diff --git a/backend/src/__tests__/scheduled-tasks-rbac.test.ts b/backend/src/__tests__/scheduled-tasks-rbac.test.ts
index 2cc2a91f..bd4e2133 100644
--- a/backend/src/__tests__/scheduled-tasks-rbac.test.ts
+++ b/backend/src/__tests__/scheduled-tasks-rbac.test.ts
@@ -7,6 +7,8 @@
 import { describe, it, expect, beforeAll, afterAll, beforeEach, vi } from 'vitest';
 import request from 'supertest';
 import bcrypt from 'bcrypt';
+import fs from 'fs';
+import path from 'path';
 import { setupTestDb, cleanupTestDb, loginAsTestAdmin } from './helpers/setupTestDb';
 
 let tmpDir: string;
@@ -67,6 +69,13 @@ beforeAll(async () => {
     else auditorCookie = c;
   }
 
+  // Seed real stack directories so existence validators pass.
+  const composeDir = path.join(tmpDir, 'compose');
+  for (const name of ['web', 'api']) {
+    fs.mkdirSync(path.join(composeDir, name), { recursive: true });
+    fs.writeFileSync(path.join(composeDir, name, 'compose.yaml'), 'version: "3"\n');
+  }
+
   // Insert a local node for stack-target fixtures
   for (const [nodeName, nodeType] of [['local-test', 'local'], ['remote-test', 'remote']] as const) {
     const existing = db.getDb().prepare('SELECT id FROM nodes WHERE name = ?').get(nodeName) as { id: number } | undefined;
diff --git a/backend/src/__tests__/scheduled-tasks-routes.test.ts b/backend/src/__tests__/scheduled-tasks-routes.test.ts
index aff7d149..bc5d32d6 100644
--- a/backend/src/__tests__/scheduled-tasks-routes.test.ts
+++ b/backend/src/__tests__/scheduled-tasks-routes.test.ts
@@ -6,6 +6,8 @@
 import { describe, it, expect, beforeAll, afterAll, beforeEach, vi } from 'vitest';
 import request from 'supertest';
 import bcrypt from 'bcrypt';
+import fs from 'fs';
+import path from 'path';
 import { setupTestDb, cleanupTestDb, loginAsTestAdmin } from './helpers/setupTestDb';
 
 let tmpDir: string;
@@ -30,6 +32,24 @@ beforeAll(async () => {
   const viewerRes = await request(app).post('/api/auth/login').send({ username: 'sched-viewer', password: 'viewerpass' });
   const cookies = viewerRes.headers['set-cookie'] as string | string[];
   viewerCookie = Array.isArray(cookies) ? cookies[0] : cookies;
+
+  // Seed real stack directories so existence validators pass.
+  const composeDir = path.join(tmpDir, 'compose');
+  for (const name of ['my-stack', 's']) {
+    fs.mkdirSync(path.join(composeDir, name), { recursive: true });
+    fs.writeFileSync(path.join(composeDir, name, 'compose.yaml'), 'version: "3"\n');
+  }
+
+  // Mock DockerController.findContainerByName so container existence checks pass
+  // in tests (no real Docker daemon available). Only resolve for names used by
+  // the test fixtures; everything else returns null to exercise the 400 path.
+  const { default: DockerController } = await import('../services/DockerController');
+  const containerFixture = { id: 'abc123test', name: 'test-container', state: 'running', image: 'test:latest', stackProject: null };
+  vi.spyOn(DockerController.prototype, 'findContainerByName')
+    .mockImplementation(async (name: string) => {
+      if (name === 'watchtower' || name === 'sidecar') return { ...containerFixture, name };
+      return null;
+    });
 });
 
 afterAll(() => cleanupTestDb(tmpDir));
@@ -178,6 +198,15 @@ describe('POST /api/scheduled-tasks', () => {
     expect(res.status).toBe(403);
   });
 
