diff --git a/tests/unit/atomic-checkout.test.ts b/tests/unit/atomic-checkout.test.ts
new file mode 100644
index 00000000..31d2144b
--- /dev/null
+++ b/tests/unit/atomic-checkout.test.ts
@@ -0,0 +1,300 @@
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { mkdtemp, rm } from "node:fs/promises";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("electron", () => ({
+  safeStorage: {
+    isEncryptionAvailable: () => false,
+    getSelectedStorageBackend: () => "basic_text",
+    encryptString: (value: string) => Buffer.from(value, "utf8"),
+    decryptString: (value: Buffer) => value.toString("utf8"),
+  },
+}));
+
+import { AppDatabase, STALE_LOCK_TIMEOUT_MS } from "../../src/main/database";
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function createTestCompany(db: AppDatabase): string {
+  return db.saveCompany({
+    name: "Test Company",
+    description: "Atomic checkout tests",
+    status: "active",
+  });
+}
+
+function createTestConnector(db: AppDatabase): void {
+  db.saveConnector({
+    id: "codex_local",
+    command: "codex",
+    model: "gpt-5",
+    envBindingText: "",
+    notes: "Test connector",
+  });
+}
+
+function createTestWorkspace(db: AppDatabase, companyId: string): string {
+  return db.saveWorkspace({
+    companyId,
+    name: "Test Workspace",
+    localPath: "/tmp/test-workspace",
+    repoUrl: "https://github.com/example/test",
+    repoRef: "main",
+    isPrimary: true,
+  });
+}
+
+function createTestAgent(db: AppDatabase, companyId: string, workspaceId: string, name = "Agent"): string {
+  return db.saveAgent({
+    companyId,
+    name,
+    role: "IC",
+    title: name,
+    status: "active",
+    reportsTo: null,
+    connectorId: "codex_local",
+    workspaceId,
+    model: null,
+    capabilities: "Executes tasks",
+    budgetMonthlyUsd: 100,
+  });
+}
+
+function createTestTask(db: AppDatabase, companyId: string, agentId: string | null, workspaceId: string): string {
+  return db.saveTask({
+    companyId,
+    title: "Test Task",
+    description: "A test task for checkout",
+    assigneeAgentId: agentId,
+    workspaceId,
+    priority: "medium",
+    status: "todo",
+  });
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe("CAS checkout", () => {
+  let tempDir: string;
+  let db: AppDatabase;
+
+  beforeEach(async () => {
+    tempDir = await mkdtemp(join(tmpdir(), "agentcompany-checkout-"));
+    db = new AppDatabase(tempDir);
+    db.init();
+  });
+
+  afterEach(async () => {
+    db.close();
+    await rm(tempDir, { recursive: true, force: true });
+  });
+
+  it("CAS checkout succeeds for first claimer", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    const { id: runId } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    const result = db.checkoutTask(taskId, runId);
+    expect(result).toBe(true);
+
+    const task = db.getTask(taskId);
+    expect(task.status).toBe("in_progress");
+  });
+
+  it("CAS checkout fails for second claimer", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    const { id: runId1 } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+    const { id: runId2 } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    expect(db.checkoutTask(taskId, runId1)).toBe(true);
+    expect(db.checkoutTask(taskId, runId2)).toBe(false);
+  });
+
+  it("CAS checkout rejects already-in-progress tasks", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    // Manually set task to in_progress
+    (db as any).db.prepare("UPDATE tasks SET status = 'in_progress' WHERE id = ?").run(taskId);
+
+    const { id: runId } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    expect(db.checkoutTask(taskId, runId)).toBe(false);
+  });
+
+  it("lock release in finishRun clears checkout fields", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    const { id: runId } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    db.checkoutTask(taskId, runId);
+    db.markRunStatus(runId, "running", "");
+    db.finishRun({
+      runId,
+      status: "completed",
+      summary: "Done",
+      errorMessage: null,
+      exitCode: 0,
+      signal: null,
+      model: null,
+      sessionDisplayId: null,
+      costUsd: 0,
+      unattributedCost: false,
+    });
+
+    const row = (db as any).db
+      .prepare("SELECT checkout_run_id, execution_locked_at FROM tasks WHERE id = ?")
+      .get(taskId) as { checkout_run_id: string | null; execution_locked_at: string | null };
+
+    expect(row.checkout_run_id).toBeNull();
+    expect(row.execution_locked_at).toBeNull();
+  });
+
+  it("lock release in cancelRunForDeletion clears checkout fields", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    const { id: runId } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    db.checkoutTask(taskId, runId);
+    db.markRunStatus(runId, "running", "");
+
+    // Deleting the agent triggers cancelRunForDeletion for active runs
+    db.deleteAgent(agentId, companyId);
+
+    const row = (db as any).db
+      .prepare("SELECT checkout_run_id, execution_locked_at FROM tasks WHERE id = ?")
+      .get(taskId) as { checkout_run_id: string | null; execution_locked_at: string | null };
+
+    expect(row.checkout_run_id).toBeNull();
+    expect(row.execution_locked_at).toBeNull();
+  });
+
+  it("stale lock recovery resets tasks older than 15 minutes", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    const { id: runId } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    db.checkoutTask(taskId, runId);
+
+    // Backdate execution_locked_at to 20 minutes ago
+    const backdated = new Date(Date.now() - 20 * 60 * 1000).toISOString();
+    (db as any).db
+      .prepare("UPDATE tasks SET execution_locked_at = ? WHERE id = ?")
+      .run(backdated, taskId);
+
+    const recovered = db.recoverStaleLocks();
+
+    expect(recovered.length).toBe(1);
+    expect(recovered[0].taskId).toBe(taskId);
+    expect(recovered[0].companyId).toBe(companyId);
+
+    const task = db.getTask(taskId);
+    expect(task.status).toBe("todo");
+
+    const row = (db as any).db
+      .prepare("SELECT checkout_run_id, execution_locked_at FROM tasks WHERE id = ?")
+      .get(taskId) as { checkout_run_id: string | null; execution_locked_at: string | null };
+
+    expect(row.checkout_run_id).toBeNull();
+    expect(row.execution_locked_at).toBeNull();
+  });
+
+  it("stale lock recovery ignores tasks within 15 minute window", () => {
+    const companyId = createTestCompany(db);
+    createTestConnector(db);
+    const workspaceId = createTestWorkspace(db, companyId);
+    const agentId = createTestAgent(db, companyId, workspaceId);
+    const taskId = createTestTask(db, companyId, null, workspaceId);
+
+    const { id: runId } = db.createRun({
+      companyId,
+      taskId,
+      agentId,
+      workspaceId,
+      connectorId: "codex_local",
+    });
+
+    db.checkoutTask(taskId, runId);
+
+    const recovered = db.recoverStaleLocks();
+    expect(recovered.length).toBe(0);
+
+    const task = db.getTask(taskId);
+    expect(task.status).toBe("in_progress");
+  });
+
+  it("STALE_LOCK_TIMEOUT_MS is 15 minutes", () => {
+    expect(STALE_LOCK_TIMEOUT_MS).toBe(15 * 60 * 1000);
+  });
+});