.net', memDb)!;
+
+    expect(() =>
+      TaskService.createTask(
+        {
+          description: 'Tarea con fallo',
+          created_by: creator,
+          due_date: null,
+          group_id: null,
+        },
+        [
+          // Usuario no existente: no llamamos ensureUserExists
+          { user_id: '555099999', assigned_by: creator },
+        ]
+      )
+    ).toThrow();
+
+    const counts = memDb
+      .prepare(
+        `SELECT 
+           (SELECT COUNT(*) FROM tasks) AS tasks_count,
+           (SELECT COUNT(*) FROM task_assignments) AS assigns_count`
+      )
+      .get() as any;
+
+    expect(counts.tasks_count).toBe(0);
+    expect(counts.assigns_count).toBe(0);
+  });
+
+  it('lanza error si created_by no existe (FK) y no inserta la tarea', () => {
+    // No llamamos a ensureUserExists para este created_by
+    const nonExisting = '555123123';
+
+    expect(() =>
+      TaskService.createTask(
+        {
+          description: 'No debería insertarse',
+          created_by: nonExisting,
+          due_date: null,
+          group_id: null,
+        },
+        []
+      )
+    ).toThrow();
+
+    const count = memDb
+      .prepare(`SELECT COUNT(*) AS c FROM tasks`)
+      .get() as any;
+
+    expect(count.c).toBe(0);
+  });
+});