diff --git a/AGENTS.md b/AGENTS.md
index c1198fb..adf57ed 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -8,9 +8,11 @@ A daemon that bidirectionally syncs Docmost articles with a local Markdown git
 vault (git is the state store). It reuses the sibling project **docmost-mcp** as
 a library (DocmostClient, ProseMirror ↔ Markdown converter, collab-write).
 
-**Status: scaffold only — the sync engine is NOT implemented yet.** `src/index.ts`
-is a thin stub that validates config and exits. See `SPEC.md` for the full design
-and the phased plan before adding engine logic.
+**Status: Increment 1.** `src/index.ts` is still a thin config-validating entry,
+but the engine now has a working READ-ONLY `pull` (Docmost -> FS mirror) and a
+Phase-0 round-trip idempotency harness. Bidirectional sync, conflict handling,
+and git are NOT implemented yet (see the SPEC phases). See `SPEC.md` for the full
+design and the phased plan before adding engine logic.
 
 ## Project structure
 
@@ -27,6 +29,10 @@ build` builds the lib first, then compiles the app to `build/`.
   - `src/config-errors.ts` — `loadSettingsOrExit` turns a config error into a
     clear startup message that names the missing/invalid variable, then exits.
   - `src/index.ts` — thin entry point.
+  - `src/sanitize.ts` — filesystem-safe filename sanitization (SPEC §12).
+  - `src/layout.ts` — pure page-tree -> vault path mapping.
+  - `src/roundtrip.ts` — Phase-0 idempotency harness (SPEC §11).
+  - `src/pull.ts` — read-only Docmost -> FS mirror (SPEC §6).
 - `test/` — vitest tests (`*.test.ts`).
 - `data/` — all mutable runtime state (the git vault lives here). Gitignored;
   mounted as a docker volume in production. Never put code/static assets here.
@@ -46,6 +52,8 @@ Relative imports inside `src/` use the `.js` extension (NodeNext), e.g.
 - `make test` — run the test suite (vitest).
 - `make run` — build and run the app.
 - `make dev` — run in watch mode (tsx).
+- `make roundtrip` — run the offline round-trip idempotency harness.
+- `make pull` — mirror the configured space into the vault (read-only).
 
 `make` (or `make help`) lists all targets.
 