+  it('returns 403 (not 400) for unauthorized caller on nonexistent target', async () => {
+    // Permissions check runs first; an unauthorized caller must not learn
+    // whether a stack exists through the error code difference.
+    const res = await request(app).post('/api/scheduled-tasks').set('Cookie', viewerCookie).send({
+      ...basePayload, target_id: 'nonexistent-stack',
+    });
+    expect(res.status).toBe(403);
+  });
+
   it('creates a task and returns the new record', async () => {
     const res = await request(app).post('/api/scheduled-tasks').set('Cookie', adminCookie).send(basePayload);
     expect(res.status).toBe(201);
@@ -202,6 +231,52 @@ describe('POST /api/scheduled-tasks', () => {
     expect(res.body.error).toMatch(/5 fields/);
   });
 
+  it('rejects a nonexistent node_id with 400', async () => {
+    const res = await request(app).post('/api/scheduled-tasks').set('Cookie', adminCookie).send({
+      ...basePayload, node_id: 9999,
+    });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/require an existing node/);
+  });
+
+  it('returns 403 (not 400) for unauthorized caller probing nonexistent node via fleet update', async () => {
+    // A viewer must never learn whether a node ID exists through the error
+    // code difference (400 "node doesn't exist" vs 403 "permission denied").
+    const res = await request(app).post('/api/scheduled-tasks').set('Cookie', viewerCookie).send({
+      name: 'probe-node',
+      action: 'update',
+      target_type: 'fleet',
+      node_id: 999999,
+      cron_expression: '0 0 * * *',
+      enabled: true,
+    });
+    expect(res.status).toBe(403);
+  });
+
+  it('returns same 403 for unauthorized caller regardless of node existence', async () => {
+    const resNonexistent = await request(app).post('/api/scheduled-tasks')
+      .set('Cookie', viewerCookie).send({
+        name: 'probe-nonexistent', action: 'update', target_type: 'fleet',
+        node_id: 999999, cron_expression: '0 0 * * *', enabled: true,
+      });
+    const resExisting = await request(app).post('/api/scheduled-tasks')
+      .set('Cookie', viewerCookie).send({
+        name: 'probe-existing', action: 'update', target_type: 'fleet',
+        node_id: 1, cron_expression: '0 0 * * *', enabled: true,
+      });
+    expect(resNonexistent.status).toBe(403);
+    expect(resExisting.status).toBe(403);
+    expect(resNonexistent.body.error).toBe(resExisting.body.error);
+  });
+
+  it('rejects a nonexistent stack target with 400', async () => {
+    const res = await request(app).post('/api/scheduled-tasks').set('Cookie', adminCookie).send({
+      ...basePayload, target_id: 'nonexistent-stack',
+    });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/not found on the target node/);
+  });
+
   it('rejects a missing cron expression with a clear message', async () => {
     const res = await request(app).post('/api/scheduled-tasks').set('Cookie', adminCookie).send({
       ...basePayload, cron_expression: undefined,
@@ -616,6 +691,23 @@ describe('POST /api/scheduled-tasks - container lifecycle', () => {
     expect(res.body.action).toBe('restart');
   });
 
+  it('rejects a nonexistent container target with 400', async () => {
+    const res = await request(app)
+      .post('/api/scheduled-tasks')
+      .set('Cookie', adminCookie)
+      .send({
+        name: 'missing-ctr',
+        target_type: 'container',
+        target_id: 'nonexistent-container',
+        node_id: 1,
+        action: 'restart',
+        cron_expression: '0 3 * * *',
+        enabled: true,
+      });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/not found on the target node/);
+  });
+
   it('rejects invalid container names', async () => {
     const res = await request(app)
       .post('/api/scheduled-tasks')
diff --git a/backend/src/routes/scheduledTasks.ts b/backend/src/routes/scheduledTasks.ts
index bdc7a9cc..5ed9e0cb 100644
--- a/backend/src/routes/scheduledTasks.ts
+++ b/backend/src/routes/scheduledTasks.ts
@@ -13,6 +13,8 @@ import {
 } from '../services/scheduledActionRegistry';
 import { SchedulerService } from '../services/SchedulerService';
 import { NotificationService } from '../services/NotificationService';
+import DockerController from '../services/DockerController';
+import { FileSystemService } from '../services/FileSystemService';
 import { checkPermission, requirePermission } from '../middleware/permissions';
 import { escapeCsvField } from '../utils/csv';
 import { getErrorMessage } from '../utils/errors';
@@ -108,6 +110,29 @@ function validateContainerTarget(targetType: TargetType, targetId: unknown, node
   return null;
 }
 
