diff --git a/src/core/brain-writer.ts b/src/core/brain-writer.ts
index d7ff74082..1d963dbbc 100644
--- a/src/core/brain-writer.ts
+++ b/src/core/brain-writer.ts
@@ -139,8 +139,21 @@ export function autoFixFrontmatter(
     fixes.push({ code: 'NULL_BYTES', description: 'Stripped null bytes' });
   }
 
-  // 2. MISSING_CLOSE — if there's an opener but no closer before a heading,
-  //    insert `---` immediately before the heading. Walk lines once.
+  // 2. MISSING_CLOSE — if there's an opener but no closer at all, insert
+  //    `---` immediately before the first heading-shaped line (best-effort
+  //    guess at where the frontmatter was meant to end).
+  //
+  //    Find the closer FIRST, scanning the full zone — do not stop at the
+  //    first `#`-prefixed line. A `#` line between the opening and closing
+  //    `---` is a YAML comment (comments are valid anywhere in a YAML
+  //    document), not a markdown heading; only the genuine absence of a
+  //    closing `---` counts as MISSING_CLOSE. Mirrors the fix applied to
+  //    the parseMarkdown validator in #2153 — this is the sibling
+  //    reimplementation in the auto-fixer and had the same bug (it broke
+  //    out of the scan on the first heading-shaped line, so a `#` comment
+  //    appearing before a real closing fence was misdetected as
+  //    MISSING_CLOSE and the fix inserted a spurious `---` that split
+  //    valid frontmatter in two, pushing the real keys into the body).
   {
     const lines = working.split('\n');
     let firstNonEmpty = -1;
@@ -149,24 +162,27 @@ export function autoFixFrontmatter(
     }
     if (firstNonEmpty >= 0 && lines[firstNonEmpty].trim() === '---') {
       let closeIdx = -1;
-      let headingIdx = -1;
       for (let i = firstNonEmpty + 1; i < lines.length; i++) {
-        const t = lines[i].trim();
-        if (t === '---') { closeIdx = i; break; }
-        if (/^#{1,6}\s/.test(t)) { headingIdx = i; break; }
+        if (lines[i].trim() === '---') { closeIdx = i; break; }
       }
-      if (closeIdx === -1 && headingIdx >= 0) {
-        const fixed = [
-          ...lines.slice(0, headingIdx),
-          '---',
-          '',
-          ...lines.slice(headingIdx),
-        ];
-        working = fixed.join('\n');
-        fixes.push({
-          code: 'MISSING_CLOSE',
-          description: `Inserted closing --- before heading at line ${headingIdx + 1}`,
-        });
+      if (closeIdx === -1) {
+        let headingIdx = -1;
+        for (let i = firstNonEmpty + 1; i < lines.length; i++) {
+          if (/^#{1,6}\s/.test(lines[i].trim())) { headingIdx = i; break; }
+        }
+        if (headingIdx >= 0) {
+          const fixed = [
+            ...lines.slice(0, headingIdx),
+            '---',
+            '',
+            ...lines.slice(headingIdx),
+          ];
+          working = fixed.join('\n');
+          fixes.push({
+            code: 'MISSING_CLOSE',
+            description: `Inserted closing --- before heading at line ${headingIdx + 1}`,
+          });
+        }
       }
     }
   }
diff --git a/test/brain-writer.test.ts b/test/brain-writer.test.ts
index 611d69fff..733100f63 100644
--- a/test/brain-writer.test.ts
+++ b/test/brain-writer.test.ts
@@ -32,6 +32,43 @@ describe('autoFixFrontmatter', () => {
     expect(idxClose).toBeLessThan(idxHeading);
   });
 
+  // Regression for #3225: a `#`-prefixed line inside an already-closed
+  // frontmatter fence is a YAML comment, not a markdown heading. The old
+  // MISSING_CLOSE scan broke out on the first heading-shaped line without
+  // continuing to look for the real closer, so it inserted a spurious
+  // `---` before the comment and split valid frontmatter in two — pushing
+  // the real keys (title, pubDate, ...) into the document body.
+  test('does not corrupt closed frontmatter containing a YAML comment line', () => {
+    const input = `${fence}\n# a YAML comment inside the frontmatter block\ntitle: "Real Title"\npubDate: 2026-06-29\n${fence}\nBody...`;
+    const { content, fixes } = autoFixFrontmatter(input);
+    expect(content).toBe(input);
+    expect(fixes).toEqual([]);
+  });
+
+  test('does not corrupt closed frontmatter that is comment-only', () => {
+    const input = `${fence}\n# just a comment\n# another comment\n${fence}\nBody`;
+    const { content, fixes } = autoFixFrontmatter(input);
+    expect(content).toBe(input);
+    expect(fixes).toEqual([]);
+  });
+
+  test('does not corrupt closed frontmatter with an indented `#` line inside a YAML block scalar', () => {
+    const input = `${fence}\ndescription: |\n  # not a heading, just literal block-scalar text\ntitle: ok\n${fence}\nBody`;
+    const { content, fixes } = autoFixFrontmatter(input);
+    expect(content).toBe(input);
+    expect(fixes).toEqual([]);
+  });
+
+  test('YAML comment before close does not suppress an unrelated real fix (SLUG_MISMATCH)', () => {
+    const input = `${fence}\n# a YAML comment\ntitle: hi\nslug: wrong-slug\n${fence}\nBody`;
+    const { content, fixes } = autoFixFrontmatter(input, { filePath: 'people/jane-doe.md' });
+    expect(fixes.some(f => f.code === 'MISSING_CLOSE')).toBe(false);
+    expect(fixes.some(f => f.code === 'SLUG_MISMATCH')).toBe(true);
+    // The frontmatter fence itself must stay intact — only the slug line
+    // is removed, the comment/title/close survive unchanged.
+    expect(content).toBe(`${fence}\n# a YAML comment\ntitle: hi\n\n${fence}\nBody`);
+  });
+
   test('rewrites nested-quote title to single-quoted', () => {
     const input = `${fence}\ntype: concept\ntitle: "Phil "Nick" Last"\n${fence}\n\nbody`;
     const { content, fixes } = autoFixFrontmatter(input);