diff --git a/packages/docmost-client/src/client.ts b/packages/docmost-client/src/client.ts
index 109c1be..5229cf4 100644
--- a/packages/docmost-client/src/client.ts
+++ b/packages/docmost-client/src/client.ts
@@ -2704,6 +2704,11 @@ export class DocmostClient {
  * collecting items strictly newer than sinceIso and stopping at the first item
  * with updatedAt <= sinceIso. `fetchPage(cursor)` returns one page; dedup by id
  * guards a server that ignores the cursor; hardPageCap bounds the walk.
+ *
+ * Precondition: `sinceIso` and each `item.updatedAt` MUST be the SAME UTC
+ * ISO-8601 format that Docmost emits, because the cutoff comparison is purely
+ * lexicographic (string `<=`); mixed formats or non-UTC offsets would compare
+ * incorrectly.
  */
 export async function collectRecentSince(
   fetchPage: (cursor: string | null) => Promise<{ items: any[]; nextCursor: string | null }>,
diff --git a/src/layout.ts b/src/layout.ts
new file mode 100644
index 0000000..b3b21cf
--- /dev/null
+++ b/src/layout.ts
@@ -0,0 +1,177 @@
+/**
+ * Pure page-tree -> vault path mapping (SPEC §12).
+ *
+ * Given the flat list of page nodes for a space (as returned by
+ * `listAllSpacePages`), compute for every page a deterministic, collision-free
+ * destination: a folder path (root -> leaf ancestors) plus a file stem (the
+ * page's own name, no extension). This module is intentionally PURE and
+ * dependency-free apart from the sanitization helpers, so the whole tree ->
+ * path logic is unit-testable without any I/O. The names are COSMETIC; identity
+ * lives in each file's meta block (pageId / slugId).
+ */
+
+import { sanitizeTitle, disambiguate } from "./sanitize.js";
+
+/** Flat page node as returned by `listAllSpacePages` (no content). */
+export interface PageNode {
+  id: string;
+  title?: string;
+  slugId?: string;
+  parentPageId?: string | null;
+  hasChildren?: boolean;
+}
+
+/** A page's resolved vault destination: folder path + file stem. */
+export interface VaultEntry {
+  /** Folder path, root -> leaf (the page's ancestors). Empty for a root page. */
+  segments: string[];
+  /** The page's own file name without extension. */
+  stem: string;
+}
+
+/**
+ * Build the full vault layout for a space.
+ *
+ * Returns a Map keyed by pageId -> `{ segments, stem }`. The result is
+ * deterministic for a given input and guarantees every full destination path
+ * (`[...segments, stem].join("/")`) is unique, so no page can silently overwrite
+ * another.
+ *
+ * Disambiguation is layered:
+ *   1. Sibling collisions (same sanitized title under the same parent) are
+ *      resolved with a stable ` ~<slugId>` suffix (the suffix is itself
+ *      sanitized, since slugId/id is untrusted data that must never inject a
+ *      path separator).
+ *   2. A final full-path pass catches residual collisions that sibling-scoping
+ *      cannot see — e.g. two pages whose parents are BOTH outside the input set
+ *      both bucket at the root with `segments: []`.
+ */
+export function buildVaultLayout(pages: PageNode[]): Map<string, VaultEntry> {
+  // Index pages by id so the parent chain can be walked. Guard against
+  // duplicate ids in the input (first one wins).
+  const byId = new Map<string, PageNode>();
+  for (const p of pages) {
+    if (p && p.id && !byId.has(p.id)) byId.set(p.id, p);
+  }
+
+  // Resolve each node's display name once, deterministically, tracking sibling
+  // collisions per parent. `usedBySibling` maps a parent key -> set of names
+  // already taken under that parent. The bucket key is the node's parent ONLY
+  // when that parent is actually present in `byId`; otherwise (null parent, or
+  // an orphan whose parent is outside the input set) the node buckets at
+  // `"__root__"`. This is critical: orphans land at the vault root (see
+  // `folderSegmentsFor`), so they MUST share the root bucket with real root
+  // pages to be disambiguated against each other here — making `nameById` final
+  // before any `segments` are computed, so no ancestor name can drift later.
+  const usedBySibling = new Map<string, Set<string>>();
+  const nameById = new Map<string, string>();
+  for (const p of pages) {
+    if (p && p.id && !nameById.has(p.id)) {
+      const parentKey =
+        p.parentPageId && byId.has(p.parentPageId) ? p.parentPageId : "__root__";
+      nameById.set(p.id, nameForNode(p, parentKey, usedBySibling));
+    }
+  }
+
+  // Every id we index above MUST get a resolved name; this helper returns it
+  // and THROWS if it is somehow absent, rather than silently recomputing a
+  // DIFFERENT, non-disambiguated name (which would desync a folder segment from
+  // its target file).
+  const nameOf = (id: string): string => {
+    const name = nameById.get(id);
+    if (name === undefined) {
+      throw new Error(`buildVaultLayout: no resolved name for page id ${id}`);
+    }
+    return name;
+  };
+
