diff --git a/playwright.config.js b/playwright.config.js
index 8216626..8199f31 100644
--- a/playwright.config.js
+++ b/playwright.config.js
@@ -43,6 +43,10 @@ export default defineConfig({
       name: 'transmittal-drag-drop',
       testMatch: 'transmittal-drag-drop.spec.js',
     },
+    {
+      name: 'transmittal-validation',
+      testMatch: 'transmittal-validation.spec.js',
+    },
     {
       name: 'classifier',
       testMatch: 'classifier.spec.js',
diff --git a/tests/transmittal-validation.spec.js b/tests/transmittal-validation.spec.js
new file mode 100644
index 0000000..15c605e
--- /dev/null
+++ b/tests/transmittal-validation.spec.js
@@ -0,0 +1,82 @@
+import { test, expect } from '@playwright/test';
+import { MOCK_FS_INIT_SCRIPT } from './fixtures/mock-fs-api.js';
+import * as path from 'path';
+
+const HTML_PATH = path.resolve('transmittal/dist/transmittal.html');
+
+// Covers the transmittal validation module (transmittal/js/validation.js),
+// which had no in-browser coverage:
+//   - the live #tracking-number aria-invalid binding (whitespace / underscore)
+//   - validateBeforePublish(): the publish-time gate that rejects a tracking
+//     number or a per-file tracking number / revision containing spaces or
+//     underscores.
+test.describe('Transmittal – validation', () => {
+    test.beforeEach(async ({ page }) => {
+        await page.addInitScript(MOCK_FS_INIT_SCRIPT);
+        await page.goto(`file://${HTML_PATH}`, { waitUntil: 'networkidle' });
+        await page.waitForSelector('#tracking-number');
+    });
+
+    test('tracking-number field flags whitespace and underscores live', async ({ page }) => {
+        const input = page.locator('#tracking-number');
+
+        await input.fill('123456-EL-TRX-0001');
+        await expect(input).toHaveAttribute('aria-invalid', 'false');
+
+        await input.fill('123456 EL TRX'); // space
+        await expect(input).toHaveAttribute('aria-invalid', 'true');
+        await expect(input).toHaveClass(/border-red-500/);
+
+        await input.fill('123456_EL_TRX'); // underscore
+        await expect(input).toHaveAttribute('aria-invalid', 'true');
+
+        await input.fill('123456-EL-TRX-0002'); // clean again — clears the flag
+        await expect(input).toHaveAttribute('aria-invalid', 'false');
+        await expect(input).not.toHaveClass(/border-red-500/);
+    });
+
+    test('validateBeforePublish passes for a clean transmittal', async ({ page }) => {
+        const result = await page.evaluate(() => {
+            const app = window.transmittalApp;
+            app.dom.qs('#tracking-number').value = '123456-EL-TRX-0001';
+            app.data.files = [{
+                trackingNumber: '123456-EL-SPC-2623', revision: 'A',
+                status: 'IFC', extension: 'pdf', title: 'Spec',
+                name: 'x', path: '', size: 0, fileSize: 0, sha256: '',
+            }];
+            return app.modules.validation.validateBeforePublish();
+        });
+        expect(result.ok).toBe(true);
+        expect(result.message).toBe('');
+    });
+
+    test('validateBeforePublish rejects a tracking number with spaces/underscores and focuses it', async ({ page }) => {
+        const result = await page.evaluate(() => {
+            const app = window.transmittalApp;
+            app.dom.qs('#tracking-number').value = 'BAD NUMBER_01';
+            app.data.files = [];
+            const r = app.modules.validation.validateBeforePublish();
+            return { ok: r.ok, message: r.message, focusId: r.focusEl ? r.focusEl.id : null };
+        });
+        expect(result.ok).toBe(false);
+        expect(result.message).toContain('spaces or underscores');
+        expect(result.focusId).toBe('tracking-number'); // gate points the user at the bad field
+    });
+
+    test('validateBeforePublish flags per-file bad tracking numbers and revisions by row', async ({ page }) => {
+        const result = await page.evaluate(() => {
+            const app = window.transmittalApp;
+            app.dom.qs('#tracking-number').value = '123456-EL-TRX-0001'; // header is clean
+            app.data.files = [
+                { trackingNumber: '123456 EL SPC', revision: 'A' },  // row 1: space in tracking
+                { trackingNumber: '123456-EL-DRW', revision: 'B 1' }, // row 2: space in revision
+            ];
+            return app.modules.validation.validateBeforePublish();
+        });
+        expect(result.ok).toBe(false);
+        expect(result.message).toContain('Row 1');
+        expect(result.message).toContain('Row 2');
+        expect(result.message).toMatch(/tracking number must not contain spaces or underscores/i);
+        expect(result.message).toMatch(/revision must not contain spaces/i);
+    });
+});