diff --git a/src/cli/commands/dev/__tests__/harness-logs.test.ts b/src/cli/commands/dev/__tests__/harness-logs.test.ts
new file mode 100644
index 000000000..974c7af64
--- /dev/null
+++ b/src/cli/commands/dev/__tests__/harness-logs.test.ts
@@ -0,0 +1,73 @@
+// Regression test for #1406: `dev --logs` on a harness-only project must exit 1
+// (ValidationError), not print guidance and exit 0.
+import { registerDev } from '../command.js';
+import * as devOps from '../../../operations/dev';
+import { Command } from '@commander-js/extra-typings';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('../../../tui/guards', () => ({
+  requireProject: vi.fn(),
+  requireTTY: vi.fn(),
+}));
+
+vi.mock('../../../telemetry/cli-command-run.js', () => ({
+  withCommandRunTelemetry: vi.fn((_key: string, _attrs: unknown, fn: (recorder: { set: () => void }) => unknown) =>
+    fn({ set: vi.fn() })
+  ),
+}));
+
+vi.mock('../../../operations/dev', async importOriginal => ({
+  ...(await importOriginal<typeof import('../../../operations/dev')>()),
+  loadProjectConfig: vi.fn(),
+  getDevSupportedAgents: vi.fn(),
+}));
+
+describe('dev --logs on a harness-only project (#1406)', () => {
+  let exitCodes: (number | undefined)[];
+  let errors: string[];
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    exitCodes = [];
+    errors = [];
+    // Harness-only project: no runtimes, one harness, zero dev-supported agents.
+    vi.mocked(devOps.loadProjectConfig).mockResolvedValue({
+      runtimes: [],
+      harnesses: [{ name: 'my-harness' }],
+    } as never);
+    vi.mocked(devOps.getDevSupportedAgents).mockReturnValue([]);
+    vi.spyOn(console, 'log').mockImplementation(() => undefined);
+    vi.spyOn(console, 'error').mockImplementation((...args: unknown[]) => {
+      errors.push(args.join(' '));
+    });
+    vi.spyOn(process, 'exit').mockImplementation((code?: string | number | null) => {
+      exitCodes.push(typeof code === 'number' ? code : undefined);
+      return undefined as never;
+    });
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('exits 1 (ValidationError) instead of returning success and exiting 0', async () => {
+    const program = new Command();
+    program.exitOverride();
+    registerDev(program);
+
+    await program.parseAsync(['dev', '--logs', '--skip-deploy', '--no-traces'], { from: 'user' }).catch(() => undefined);
+
+    expect(exitCodes).toContain(1);
+    expect(exitCodes).not.toContain(0);
+  });
+
+  it('errors directing the user to `agentcore invoke --harness <name>`', async () => {
+    const program = new Command();
+    program.exitOverride();
+    registerDev(program);
+
+    await program.parseAsync(['dev', '--logs', '--skip-deploy', '--no-traces'], { from: 'user' }).catch(() => undefined);
+
+    expect(errors.join('\n')).toContain('agentcore invoke --harness my-harness');
+  });
+});
diff --git a/src/cli/commands/dev/command.tsx b/src/cli/commands/dev/command.tsx
index e680899d7..422f86e65 100644
--- a/src/cli/commands/dev/command.tsx
+++ b/src/cli/commands/dev/command.tsx
@@ -359,21 +359,13 @@ export const registerDev = (program: Command) => {
 
             // --logs: non-interactive server mode
             if (opts.logs) {
-              // Harness-only projects need deploy then print invoke instructions
+              // Harness-only projects have no local dev server to tail logs from.
               if (supportedAgents.length === 0 && hasHarnesses) {
                 recorder.set({ agent_environment: 'harness' as const });
-                if (!opts.skipDeploy) {
-                  await runCliDeploy();
-                }
-                const harnessNames = (project.harnesses ?? []).map(h => h.name);
-                console.log('Harness dev runs against the deployed service (no local server).');
-                console.log(`If you changed the harness config, redeploy to pick up changes: agentcore deploy`);
-                console.log(`\nInvoke your harness:`);
-                for (const name of harnessNames) {
-                  console.log(`  agentcore invoke --harness ${name} "your prompt"`);
-                }
-                console.log(`\nOr use the interactive TUI: agentcore dev`);
-                return { success: true as const, blockingPromise: Promise.resolve() };
+                const firstHarness = project.harnesses?.[0]?.name ?? '<name>';
+                throw new ValidationError(
+                  `Harness projects do not support local dev. Use \`agentcore invoke --harness ${firstHarness}\` instead.`
+                );
               }
 
               if (project.runtimes.length > 1 && !opts.runtime) {