+  // Build the folder path for a page by walking parentPageId to the root. The
+  // page's OWN name is the file stem; its ancestors become folders. A `visited`
+  // guard prevents an infinite loop on a malformed parent cycle.
+  const folderSegmentsFor = (node: PageNode): string[] => {
+    const ancestors: string[] = [];
+    const visited = new Set<string>();
+    let current: PageNode | undefined = node.parentPageId
+      ? byId.get(node.parentPageId)
+      : undefined;
+    while (current && current.id && !visited.has(current.id)) {
+      visited.add(current.id);
+      ancestors.unshift(nameOf(current.id));
+      current = current.parentPageId
+        ? byId.get(current.parentPageId)
+        : undefined;
+    }
+    return ancestors;
+  };
+
+  // First pass: compute the provisional { segments, stem } for every node.
+  const layout = new Map<string, VaultEntry>();
+  for (const p of pages) {
+    if (!p || !p.id || layout.has(p.id)) continue;
+    layout.set(p.id, {
+      segments: folderSegmentsFor(p),
+      stem: nameOf(p.id),
+    });
+  }
+
+  // Final full-path uniqueness pass — a belt-and-suspenders safety net. Note
+  // that cross-bucket (orphan/root) collisions are now resolved in the name pass
+  // above (orphans share the "__root__" bucket), so ancestor names are final
+  // before `segments` are built and this pass should rarely/never re-stem an
+  // ancestor. It only re-stems the colliding LATER leaf via the sanitized
+  // slugId/id, then (if still colliding) appends the id.
+  const usedPaths = new Set<string>();
+  const seenIds = new Set<string>();
+  const pathKey = (e: VaultEntry): string => [...e.segments, e.stem].join("/");
+  for (const p of pages) {
+    if (!p || !p.id || seenIds.has(p.id)) continue;
+    seenIds.add(p.id);
+    const entry = layout.get(p.id);
+    if (!entry) continue;
+
+    if (usedPaths.has(pathKey(entry))) {
+      // First attempt: disambiguate the stem with the sanitized slugId (or id).
+      entry.stem = disambiguate(entry.stem, sanitizeTitle(p.slugId ?? p.id));
+      if (usedPaths.has(pathKey(entry))) {
+        // Still colliding: append the (sanitized) id as a last resort. The id
+        // is globally unique, so this always resolves the collision.
+        entry.stem = disambiguate(entry.stem, sanitizeTitle(p.id));
+      }
+    }
+    usedPaths.add(pathKey(entry));
+  }
+
+  return layout;
+}
+
+/**
+ * Compute a deterministic, collision-free name for a node among its SIBLINGS.
+ * `usedBySibling` maps a parent key -> set of names already taken, so two
+ * siblings that sanitize to the same name get a stable ` ~slugId` suffix
+ * (SPEC §12). The suffix is itself passed through `sanitizeTitle`, because the
+ * slugId/id is a second untrusted-data channel that must never leak a path
+ * separator into the name. `parentKey` is supplied by the caller (it resolves
+ * to `"__root__"` for root pages AND for orphans whose parent is outside the
+ * input set, so they share one bucket). The name is COSMETIC; identity lives in
+ * the meta block.
+ */
+function nameForNode(
+  node: PageNode,
+  parentKey: string,
+  usedBySibling: Map<string, Set<string>>,
+): string {
+  let used = usedBySibling.get(parentKey);
+  if (!used) {
+    used = new Set<string>();
+    usedBySibling.set(parentKey, used);
+  }
+
+  let name = sanitizeTitle(node.title ?? "");
+  if (used.has(name)) {
+    // Sibling collision: disambiguate with the stable, sanitized slugId (fall
+    // back to the sanitized pageId if no slugId is present).
+    name = disambiguate(name, sanitizeTitle(node.slugId ?? node.id));
+  }
+  used.add(name);
+  return name;
+}
diff --git a/src/pull.ts b/src/pull.ts
index be6d0ed..da9b6b1 100644
--- a/src/pull.ts
+++ b/src/pull.ts
@@ -4,8 +4,12 @@
  * Walks the configured space's page tree and writes one self-contained `.md`
  * per page under `<vaultPath>/<...ancestors>/<Title>.md`. This increment is
  * READ-ONLY toward Docmost (no writes, no git) — it only fetches and writes
- * local files. The meta block inside each file carries pageId/slugId/
- * parentPageId (identity), so no external map file is needed.
+ * local files. The meta block inside each file carries
+ * `{ version, pageId, slugId, title, spaceId, parentPageId }` (identity), so no
+ * external map file is needed.
+ *
+ * The pure tree -> path mapping lives in `./layout.js`; this file is a thin,
+ * fault-tolerant I/O loop around it.
  *
  * Requires a `.env` with real Docmost credentials. This file must COMPILE and
  * be correct, but is not expected to be run without live access.
@@ -17,43 +21,13 @@ import { join } from "node:path";
 import { pathToFileURL } from "node:url";
 import { DocmostClient } from "docmost-client";
 import { loadSettings } from "./settings.js";
-import { sanitizeTitle, disambiguate } from "./sanitize.js";
+import { buildVaultLayout, type PageNode } from "./layout.js";
 
