diff --git a/packages/pen-core/src/__tests__/element-builders-no-css-padding.test.ts b/packages/pen-core/src/__tests__/element-builders-no-css-padding.test.ts
new file mode 100644
index 000000000..3d1ec5e25
--- /dev/null
+++ b/packages/pen-core/src/__tests__/element-builders-no-css-padding.test.ts
@@ -0,0 +1,68 @@
+/**
+ * Static guard: element-builder source files must not use the CSS-side
+ * padding shorthands `paddingTop` / `paddingRight` / `paddingBottom`
+ * / `paddingLeft` as object property assignments.
+ *
+ * The layout engine's `resolvePadding` (`packages/pen-core/src/layout/
+ * engine.ts`) only reads the unified `padding` field
+ * (`number | [T,R] | [T,R,B,L] | string`); the CSS-side siblings
+ * are silently dropped, so nodes that declared them rendered with
+ * zero insets. Codex caught this on `add_sidebar_nav_v0` (commit
+ * 5c88aca6) and a 14-builder follow-up sweep landed in 5c91ff19.
+ *
+ * This test makes the trap loud — any future builder that re-introduces
+ * the shorthand fails CI before reaching review. References to the
+ * field names in JSDoc / line comments are still allowed (`* `, `//`)
+ * because that's how the warning is documented in builder doc blocks.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { readFileSync, readdirSync } from 'node:fs';
+import { join } from 'node:path';
+
+const BUILDERS_DIR = join(__dirname, '..', 'element-builders');
+
+const FORBIDDEN = ['paddingTop', 'paddingRight', 'paddingBottom', 'paddingLeft'] as const;
+
+function isCommentLine(line: string): boolean {
+  const trimmed = line.trimStart();
+  return (
+    trimmed.startsWith('//') ||
+    trimmed.startsWith('*') ||
+    trimmed.startsWith('/*') ||
+    trimmed.startsWith('*/')
+  );
+}
+
+describe('element-builders — no CSS-side padding shorthands', () => {
+  it('every builder uses padding: [T,R,B,L] / [T,R] / number, never paddingTop/Right/Bottom/Left', () => {
+    const offenders: string[] = [];
+    const files = readdirSync(BUILDERS_DIR).filter(
+      (f) => f.endsWith('.ts') && !f.endsWith('.d.ts'),
+    );
+    for (const file of files) {
+      const src = readFileSync(join(BUILDERS_DIR, file), 'utf-8');
+      const lines = src.split('\n');
+      for (let i = 0; i < lines.length; i += 1) {
+        const line = lines[i];
+        if (isCommentLine(line)) continue;
+        for (const field of FORBIDDEN) {
+          // Match `field:` as a property key (allow leading whitespace,
+          // optional quotes). Avoids matching inside strings of comments
+          // already filtered above; identifier boundary in front prevents
+          // matching `notPaddingTop`.
+          const re = new RegExp(`(?:^|[\\s,{('"\`])${field}\\s*:`);
+          if (re.test(line)) {
+            offenders.push(`${file}:${i + 1}: ${line.trim()}`);
+          }
+        }
+      }
+    }
+    expect(
+      offenders,
+      `Builders using CSS-side padding shorthand silently render with 0 inset. ` +
+        `Switch to the unified padding field (number | [T,R] | [T,R,B,L]).\n` +
+        offenders.join('\n'),
+    ).toEqual([]);
+  });
+});