diff --git a/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-errors.test.ts b/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-errors.test.ts
new file mode 100644
index 0000000..d286c9c
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-errors.test.ts
@@ -0,0 +1,91 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import {
+  batchQuickAssignPermissions,
+  batchQuickAssignRole,
+  buildBaseProject,
+  buildBaseResource,
+  createBatchQuickAssignInput,
+  createToolContext,
+  executeTool,
+} from "./assistant-tools-timeline-batch-quick-assign-test-helpers.js";
+
+describe("assistant timeline batch quick-assign error tools", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("returns a structured batch assignment resolver error for an unknown resource", async () => {
+    const db = {
+      project: {
+        findUnique: vi.fn().mockResolvedValue({
+          ...buildBaseProject(),
+          startDate: new Date("2026-03-16"),
+          endDate: new Date("2026-03-20"),
+        }),
+        findFirst: vi.fn().mockResolvedValue(null),
+      },
+      resource: {
+        findUnique: vi.fn().mockResolvedValue(null),
+        findFirst: vi.fn().mockResolvedValue(null),
+        findMany: vi.fn().mockResolvedValue([]),
+      },
+      assignment: {
+        create: vi.fn(),
+      },
+    };
+    const ctx = createToolContext(db, batchQuickAssignPermissions, batchQuickAssignRole);
+
+    const result = await executeTool(
+      "batch_quick_assign_timeline_resources",
+      JSON.stringify(
+        createBatchQuickAssignInput([
+          {
+            resourceIdentifier: "missing_resource",
+          },
+        ]),
+      ),
+      ctx,
+    );
+
+    expect(result.action).toBeUndefined();
+    expect(JSON.parse(result.content)).toEqual({
+      error: "assignments[0].resourceIdentifier: Resource not found: missing_resource",
+      field: "assignments[0].resourceIdentifier",
+      index: 0,
+    });
+    expect(db.assignment.create).not.toHaveBeenCalled();
+  });
+
+  it("returns a stable validation error for batch timeline mutation date ranges", async () => {
+    const ctx = createToolContext(
+      {
+        project: {
+          findUnique: vi.fn().mockResolvedValue(buildBaseProject()),
+        },
+        resource: {
+          findUnique: vi.fn().mockResolvedValue(buildBaseResource()),
+        },
+      },
+      batchQuickAssignPermissions,
+      batchQuickAssignRole,
+    );
+
+    const result = await executeTool(
+      "batch_quick_assign_timeline_resources",
+      JSON.stringify(
+        createBatchQuickAssignInput([
+          {
+            startDate: "2026-03-20",
+            endDate: "2026-03-16",
+          },
+        ]),
+      ),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual({
+      error: "End date must be after start date",
+    });
+  });
+});
diff --git a/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-success.test.ts b/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-success.test.ts
new file mode 100644
index 0000000..2f24ba4
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-success.test.ts
@@ -0,0 +1,60 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import {
+  batchQuickAssignPermissions,
+  batchQuickAssignRole,
+  buildBaseProject,
+  buildBaseResource,
+  createBatchQuickAssignInput,
+  createToolContext,
+  executeTool,
+} from "./assistant-tools-timeline-batch-quick-assign-test-helpers.js";
+
+describe("assistant timeline batch quick-assign tools", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("batch quick-assigns timeline resources through the real timeline router mutation", async () => {
+    const db = {
+      project: {
+        findUnique: vi.fn().mockResolvedValue(buildBaseProject()),
+      },
+      resource: {
+        findUnique: vi
+          .fn()
+          .mockImplementation(async ({ where }: { where: { id: string } }) => buildBaseResource(where.id)),
+      },
+      assignment: {
+        findMany: vi.fn().mockResolvedValue([]),
+        create: vi
+          .fn()
+          .mockResolvedValueOnce({ id: "assignment_batch_1" })
+          .mockResolvedValueOnce({ id: "assignment_batch_2" }),
+      },
+      auditLog: {
+        create: vi.fn().mockResolvedValue({}),
+      },
+      vacation: {
+        findMany: vi.fn().mockResolvedValue([]),
+      },
+      $transaction: vi.fn(async (callback: (tx: unknown) => unknown) => callback(db)),
+    };
+    const ctx = createToolContext(db, batchQuickAssignPermissions, batchQuickAssignRole);
+
+    const result = await executeTool(
+      "batch_quick_assign_timeline_resources",
+      JSON.stringify(createBatchQuickAssignInput()),
+      ctx,
+    );
+
+    expect(result.action).toEqual({ type: "invalidate", scope: ["allocation", "timeline", "project"] });
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        success: true,
+        count: 2,
+      }),
+    );
+    expect(db.assignment.create).toHaveBeenCalledTimes(2);
+  });
+});
diff --git a/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-test-helpers.ts b/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-test-helpers.ts
new file mode 100644
index 0000000..255b40c
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-timeline-batch-quick-assign-test-helpers.ts
@@ -0,0 +1,72 @@
+import { PermissionKey, SystemRole } from "@capakraken/shared";
+import { vi } from "vitest";
+
+vi.mock("@capakraken/application", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@capakraken/application")>();
+  return {
+    ...actual,
+    getDashboardBudgetForecast: vi.fn().mockResolvedValue([]),
+    getDashboardPeakTimes: vi.fn().mockResolvedValue([]),
+    listAssignmentBookings: vi.fn().mockResolvedValue([]),
+  };
+});
+
+vi.mock("../sse/event-bus.js", () => ({
+  emitAllocationCreated: vi.fn(),
+  emitAllocationDeleted: vi.fn(),
+  emitAllocationUpdated: vi.fn(),
+  emitProjectShifted: vi.fn(),
+}));
+
+vi.mock("../lib/budget-alerts.js", () => ({
+  checkBudgetThresholds: vi.fn(),
+}));
+
+vi.mock("../lib/cache.js", () => ({
+  invalidateDashboardCache: vi.fn(),
+}));
+
+import { executeTool as executeAssistantTool } from "../router/assistant-tools.js";
+
+export { createToolContext } from "./assistant-tools-advanced-timeline-test-helpers.js";
+export {
+  buildBaseProject,
+  buildBaseResource,
+} from "./assistant-tools-timeline-allocation-mutation-test-helpers.js";
+
+export const batchQuickAssignPermissions = [
+  PermissionKey.MANAGE_ALLOCATIONS,
+  PermissionKey.USE_ASSISTANT_ADVANCED_TOOLS,
+];
+
+export const batchQuickAssignRole = SystemRole.MANAGER;
+
+export function createBatchQuickAssignInput(
+  overrides: Array<Record<string, unknown>> = [],
+) {
+  const baseAssignments = [
+    {
+      resourceIdentifier: "resource_1",
+      projectIdentifier: "project_1",
+      startDate: "2026-03-16",
+      endDate: "2026-03-20",
+      hoursPerDay: 8,
+    },
+    {
+      resourceIdentifier: "resource_2",
+      projectIdentifier: "project_1",
+      startDate: "2026-03-23",
+      endDate: "2026-03-27",
+      hoursPerDay: 6,
+    },
+  ];
+
+  return {
+    assignments: baseAssignments.map((assignment, index) => ({
+      ...assignment,
+      ...(overrides[index] ?? {}),
+    })),
+  };
+}
+
+export const executeTool = executeAssistantTool;