-/** Flat page node as returned by listAllSpacePages (no content). */
-interface PageNode {
-  id: string;
-  title?: string;
-  slugId?: string;
-  parentPageId?: string | null;
-  hasChildren?: boolean;
-}
-
-/**
- * Compute a deterministic, collision-free folder/file name for a node among its
- * siblings. `usedBySibling` maps a parent key -> set of names already taken, so
- * two siblings that sanitize to the same name get a stable ` ~slugId` suffix
- * (SPEC §12). The name is COSMETIC; identity lives in the file's meta block.
- */
-function nameForNode(
-  node: PageNode,
-  usedBySibling: Map<string, Set<string>>,
-): string {
-  const parentKey = node.parentPageId ?? "__root__";
-  let used = usedBySibling.get(parentKey);
-  if (!used) {
-    used = new Set<string>();
-    usedBySibling.set(parentKey, used);
-  }
-
-  let name = sanitizeTitle(node.title ?? "");
-  if (used.has(name)) {
-    // Sibling collision: disambiguate with the stable slugId (fall back to the
-    // pageId if no slugId is present).
-    name = disambiguate(name, node.slugId ?? node.id);
-  }
-  used.add(name);
-  return name;
-}
+// Number of pages fetched/written concurrently. Bounded so a large space does
+// not open thousands of simultaneous requests/file handles.
+const CONCURRENCY = 6;
+// How often to log incremental progress (every N completed pages).
+const PROGRESS_EVERY = 25;
 
