@@ -701,3 +701,26 @@ function singleStartingQuote(text: string) {
 function isSimpleQuote(text: string) {
   return text.startsWith('"') && text.endsWith('"') && text.split('"').length === 3
 }
+/**
+ * Write a Markdown page back out, preserving the original frontmatter text verbatim
+ * whenever the frontmatter data itself didn't change.
+ *
+ * Round-tripping frontmatter through the YAML serializer reflows values that were
+ * never touched: long `intro` strings become block scalars, `redirect_from` entries get
+ * rewrapped, and quote styles change. That churn dwarfs the actual link fixes and makes
+ * a bulk run unreviewable, which is why this only reserializes when it has to.
+ */
+export function serializeMarkdown(
+  rawContent: string,
+  content: string,
+  newContent: string,
+  newData: Record<string, unknown> | undefined,
+  differentData: boolean,
+): string {
+  // `content` is the tail of the file, so everything before it is the frontmatter
+  // block exactly as the author wrote it, delimiters and all.
+  if (!differentData && rawContent.endsWith(content)) {
+    return rawContent.slice(0, rawContent.length - content.length) + newContent
+  }
+  return frontmatter.stringify(newContent, newData || {})
+}
@@ -14,8 +14,7 @@ import { program } from 'commander'
 import chalk from 'chalk'
 import { dump } from 'js-yaml'
 
-import { updateInternalLinks } from '@/links/lib/update-internal-links'
-import frontmatter from '@/frame/lib/read-frontmatter'
+import { updateInternalLinks, serializeMarkdown } from '@/links/lib/update-internal-links'
 import walkFiles from '@/workflows/walk-files'
 
 program
@@ -158,7 +157,11 @@ async function main(files: string[], opts: Options) {
           } else {
             // Remember the `content` and `newContent` is the "meat" of the
             // Markdown page. To save it you need the frontmatter data too.
-            fs.writeFileSync(file, frontmatter.stringify(newContent || '', newData || {}), 'utf-8')
+            fs.writeFileSync(
+              file,
+              serializeMarkdown(rawContent, content, newContent, newData, differentData),
+              'utf-8',
+            )
           }
         }
       }
 
@@ -0,0 +1,119 @@
+import { describe, expect, test } from 'vitest'
+
+import frontmatter from '@/frame/lib/read-frontmatter'
+import { serializeMarkdown } from '@/links/lib/update-internal-links'
+
+const PAGE = `---
+title: About merge methods
+intro: 'You can allow contributors with push access to merge their pull requests on {% data variables.product.prodname_dotcom %} with different merge options.'
+redirect_from:
+  - /articles/about-merge-methods-on-github
+  - /github/administering-a-repository/about-merge-methods-on-github
+versions:
+  fpt: '*'
+  ghes: '*'
+---
+
+Body text with a [link](/en/old-path).
+`
+
+// `frontmatter()` only omits `content` when the YAML fails to parse, which none of
+// these fixtures do. Narrow it once here so each test can stay readable.
+function parse(raw: string): { content: string; data: Record<string, unknown> } {
+  const { content, data } = frontmatter(raw)
+  if (content === undefined) throw new Error('fixture failed to parse')
+  return { content, data: data || {} }
+}
+
+// The whole fix rests on gray-matter's `content` being an exact suffix of the raw file.
+// Test that against the real parser, not a hand-rolled stand-in.
+describe('frontmatter parse invariant', () => {
+  const cases: [string, string][] = [
+    ['standard page', PAGE],
+    ['CRLF line endings', PAGE.replace(/\n/g, '\r\n')],
+    ['byte order mark', `\uFEFF${PAGE}`],
+    ['no frontmatter', 'Just a body with a [link](/en/old-path).\n'],
+    ['empty frontmatter', '---\n---\n\nBody.\n'],
+    ['thematic break in body', '---\ntitle: T\n---\n\nBefore.\n\n---\n\nAfter.\n'],
+    ['empty body', '---\ntitle: T\n---\n'],
+    ['no trailing newline', '---\ntitle: T\n---\n\nBody.'],
+  ]
+
+  test.each(cases)('%s: content is a suffix of rawContent', (_name, raw) => {
+    const { content } = parse(raw)
+
+    expect(raw.endsWith(content)).toBe(true)
+  })
+
+  test.each(cases)('%s: round-trips unchanged when nothing changed', (_name, raw) => {
+    const { content, data } = parse(raw)
+
+    expect(serializeMarkdown(raw, content, content, data, false)).toBe(raw)
+  })
+
+  test.each(cases)('%s: preserves the frontmatter block byte-for-byte', (_name, raw) => {
+    const { content, data } = parse(raw)
+    const newContent = content.replace('/en/old-path', '/en/new-path')
+
+    const result = serializeMarkdown(raw, content, newContent, data, false)
+
+    const originalFrontmatter = raw.slice(0, raw.length - content.length)
+    expect(result).toBe(originalFrontmatter + newContent)
+  })
+})
+
+describe('serializeMarkdown', () => {
+  test('rewrites the body without reflowing untouched YAML', () => {
+    const { content, data } = parse(PAGE)
+    const newContent = content.replace('/en/old-path', '/en/new-path')
+
+    const result = serializeMarkdown(PAGE, content, newContent, data, false)
+
+    expect(result).toContain('/en/new-path')
+    expect(result).not.toContain('/en/old-path')
+    // The single-quoted intro and the list-style redirect_from must survive untouched.
+    expect(result).toContain(
+      "intro: 'You can allow contributors with push access to merge their pull requests",
+    )
+    expect(result).toContain('  - /articles/about-merge-methods-on-github')
+  })
+
+  test('reserializes when the frontmatter data actually changed', () => {
+    const { content } = parse(PAGE)
+
+    const result = serializeMarkdown(
+      PAGE,
+      content,
+      content,
+      { title: 'About merge methods', redirect_from: ['/new-redirect'] },
+      true,
+    )
+
+    expect(result).toContain('/new-redirect')
+    expect(result).not.toContain('/articles/about-merge-methods-on-github')
+    expect(result).toContain('Body text with a [link](/en/old-path).')
+  })
+
+  test('falls back to reserializing if content is not a clean tail of the raw file', () => {
+    const result = serializeMarkdown(
+      PAGE,
+      'not a suffix of the raw file',
+      'New body.',
+      { title: 'Fallback' },
+      false,
+    )
+
+    expect(result).toContain('title: Fallback')
+    expect(result).toContain('New body.')
+  })
+
+  test('is idempotent across repeated runs', () => {
+    const { content, data } = parse(PAGE)
+    const newContent = content.replace('/en/old-path', '/en/new-path')
+
+    const once = serializeMarkdown(PAGE, content, newContent, data, false)
+    const twice = serializeMarkdown(once, parse(once).content, newContent, data, false)
+
+    expect(twice).toBe(once)
+  })
+})