+/**
+ * Validate that a stack or container target actually exists on the target
+ * node. Skipped for remote nodes (would require a proxy call).
+ */
+async function validateTargetExists(
+  targetType: TargetType,
+  targetId: string | null,
+  nodeId: number | null,
+): Promise<string | null> {
+  const isStack = targetType === 'stack';
+  if ((!isStack && targetType !== 'container') || !targetId || nodeId == null) return null;
+  const node = DatabaseService.getInstance().getNode(nodeId);
+  if (!node) return `${isStack ? 'Stack' : 'Container'} operations require an existing node.`;
+  if (node.type === 'remote') return null; // Skip existence check (would need proxy).
+  const exists = isStack
+    ? (await FileSystemService.getInstance(nodeId).getStacks()).includes(targetId)
+    : (await DockerController.getInstance(nodeId).findContainerByName(targetId)) != null;
+  if (!exists) {
+    return `${isStack ? 'Stack' : 'Container'} "${targetId}" not found on the target node.`;
+  }
+  return null;
+}
+
 /**
  * Shared guard for non-stack actions that require a node. Stack actions use
  * validateStackTarget because they also require target_id. Label-targeted
@@ -121,6 +146,7 @@ function validateActionNode(
   selectorType?: unknown,
 ): string | null {
   if (targetType === 'stack' || targetType === 'container') return null;
+
   const def = getScheduledActionDefinition(action);
   if (!def?.requiresNode) return null;
 
@@ -137,11 +163,14 @@ function validateActionNode(
 
   const parsedNodeId = parsePositiveNodeId(nodeId);
   if (parsedNodeId === null) return `${labelSingular} action requires a valid node_id.`;
-  if (def.nodeScope !== 'local') return null;
 
+  // Validate node existence for every action, not only local-scoped ones.
   const node = DatabaseService.getInstance().getNode(parsedNodeId);
-  if (!node) return `${labelPlural} require an existing local node.`;
-  if (node.type === 'remote') return `${labelPlural} currently require a local node.`;
+  if (!node) return `${labelSingular} action requires an existing node.`;
+
+  if (def.nodeScope === 'local' && node.type === 'remote') {
+    return `${labelPlural} currently require a local node.`;
+  }
   return null;
 }
 
@@ -345,7 +374,7 @@ scheduledTasksRouter.get('/', (req: Request, res: Response): void => {
   }
 });
 
-scheduledTasksRouter.post('/', (req: Request, res: Response): void => {
+scheduledTasksRouter.post('/', async (req: Request, res: Response): Promise<void> => {
   try {
     const {
       name, target_type, target_id, node_id, action, cron_expression, enabled,
@@ -366,8 +395,6 @@ scheduledTasksRouter.post('/', (req: Request, res: Response): void => {
     const targetErr = validateActionTarget(action, target_type);
     if (targetErr) { res.status(400).json({ error: targetErr }); return; }
 
-    const nodeErr = validateActionNode(action, target_type, node_id, selector_type);
-    if (nodeErr) { res.status(400).json({ error: nodeErr }); return; }
     const stackTargetErr = validateStackTarget(target_type, target_id, node_id);
     if (stackTargetErr) { res.status(400).json({ error: stackTargetErr }); return; }
     const containerTargetErr = validateContainerTarget(target_type, target_id, node_id);
@@ -384,6 +411,7 @@ scheduledTasksRouter.post('/', (req: Request, res: Response): void => {
     const runAtErr = validateRunAt(run_at);
     if (runAtErr) { res.status(400).json({ error: runAtErr }); return; }
 
+    // Compute normalized IDs early so existence validators can use them.
     const labelSelector = usesStackLabelSelector(action, target_type, selector_type);
     const normalizedNodeId = labelSelector
       ? (node_id == null || node_id === '' ? null : parsePositiveNodeId(node_id))
@@ -394,7 +422,9 @@ scheduledTasksRouter.post('/', (req: Request, res: Response): void => {
     const normalizedTargetId =
       target_type === 'stack' || target_type === 'container' ? target_id : null;
 