 async function main(): Promise<void> {
   const s = loadSettings();
@@ -67,64 +41,74 @@ async function main(): Promise<void> {
   const vaultRoot = s.vaultPath;
 
   const pages: PageNode[] = await client.listAllSpacePages(spaceId);
+  const layout = buildVaultLayout(pages);
 
-  // Index pages by id so the parent chain can be walked.
-  const byId = new Map<string, PageNode>();
-  for (const p of pages) {
-    if (p && p.id) byId.set(p.id, p);
-  }
+  const total = pages.length;
+  let written = 0;
+  let failed = 0;
+  let completed = 0;
+  let nextIndex = 0;
 
-  // Resolve each node's display name once, deterministically, tracking sibling
-  // collisions per parent.
-  const usedBySibling = new Map<string, Set<string>>();
-  const nameById = new Map<string, string>();
-  for (const p of pages) {
-    if (p && p.id) nameById.set(p.id, nameForNode(p, usedBySibling));
-  }
-
-  // Build the folder path for a page by walking parentPageId to the root. The
-  // page's OWN name is the file stem; its ancestors become folders. A `visited`
-  // guard prevents an infinite loop on a malformed parent cycle.
-  const folderSegmentsFor = (node: PageNode): string[] => {
-    const ancestors: string[] = [];
-    const visited = new Set<string>();
-    let current: PageNode | undefined = node.parentPageId
-      ? byId.get(node.parentPageId)
-      : undefined;
-    while (current && current.id && !visited.has(current.id)) {
-      visited.add(current.id);
-      ancestors.unshift(
-        nameById.get(current.id) ?? sanitizeTitle(current.title ?? ""),
+  // Pull + write a single page. Each call is wrapped so one bad page (network
+  // error, page deleted between the walk and the fetch, body conversion
+  // failure) NEVER aborts the whole pull — it is counted as a failure and the
+  // pool moves on. Mirrors the deliberately fault-tolerant enumerateSpacePages.
+  const pullOne = async (page: PageNode): Promise<void> => {
+    if (!page || !page.id) return;
+    const entry = layout.get(page.id);
+    if (!entry) return; // no layout entry (e.g. duplicate/skipped id)
+    try {
+      const dir = join(vaultRoot, ...entry.segments);
+      await mkdir(dir, { recursive: true });
+      // Body + meta only (no comments block) — SPEC §3.
+      const md = await client.exportPageBody(page.id);
+      await writeFile(join(dir, `${entry.stem}.md`), md, "utf8");
+      written++;
+    } catch (err) {
+      failed++;
+      console.error(
+        `pull: failed page ${page.id}:`,
+        err instanceof Error ? err.message : String(err),
       );
-      current = current.parentPageId
-        ? byId.get(current.parentPageId)
-        : undefined;
+    } finally {
+      completed++;
+      if (completed % PROGRESS_EVERY === 0) {
+        console.log(`pulled ${completed}/${total}`);
+      }
     }
-    return ancestors;
   };
 
-  let written = 0;
-  for (const p of pages) {
-    if (!p || !p.id) continue;
-    const segments = folderSegmentsFor(p);
-    const fileStem = nameById.get(p.id) ?? sanitizeTitle(p.title ?? "");
-    const dir = join(vaultRoot, ...segments);
-    await mkdir(dir, { recursive: true });
+  // A small dependency-free bounded-concurrency pool: a fixed set of runners
+  // each pull the next index until the list is exhausted.
+  const runner = async (): Promise<void> => {
+    while (true) {
+      const i = nextIndex++;
+      if (i >= pages.length) return;
+      await pullOne(pages[i]);
+    }
+  };
 
