diff --git a/apps/server/package.json b/apps/server/package.json
index 517f1b3d..86745b57 100644
--- a/apps/server/package.json
+++ b/apps/server/package.json
@@ -43,6 +43,7 @@
     "@clickhouse/client": "^1.18.2",
     "@docmost/mcp": "workspace:*",
     "@docmost/pdf-inspector": "1.9.6",
+    "@docmost/prosemirror-markdown": "workspace:*",
     "@fastify/cookie": "^11.0.2",
     "@fastify/multipart": "^10.0.0",
     "@fastify/static": "^9.1.3",
@@ -175,7 +176,7 @@
       "/node_modules/"
     ],
     "transform": {
-      "happy-dom.+\\.js$": [
+      "(happy-dom.+|prosemirror-markdown/build/.+)\\.js$": [
         "babel-jest",
         {
           "presets": [
@@ -193,7 +194,7 @@
       "^.+\\.(t|j)sx?$": "ts-jest"
     },
     "transformIgnorePatterns": [
-      "/node_modules/(?!(\\.pnpm/)?(nanoid|uuid|image-dimensions|marked|happy-dom|lib0)(@|/))"
+      "/node_modules/(?!(\\.pnpm/)?(nanoid|uuid|image-dimensions|marked|happy-dom|lib0|@docmost/prosemirror-markdown)(@|/))"
     ],
     "collectCoverageFrom": [
       "**/*.(t|j)s"
@@ -204,7 +205,8 @@
       "^@docmost/db/(.*)$": "<rootDir>/database/$1",
       "^@docmost/transactional/(.*)$": "<rootDir>/integrations/transactional/$1",
       "^@docmost/ee/(.*)$": "<rootDir>/ee/$1",
-      "^src/(.*)$": "<rootDir>/$1"
+      "^src/(.*)$": "<rootDir>/$1",
+      "^@tiptap/react$": "<rootDir>/../test/stubs/tiptap-react.js"
     }
   }
 }
diff --git a/apps/server/src/collaboration/collaboration.util.ts b/apps/server/src/collaboration/collaboration.util.ts
index 7970051b..50e961ce 100644
--- a/apps/server/src/collaboration/collaboration.util.ts
+++ b/apps/server/src/collaboration/collaboration.util.ts
@@ -43,7 +43,6 @@ import {
   Column,
   Status,
   addUniqueIdsToDoc,
-  htmlToMarkdown,
   TransclusionSource,
   TransclusionReference,
   FootnoteReference,
@@ -51,6 +50,7 @@ import {
   FootnoteDefinition,
   PageEmbed,
 } from '@docmost/editor-ext';
+import { convertProseMirrorToMarkdown } from '@docmost/prosemirror-markdown';
 import { generateText, getSchema, JSONContent } from '@tiptap/core';
 import { generateHTML, generateJSON } from '../common/helpers/prosemirror/html';
 // @tiptap/html library works best for generating prosemirror json state but not HTML
@@ -239,6 +239,10 @@ export function prosemirrorNodeToYElement(node: any): Y.XmlElement | Y.XmlText {
 }
 
