diff --git a/test/cso-preserved.test.ts b/test/cso-preserved.test.ts
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+/**
+ * cso security-guidance preservation test (v1.45.0.0 T6).
+ *
+ * The cso skill carries load-bearing security prose: OWASP Top 10 mappings,
+ * STRIDE threat-model phrasing, "do not auto-fix without user approval"
+ * gates. Codex 2nd-pass critique #9: "cso exemption too broad ... should
+ * still get resolver dedup, catalog trim, sectioning if safe, and targeted
+ * evals around must-not-miss checks."
+ *
+ * This test pins the must-not-miss checks. cso gets the same resolver gate
+ * (T2), jargon dedup (T3), and catalog trim (T4) as every other skill — but
+ * its security-guidance body content stays intact. Future compression work
+ * that would strip this content fails CI here.
+ */
+
+import { describe, test, expect } from 'bun:test';
+import * as fs from 'fs';
+import * as path from 'path';
+
+const REPO_ROOT = path.resolve(import.meta.dir, '..');
+const CSO_SKILL = path.join(REPO_ROOT, 'cso', 'SKILL.md');
+
+const MUST_PRESERVE_PHRASES = [
+  // OWASP / STRIDE positioning
+  'OWASP',
+  'STRIDE',
+  // Mode discipline
+  'daily',
+  'comprehensive',
+  // Severity language
+  'confidence',
+  // Active verification requirement (codex critique: "active verification")
+  'verif', // covers "verify", "verification", "verified"
+];
+
+const MUST_PRESERVE_HEADINGS = [
+  '## Preamble',  // from PREAMBLE resolver
+];
+
+describe('cso skill preserves load-bearing security guidance', () => {
+  test('cso/SKILL.md exists and is non-trivial', () => {
+    expect(fs.existsSync(CSO_SKILL)).toBe(true);
+    const content = fs.readFileSync(CSO_SKILL, 'utf-8');
+    // cso is a content-heavy security skill; under 30 KB suggests stripping went too far.
+    expect(content.length).toBeGreaterThan(30_000);
+  });
+
+  test('cso preserves required security phrases (case-insensitive)', () => {
+    const content = fs.readFileSync(CSO_SKILL, 'utf-8').toLowerCase();
+    const missing: string[] = [];
+    for (const phrase of MUST_PRESERVE_PHRASES) {
+      if (!content.includes(phrase.toLowerCase())) missing.push(phrase);
+    }
+    if (missing.length > 0) {
+      throw new Error(
+        `cso/SKILL.md is missing required security phrases: ${missing.join(', ')}. ` +
+        `These are load-bearing for the skill's audit posture. If you intentionally ` +
+        `removed them, update this test with the new phrasing.`,
+      );
+    }
+  });
+
+  test('cso preserves required headings', () => {
+    const content = fs.readFileSync(CSO_SKILL, 'utf-8');
+    for (const heading of MUST_PRESERVE_HEADINGS) {
+      expect(content).toContain(heading);
+    }
+  });
+
+  test('cso catalog trim landed (frontmatter description ≤ 200 chars)', () => {
+    const content = fs.readFileSync(CSO_SKILL, 'utf-8');
+    const fmMatch = content.match(/^---\n([\s\S]*?)\n---/);
+    expect(fmMatch).not.toBeNull();
+    const fm = fmMatch![1];
+    const descMatch = fm.match(/^description:\s+(.+)$/m);
+    expect(descMatch).not.toBeNull();
+    const desc = descMatch![1].trim();
+    expect(desc.length).toBeLessThanOrEqual(200);
+    expect(desc).toContain('(gstack)');
+  });
+
+  test('cso routing prose moved to "## When to invoke" body section', () => {
+    const content = fs.readFileSync(CSO_SKILL, 'utf-8');
+    expect(content).toContain('## When to invoke this skill');
+  });
+});