fix(core): improve SKILL.md frontmatter parsing robustness

- Support UTF-8 BOM at the start of SKILL.md files.
- Support optional trailing spaces after frontmatter markers.
- Improve simple parser fallback to be case-insensitive and support spaces before colons.
- Prevent simple parser from swallowing subsequent keys into descriptions.
- Added unit tests for identified edge cases.

Fixes https://github.com/google-gemini/gemini-cli/issues/25693

packages/core/src/skills/skillLoader.test.ts

@@ -271,4 +271,76 @@ description: Test sanitization
     expect(skills).toHaveLength(1);
     expect(skills[0].name).toBe('gke-prs-troubleshooter');
   });
+
+  it('should handle UTF-8 BOM in SKILL.md', async () => {
+    const skillDir = path.join(testRootDir, 'bom-skill');
+    await fs.mkdir(skillDir, { recursive: true });
+    const skillFile = path.join(skillDir, 'SKILL.md');
+    await fs.writeFile(
+      skillFile,
+      `\uFEFF---\nname: bom-skill\ndescription: A skill with a BOM\n---\n# Instructions\nBody\n`,
+    );
+
+    const skills = await loadSkillsFromDir(testRootDir);
+
+    expect(skills).toHaveLength(1);
+    expect(skills[0].name).toBe('bom-skill');
+  });
+
+  it('should handle trailing spaces after frontmatter markers', async () => {
+    const skillDir = path.join(testRootDir, 'space-skill');
+    await fs.mkdir(skillDir, { recursive: true });
+    const skillFile = path.join(skillDir, 'SKILL.md');
+    await fs.writeFile(
+      skillFile,
+      `---  \nname: space-skill\ndescription: A skill with trailing spaces\n--- \n# Instructions\nBody\n`,
+    );
+
+    const skills = await loadSkillsFromDir(testRootDir);
+
+    expect(skills).toHaveLength(1);
+    expect(skills[0].name).toBe('space-skill');
+  });
+
+  it('should handle space before colon and case-insensitivity in simple parser', async () => {
+    const skillDir = path.join(testRootDir, 'simple-parser-robustness');
+    await fs.mkdir(skillDir, { recursive: true });
+    const skillFile = path.join(skillDir, 'SKILL.md');
+    // Forces YAML failure with unquoted colon, triggers simple parser
+    await fs.writeFile(
+      skillFile,
+      `---
+Name : robust-name
+DESCRIPTION : robust:description
+---
+`,
+    );
+
+    const skills = await loadSkillsFromDir(testRootDir);
+
+    expect(skills).toHaveLength(1);
+    expect(skills[0].name).toBe('robust-name');
+    expect(skills[0].description).toBe('robust:description');
+  });
+
+  it('should not swallow other keys into description in simple parser', async () => {
+    const skillDir = path.join(testRootDir, 'simple-parser-swallow');
+    await fs.mkdir(skillDir, { recursive: true });
+    const skillFile = path.join(skillDir, 'SKILL.md');
+    // Forces YAML failure, triggers simple parser
+    await fs.writeFile(
+      skillFile,
+      `---
+description: A long description: with a colon
+  name: my-skill
+---
+`,
+    );
+
+    const skills = await loadSkillsFromDir(testRootDir);
+
+    expect(skills).toHaveLength(1);
+    expect(skills[0].name).toBe('my-skill');
+    expect(skills[0].description).toBe('A long description: with a colon');
+  });
 });

packages/core/src/skills/skillLoader.ts

@@ -32,7 +32,7 @@ export interface SkillDefinition {
 }
 
 export const FRONTMATTER_REGEX =
-  /^---\r?\n([\s\S]*?)\r?\n---(?:\r?\n([\s\S]*))?/;
+  /^\uFEFF?---[ \t]*\r?\n([\s\S]*?)\r?\n---[ \t]*(?:\r?\n([\s\S]*))?/;
 
 /**
  * Parses frontmatter content using YAML with a fallback to simple key-value parsing.
@@ -49,6 +49,8 @@ export function parseFrontmatter(
       if (typeof name === 'string' && typeof description === 'string') {
         return { name, description };
       }
+      // If they are not strings (e.g. number or boolean), fall back to simple parser
+      // which will treat them as strings.
     }
   } catch (yamlError) {
     debugLogger.debug(
@@ -75,22 +77,23 @@ function parseSimpleFrontmatter(
     const line = lines[i];
 
     // Match "name:" at the start of the line (optional whitespace)
-    const nameMatch = line.match(/^\s*name:\s*(.*)$/);
+    const nameMatch = line.match(/^\s*name\s*:\s*(.*)$/i);
     if (nameMatch) {
       name = nameMatch[1].trim();
       continue;
     }
 
     // Match "description:" at the start of the line (optional whitespace)
-    const descMatch = line.match(/^\s*description:\s*(.*)$/);
+    const descMatch = line.match(/^\s*description\s*:\s*(.*)$/i);
     if (descMatch) {
       const descLines = [descMatch[1].trim()];
 
       // Check for multi-line description (indented continuation lines)
       while (i + 1 < lines.length) {
         const nextLine = lines[i + 1];
-        // If next line is indented, it's a continuation of the description
-        if (nextLine.match(/^[ \t]+\S/)) {
+        // If next line is indented, it's a continuation of the description,
+        // UNLESS it looks like another key (e.g. "name:")
+        if (nextLine.match(/^[ \t]+\S/) && !nextLine.match(/^\s*\w+\s*:/)) {
           descLines.push(nextLine.trim());
           i++;
         } else {