 export function jsonToMarkdown(tiptapJson: any): string {
-  const html = jsonToHtml(tiptapJson);
-  return htmlToMarkdown(html);
+  // Direct ProseMirror JSON -> Markdown via the canonical converter
+  // (`@docmost/prosemirror-markdown`) — no HTML intermediate, no second
+  // editor-ext markdown layer. Same serializer as the page/space export and the
+  // git-sync vault writer, so every server PM->MD path emits identical canonical
+  // markdown (issue #345).
+  return convertProseMirrorToMarkdown(tiptapJson);
 }
diff --git a/apps/server/src/integrations/export/export-markdown.spec.ts b/apps/server/src/integrations/export/export-markdown.spec.ts
new file mode 100644
index 00000000..a242e6cb
--- /dev/null
+++ b/apps/server/src/integrations/export/export-markdown.spec.ts
@@ -0,0 +1,145 @@
+// export.service.ts imports the ESM-only @sindresorhus/slugify (not in jest's
+// transform allowlist). It is irrelevant to the markdown-serialization path under
+// test (only used for page-mention link slugs on the DB path), so it is mocked
+// out to keep the module graph loadable under ts-jest (mirrors the import specs).
+jest.mock('@sindresorhus/slugify', () => ({
+  __esModule: true,
+  default: (input: string) => String(input),
+}));
+
+import { convertProseMirrorToMarkdown } from '@docmost/prosemirror-markdown';
+import { ExportService } from './export.service';
+import { ExportFormat } from './dto/export-dto';
+
+/**
+ * STEP 1 golden test for issue #345: server MARKDOWN export runs DIRECTLY through
+ * the canonical converter (`convertProseMirrorToMarkdown`) — no HTML intermediate
+ * and no `@docmost/editor-ext` markdown layer — so the emitted markdown is in the
+ * canonical package forms and is byte-identical to the git-sync vault body.
+ *
+ * These are the goldens the swap has to satisfy: they assert the CANONICAL
+ * surface (callout `> [!type]`, inline footnote `^[…]`, lossless image
+ * `<!--img …-->`) rather than the old editor-ext forms (`:::type`, `[^id]`,
+ * lossy `![alt](src)`).
+ *
+ * `exportPage(..., singlePage=false)` takes no DB path (no mention rewriting), so
+ * the service is constructed with null collaborators and only the pure
+ * PM -> Markdown path is exercised.
+ */
+
+function makeService(): ExportService {
+  return new ExportService(
+    null as any, // pageRepo
+    null as any, // pagePermissionRepo
+    null as any, // db
+    null as any, // storageService
+    null as any, // environmentService
+    null as any, // domainService
+  );
+}
+
+// A representative page exercising the node types whose canonical markdown form
+// changed with the move off the editor-ext layer: callout, inline footnote, and a
+// lossless image carrying width/align attrs that the old layer dropped.
+const REPRESENTATIVE_DOC = {
+  type: 'doc',
+  content: [
+    {
+      type: 'paragraph',
+      content: [
+        { type: 'text', text: 'Body ' },
+        { type: 'footnoteReference', attrs: { id: 'fn-1' } },
+        { type: 'text', text: ' end.' },
+      ],
+    },
+    {
+      type: 'callout',
+      attrs: { type: 'info', icon: null },
+      content: [
+        {
+          type: 'paragraph',
+          content: [{ type: 'text', text: 'Heads up' }],
+        },
+      ],
+    },
+    {
+      type: 'image',
+      attrs: {
+        src: '/files/pic.png',
+        alt: 'Pic',
+        width: 320,
+        align: 'left',
+      },
+    },
+    {
+      type: 'footnotesList',
+      content: [
+        {
+          type: 'footnoteDefinition',
+          attrs: { id: 'fn-1' },
+          content: [
+            {
+              type: 'paragraph',
+              content: [{ type: 'text', text: 'the note' }],
+            },
+          ],
+        },
+      ],
+    },
+  ],
+};
+
+describe('ExportService — markdown export via the canonical converter (#345)', () => {
+  it('emits canonical callout, inline footnote and lossless image forms', async () => {
+    const service = makeService();
+    const md = (await service.exportPage(ExportFormat.Markdown, {
+      title: '',
+      content: REPRESENTATIVE_DOC,
+    } as any)) as string;
+
+    // Callout: Obsidian `> [!type]`, NOT the legacy `:::type`.
+    expect(md).toContain('> [!info]');
+    expect(md).not.toContain(':::');
+
+    // Inline footnote: `^[…]`, NOT the reference `[^id]` form.
+    expect(md).toContain('^[the note]');
+    expect(md).not.toMatch(/\[\^/);
+
+    // Lossless image: trailing `<!--img …-->` carrying the dropped attrs.
+    expect(md).toContain('![Pic](/files/pic.png)');
+    expect(md).toContain('<!--img');
+    expect(md).toContain('"width":"320"');
+    expect(md).toContain('"align":"left"');
+  });
+
+  it('export body is byte-identical to the git-sync vault serializer (export == vault)', async () => {
+    const service = makeService();
+    // A title-less page: exportPage prepends NO heading, so the whole output is
+    // the page BODY — exactly what git-sync serializes (git-sync stores the title
+    // in frontmatter / the filename, never as an in-body H1).
+    const exported = (await service.exportPage(ExportFormat.Markdown, {
+      title: '',
+      content: REPRESENTATIVE_DOC,
+    } as any)) as string;
+
+    // The git-sync vault writer feeds this SAME converter (git-sync
+    // `stabilizePageBody` = convertProseMirrorToMarkdown(content) at the
+    // fixpoint). For an already-stable doc the single pass IS the fixpoint, so
+    // the two are byte-identical by construction — assert it.
+    const vaultBody = convertProseMirrorToMarkdown(REPRESENTATIVE_DOC);
+    expect(exported).toBe(vaultBody);
+  });
+
+  it('prepends the page title as an H1 heading (the one documented export/vault delta)', async () => {
+    const service = makeService();
+    const md = (await service.exportPage(ExportFormat.Markdown, {
+      title: 'My Page',
+      content: { type: 'doc', content: [] },
+    } as any)) as string;
+
+    // Export makes standalone files, so it prepends the title as an H1. This is
+    // the ONE deliberate difference from the vault body (which carries the title
+    // in frontmatter). The body below the heading still serializes canonically.
+    expect(md.startsWith('# My Page')).toBe(true);
+  });
+});
diff --git a/apps/server/src/integrations/export/export.service.ts b/apps/server/src/integrations/export/export.service.ts
index b4bc73fa..c6f361b0 100644
--- a/apps/server/src/integrations/export/export.service.ts
+++ b/apps/server/src/integrations/export/export.service.ts
@@ -37,7 +37,7 @@ import {
   getAttachmentIds,
   getProsemirrorContent,
 } from '../../common/helpers/prosemirror/utils';
-import { htmlToMarkdown } from '@docmost/editor-ext';
+import { convertProseMirrorToMarkdown } from '@docmost/prosemirror-markdown';
 