-    // Permission check on the resolved action+target scope.
+    // Permission check on the resolved action+target scope. Runs before
+    // existence validators so unauthorized callers cannot probe whether a
+    // stack or container exists on a node they should not reach.
     if (!requireTaskPermission(req, res, {
       action,
       target_type,
@@ -403,6 +433,16 @@ scheduledTasksRouter.post('/', (req: Request, res: Response): void => {
       selector_type: labelSelector ? STACK_LABEL_SELECTOR : null,
     })) return;
 
+    // Node existence validation for fleet and system actions. Runs after
+    // permission so unauthorized callers cannot probe node IDs via the error
+    // code difference (400 "node doesn't exist" vs 403 "permission denied").
+    const nodeErr = validateActionNode(action, target_type, node_id, selector_type);
+    if (nodeErr) { res.status(400).json({ error: nodeErr }); return; }
+
+    // Validate target existence for stack and container targets on local nodes.
+    const targetExistErr = await validateTargetExists(target_type, normalizedTargetId, normalizedNodeId);
+    if (targetExistErr) { res.status(400).json({ error: targetExistErr }); return; }
+
     const scheduler = SchedulerService.getInstance();
     const now = Date.now();
     const pinnedRunAt = typeof run_at === 'number' ? run_at : null;
@@ -460,7 +500,7 @@ scheduledTasksRouter.get('/:id', (req: Request, res: Response): void => {
   }
 });
 
-scheduledTasksRouter.put('/:id', (req: Request, res: Response): void => {
+scheduledTasksRouter.put('/:id', async (req: Request, res: Response): Promise<void> => {
   try {
     const id = parseIntParam(req, res, 'id', 'task ID');
     if (id === null) return;
@@ -506,9 +546,6 @@ scheduledTasksRouter.put('/:id', (req: Request, res: Response): void => {
     const targetErr = validateActionTarget(finalAction, finalTargetType);
     if (targetErr) { res.status(400).json({ error: targetErr }); return; }
 
-    const nodeErr = validateActionNode(finalAction, finalTargetType, finalNodeId, finalSelectorType);
-    if (nodeErr) { res.status(400).json({ error: nodeErr }); return; }
-
     const stackTargetErr = validateStackTarget(finalTargetType, finalTargetId, finalNodeId);
     if (stackTargetErr) { res.status(400).json({ error: stackTargetErr }); return; }
 
@@ -600,14 +637,26 @@ scheduledTasksRouter.put('/:id', (req: Request, res: Response): void => {
     }
 
     // Second phase: the caller must have permission for the merged scope.
+    const parsedFinalNodeId = finalNodeId != null ? parsePositiveNodeId(finalNodeId) : null;
     if (!requireTaskPermission(req, res, {
       action: finalAction,
       target_type: finalTargetType,
       target_id: finalTargetId,
-      node_id: finalNodeId != null ? parsePositiveNodeId(finalNodeId) : null,
+      node_id: parsedFinalNodeId,
       selector_type: finalSelectorType,
     })) return;
 
+    // Node existence validation for fleet and system actions. Runs after
+    // both permission phases so unauthorized callers cannot probe node IDs.
+    const nodeErr = validateActionNode(finalAction, finalTargetType, finalNodeId, finalSelectorType);
+    if (nodeErr) { res.status(400).json({ error: nodeErr }); return; }
+
+    // Validate target existence for stack and container targets on local nodes.
+    // Runs after both permission phases so unauthorized callers cannot probe
+    // whether a stack or container exists on a target they cannot access.
+    const targetExistErr = await validateTargetExists(finalTargetType, finalTargetId, parsedFinalNodeId);
+    if (targetExistErr) { res.status(400).json({ error: targetExistErr }); return; }
+
     db.updateScheduledTask(id, updates);
     console.log(`[ScheduledTasks] Updated task id=${id}`);
     const task = db.getScheduledTask(id);