diff --git a/scripts/gen-skill-docs.ts b/scripts/gen-skill-docs.ts
index d030e79ad..fb72e4bd9 100644
--- a/scripts/gen-skill-docs.ts
+++ b/scripts/gen-skill-docs.ts
@@ -356,6 +356,28 @@ export function buildWhenToInvokeSection(parts: CatalogParts): string {
   return lines.join('\n');
 }
 
+/**
+ * Render a string as a YAML inline scalar value (the text after `key: `),
+ * quoting only when a plain scalar would be invalid or ambiguous.
+ *
+ * The bug this guards (#1778): a description like "Ship workflow: detect..."
+ * emitted as a plain scalar has an interior ": " that a strict YAML parser
+ * (Codex/OpenAI skill loading) reads as a nested mapping and rejects with
+ * "mapping values are not allowed in this context". When quoting is needed we
+ * fall back to JSON.stringify, which produces a double-quoted scalar that YAML
+ * accepts verbatim (YAML is a superset of JSON for flow scalars). Strings that
+ * are already valid plain scalars pass through unchanged to keep regen diffs small.
+ */
+export function toYamlInlineScalar(s: string): string {
+  const needsQuote =
+    s.length === 0 ||
+    s !== s.trim() ||                       // leading/trailing whitespace
+    /:(\s|$)/.test(s) ||                    // "foo: bar" / trailing colon → mapping ambiguity
+    /\s#/.test(s) ||                        // " #" → inline comment
+    /^[\s>|&*!%@`"'#,\[\]{}?-]/.test(s);    // leading YAML indicator char
+  return needsQuote ? JSON.stringify(s) : s;
+}
+
 /**
  * Apply catalog trim to a SKILL.md body:
  *  - shorten frontmatter `description:` to lead + (gstack)
@@ -397,8 +419,12 @@ export function applyCatalogTrim(content: string, skillName: string): { content:
 
   // Replace description in frontmatter — keep trailing newline so the next
   // YAML field doesn't collide on the same line as the description value.
+  // Quote the value when it would be an invalid YAML plain scalar (the common
+  // case: an interior ": " like "Ship workflow: detect..." which a strict YAML
+  // parser reads as a nested mapping and rejects — #1778). toYamlInlineScalar
+  // only quotes when needed, so descriptions without special chars stay plain.
   const newDesc = buildTrimmedDescription(parts);
-  const newFrontmatter = frontmatter.replace(descMatch[0], `description: ${newDesc}\n`);
+  const newFrontmatter = frontmatter.replace(descMatch[0], `description: ${toYamlInlineScalar(newDesc)}\n`);
   let newContent = '---\n' + newFrontmatter + content.slice(fmEnd);
 
   // Insert body section after frontmatter (after the closing ---\n and any
diff --git a/test/gen-skill-docs.test.ts b/test/gen-skill-docs.test.ts
index a405c2da9..49b5dadac 100644
--- a/test/gen-skill-docs.test.ts
+++ b/test/gen-skill-docs.test.ts
@@ -155,12 +155,39 @@ describe('gen-skill-docs', () => {
     }
   });
 
-  test('every generated SKILL.md has valid YAML frontmatter', () => {
+  // #1778: strict YAML parsers (Codex/OpenAI skill loading) reject frontmatter
+  // whose plain `description:` scalar contains an interior ": " (read as a nested
+  // mapping). Parse EVERY generated frontmatter block with a strict YAML parser,
+  // not just string-check that name:/description: exist.
+  function frontmatterBlock(content: string): string {
+    expect(content.startsWith('---\n')).toBe(true);
+    const end = content.indexOf('\n---', 4);
+    expect(end).toBeGreaterThan(0);
+    return content.slice(4, end);
+  }
+
+  test('every generated SKILL.md frontmatter parses as strict YAML', () => {
     for (const skill of CLAUDE_GENERATED_SKILLS) {
       const content = fs.readFileSync(path.join(ROOT, skill.dir, 'SKILL.md'), 'utf-8');
-      expect(content.startsWith('---\n')).toBe(true);
-      expect(content).toContain('name:');
-      expect(content).toContain('description:');
+      const fm = frontmatterBlock(content);
+      let parsed: any;
+      expect(() => { parsed = Bun.YAML.parse(fm); },
+        `frontmatter for ${skill.dir} must be valid YAML`).not.toThrow();
+      expect(typeof parsed?.name).toBe('string');
+      expect(typeof parsed?.description).toBe('string');
+    }
+  });
+
+  test('every generated Codex (.agents/skills) frontmatter parses as strict YAML', () => {
+    const agentsDir = path.join(ROOT, '.agents', 'skills');
+    if (!fs.existsSync(agentsDir)) return; // skip if external hosts not generated
+    for (const entry of fs.readdirSync(agentsDir, { withFileTypes: true })) {
+      if (!entry.isDirectory()) continue;
+      const mdPath = path.join(agentsDir, entry.name, 'SKILL.md');
+      if (!fs.existsSync(mdPath)) continue;
+      const fm = frontmatterBlock(fs.readFileSync(mdPath, 'utf-8'));
+      expect(() => Bun.YAML.parse(fm),
+        `Codex frontmatter for ${entry.name} must be valid YAML`).not.toThrow();
     }
   });