 type AllowedAttachment = { id: string; fileName: string; filePath: string };
 
@@ -79,9 +79,8 @@ export class ExportService {
       prosemirrorJson.content.unshift(titleNode);
     }
 
-    const pageHtml = jsonToHtml(prosemirrorJson);
-
     if (format === ExportFormat.HTML) {
+      const pageHtml = jsonToHtml(prosemirrorJson);
       return `<!DOCTYPE html>
       <html>
         <head>
@@ -92,11 +91,14 @@ export class ExportService {
     }
 
     if (format === ExportFormat.Markdown) {
-      const newPageHtml = pageHtml.replace(
-        /<colgroup[^>]*>[\s\S]*?<\/colgroup>/gim,
-        '',
-      );
-      return htmlToMarkdown(newPageHtml);
+      // Direct ProseMirror JSON -> Markdown via the canonical converter
+      // (`@docmost/prosemirror-markdown`). This is the SAME serializer the
+      // git-sync vault writer feeds (see git-sync `stabilizePageBody`), so an
+      // exported page body is byte-identical to its vault representation — no
+      // HTML intermediate, no second markdown layer, no format drift (issue
+      // #345). The old `<colgroup>` scrub is gone with the HTML step: the
+      // converter emits GFM tables directly and never produces `<colgroup>`.
+      return convertProseMirrorToMarkdown(prosemirrorJson);
     }
 
     return;
diff --git a/apps/server/test/stubs/tiptap-react.js b/apps/server/test/stubs/tiptap-react.js
new file mode 100644
index 00000000..e11ef884
--- /dev/null
+++ b/apps/server/test/stubs/tiptap-react.js
@@ -0,0 +1,13 @@
+// Jest stub for @tiptap/react. The server export/import code paths transitively
+// import editor-ext, whose node extensions reference `ReactNodeViewRenderer`
+// inside `addNodeView()` — code that only runs inside a live browser editor and
+// is NEVER invoked on the server. The real module eagerly pulls react-dom, which
+// throws `navigator is not defined` under jest's node environment. This stub
+// supplies the named exports the extensions bind at import time; if any were
+// actually called on the server that would (correctly) surface as a test error.
+module.exports = {
+  ReactNodeViewRenderer: () => () => ({}),
+  NodeViewWrapper: () => null,
+  NodeViewContent: () => null,
+  ReactRenderer: class {},
+};