fix(serializer): preserve canonical bullet and rule markers on round-trip

server/milkdown-headless.ts

@@ -116,6 +116,12 @@ function createSerializer(schema: Schema): (doc: ProseMirrorNode) => string {
     .use(remarkGfm)
     .use(remarkFrontmatter, ['yaml'])
     .use(remarkStringify, {
+      // Preserve canonical markdown style on round-trip. Without these, remark-stringify
+      // defaults to `*` for bullets and `***` for thematic breaks, producing cosmetic git
+      // diffs for any source that uses the `-` / `---` convention (which is the dominant
+      // style in CommonMark and GFM ecosystems).
+      bullet: '-',
+      rule: '-',
       handlers: {
         proofMark: proofMarkHandler,
       },

src/editor/index.ts

@@ -1227,6 +1227,10 @@ class ProofEditorImpl implements ProofEditor {
         // Note: remarkProofMarks is now registered via .use(remarkProofMarksPlugin)
         ctx.update(remarkStringifyOptionsCtx, (prev) => ({
           ...prev,
+          // Preserve canonical markdown style on round-trip. See server/milkdown-headless.ts
+          // for the same options applied on the headless serializer path.
+          bullet: '-',
+          rule: '-',
           handlers: {
             ...(prev.handlers ?? {}),
             proofMark: proofMarkHandler,

src/tests/milkdown-headless-serializer-style-fidelity.test.ts

@@ -0,0 +1,74 @@
+import { getHeadlessMilkdownParser, serializeMarkdown } from '../../server/milkdown-headless.js';
+
+function assert(condition: boolean, message: string): void {
+  if (!condition) throw new Error(message);
+}
+
+async function run(): Promise<void> {
+  const parser = await getHeadlessMilkdownParser();
+
+  // Canonical source: dash bullets, triple-dash thematic breaks. This is the
+  // dominant style in CommonMark/GFM ecosystems (prettier, markdownlint, GitHub).
+  const source = [
+    '# Heading',
+    '',
+    'Paragraph one.',
+    '',
+    '- bullet one',
+    '- bullet two',
+    '- bullet three',
+    '',
+    '---',
+    '',
+    'Paragraph two.',
+    '',
+    '- nested list parent',
+    '  - nested child a',
+    '  - nested child b',
+    '',
+    '---',
+    '',
+    'Paragraph three.',
+    '',
+  ].join('\n');
+
+  const doc = parser.parseMarkdown(source);
+  const out = await serializeMarkdown(doc);
+
+  // Style fidelity: the serializer must emit the same bullet and rule markers
+  // it received. Without remark-stringify { bullet: '-', rule: '-' }, output
+  // drifts to '*' bullets and '***' rules — semantically equivalent CommonMark
+  // but cosmetically destructive for any workflow that round-trips real docs.
+  assert(
+    !out.includes('* bullet'),
+    `Bullet drift: serializer emitted '*' bullets. Output was:\n${out}`,
+  );
+  assert(
+    out.includes('- bullet one') && out.includes('- bullet two') && out.includes('- bullet three'),
+    `Top-level bullets should serialize as '- '. Output was:\n${out}`,
+  );
+  assert(
+    !out.includes('***'),
+    `Thematic break drift: serializer emitted '***' rules. Output was:\n${out}`,
+  );
+  assert(
+    out.includes('\n---\n'),
+    `Thematic breaks should serialize as '---'. Output was:\n${out}`,
+  );
+
+  // Stability: serialize(parse(source)) === serialize(parse(serialize(parse(source))))
+  // (the property the existing roundtrip test checks; verifying we didn't regress it).
+  const reparsed = parser.parseMarkdown(out);
+  const out2 = await serializeMarkdown(reparsed);
+  assert(
+    out === out2,
+    `Serializer is not stable across round-trips. First output:\n${out}\nSecond output:\n${out2}`,
+  );
+
+  console.log('✓ headless Milkdown serializer style fidelity (bullets, thematic breaks)');
+}
+
+run().catch((error) => {
+  console.error(error);
+  process.exit(1);
+});