-    // Body + meta only (no comments block) — SPEC §3.
-    const fileMd = await client.exportPageBody(p.id);
-    await writeFile(join(dir, `${fileStem}.md`), fileMd, "utf8");
-    written++;
-  }
+  const runners = Array.from(
+    { length: Math.min(CONCURRENCY, pages.length) },
+    () => runner(),
+  );
+  await Promise.all(runners);
 
   console.log(
-    `pull complete: ${written} page(s) from space ${spaceId} into ${vaultRoot}`,
+    `pull complete: ${written} page(s) written, ${failed} failed, ` +
+      `out of ${total} from space ${spaceId} into ${vaultRoot}`,
   );
+
+  // Signal a partial mirror so callers/CI can react. Use process.exitCode (not
+  // a hard process.exit) so any buffered output is flushed cleanly.
+  if (failed > 0) {
+    process.exitCode = 1;
+  }
 }
 
 // Only auto-run when invoked directly as the CLI entrypoint, not when this
-// module is imported (e.g. by a unit test importing sanitizeTitle / path
-// helpers), so the import does not trigger loadSettings() + process.exit.
+// module is imported (e.g. by a unit test), so the import does not trigger
+// loadSettings() + process.exit.
 const invokedDirectly =
   typeof process.argv[1] === "string" &&
   import.meta.url === pathToFileURL(process.argv[1]).href;
diff --git a/src/roundtrip.ts b/src/roundtrip.ts
index 14baf41..87e8044 100644
--- a/src/roundtrip.ts
+++ b/src/roundtrip.ts
@@ -98,7 +98,7 @@ async function loadDoc(args: ParsedArgs): Promise<any> {
  * Find the first divergence between two values via a recursive deep compare.
  * Returns a short path + the two differing values, or null if they are equal.
  */
-function firstDivergence(
+export function firstDivergence(
   a: any,
   b: any,
   path = "$",
diff --git a/test/divergence.test.ts b/test/divergence.test.ts
new file mode 100644
index 0000000..b00ffd5
--- /dev/null
+++ b/test/divergence.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest';
+import { firstDivergence } from '../src/roundtrip.js';
+
+describe('firstDivergence', () => {
+  it('returns null for equal nested objects', () => {
+    const a = { k1: { k2: 1, k3: [1, 2, 3] }, n: 'x' };
+    const b = { k1: { k2: 1, k3: [1, 2, 3] }, n: 'x' };
+    expect(firstDivergence(a, b)).toBeNull();
+  });
+
+  it('reports the correct path for a differing leaf', () => {
+    const a = { k1: { k2: 1 } };
+    const b = { k1: { k2: 2 } };
+    const d = firstDivergence(a, b);
+    expect(d).not.toBeNull();
+    expect(d!.path).toBe('$.k1.k2');
+    expect(d!.a).toBe(1);
+    expect(d!.b).toBe(2);
+  });
+
+  it('reports an array length mismatch at $.arr.length', () => {
+    const a = { arr: [1, 2, 3] };
+    const b = { arr: [1, 2] };
+    const d = firstDivergence(a, b);
+    expect(d).not.toBeNull();
+    expect(d!.path).toBe('$.arr.length');
+    expect(d!.a).toBe(3);
+    expect(d!.b).toBe(2);
+  });
+
+  it('reports a key present only in a', () => {
+    const a = { only: 'here', shared: 1 };
+    const b = { shared: 1 };
+    const d = firstDivergence(a, b);
+    expect(d).not.toBeNull();
+    expect(d!.path).toBe('$.only');
+    expect(d!.a).toBe('here');
+    expect(d!.b).toBeUndefined();
+  });
+});
diff --git a/test/layout.test.ts b/test/layout.test.ts
new file mode 100644
index 0000000..a9c1cb2
--- /dev/null
+++ b/test/layout.test.ts
@@ -0,0 +1,144 @@
+import { describe, expect, it } from 'vitest';
+import { buildVaultLayout, type PageNode } from '../src/layout.js';
+
+describe('buildVaultLayout', () => {
+  it('disambiguates two siblings with the same sanitized title via ~slugId', () => {
+    const pages: PageNode[] = [
+      { id: 'p1', title: 'Notes', slugId: 'slug-a', parentPageId: null },
+      { id: 'p2', title: 'Notes', slugId: 'slug-b', parentPageId: null },
+    ];
+    const layout = buildVaultLayout(pages);
+    expect(layout.get('p1')).toEqual({ segments: [], stem: 'Notes' });
+    expect(layout.get('p2')).toEqual({ segments: [], stem: 'Notes ~slug-b' });
+  });
+
+  it('falls back to ~id when a colliding sibling has no slugId', () => {
+    const pages: PageNode[] = [
+      { id: 'p1', title: 'Notes', parentPageId: null },
+      { id: 'p2', title: 'Notes', parentPageId: null },
+    ];
+    const layout = buildVaultLayout(pages);
+    expect(layout.get('p1')?.stem).toBe('Notes');
+    expect(layout.get('p2')?.stem).toBe('Notes ~p2');
+  });
+
+  it('does NOT collide identical titles under DIFFERENT parents (distinct segments)', () => {
+    const pages: PageNode[] = [
+      { id: 'a', title: 'Alpha', parentPageId: null },
+      { id: 'b', title: 'Beta', parentPageId: null },
+      { id: 'a1', title: 'Notes', parentPageId: 'a' },
+      { id: 'b1', title: 'Notes', parentPageId: 'b' },
+    ];
+    const layout = buildVaultLayout(pages);
+    // Same stem, but different folder segments => no disambiguation needed.
+    expect(layout.get('a1')).toEqual({ segments: ['Alpha'], stem: 'Notes' });
+    expect(layout.get('b1')).toEqual({ segments: ['Beta'], stem: 'Notes' });
+  });
+
+  it('terminates on a 2-node parent cycle and yields a finite result', () => {
+    const pages: PageNode[] = [
+      { id: 'a', title: 'A', parentPageId: 'b' },
+      { id: 'b', title: 'B', parentPageId: 'a' },
+    ];
+    const layout = buildVaultLayout(pages);
+    // Both resolve to a finite path; the visited-guard breaks the cycle.
+    expect(layout.size).toBe(2);
+    const a = layout.get('a');
+    const b = layout.get('b');
+    expect(a).toBeDefined();
+    expect(b).toBeDefined();
+    // Each node's segment chain is bounded (no infinite walk).
+    expect(a!.segments.length).toBeLessThanOrEqual(2);
+    expect(b!.segments.length).toBeLessThanOrEqual(2);
+  });
+
+  it('maps a root page (parentPageId null) to empty segments', () => {
+    const pages: PageNode[] = [{ id: 'root', title: 'Home', parentPageId: null }];
+    const layout = buildVaultLayout(pages);
+    expect(layout.get('root')).toEqual({ segments: [], stem: 'Home' });
+  });
+
+  it('emits ancestors in root->leaf order for a deep chain', () => {
+    const pages: PageNode[] = [
+      { id: 'g', title: 'Grand', parentPageId: null },
+      { id: 'p', title: 'Parent', parentPageId: 'g' },
+      { id: 'c', title: 'Child', parentPageId: 'p' },
+    ];
+    const layout = buildVaultLayout(pages);
+    expect(layout.get('c')).toEqual({
+      segments: ['Grand', 'Parent'],
+      stem: 'Child',
+    });
+  });
+
+  it('disambiguates two orphan-parent pages with the same title at the path level', () => {
+    // Both parents are OUTSIDE the input set, so both pages bucket at the root
+    // with segments: []. Sibling-scoping cannot see this (different parentKeys),
+    // so the final full-path pass must produce DISTINCT paths.
+    const pages: PageNode[] = [
+      { id: 'x', title: 'Orphan', slugId: 'sx', parentPageId: 'missing-1' },
+      { id: 'y', title: 'Orphan', slugId: 'sy', parentPageId: 'missing-2' },
+    ];
+    const layout = buildVaultLayout(pages);
+    const ex = layout.get('x')!;
+    const ey = layout.get('y')!;
+    const pathOf = (e: { segments: string[]; stem: string }) =>
+      [...e.segments, e.stem].join('/');
+    expect(pathOf(ex)).not.toBe(pathOf(ey));
+    // The first keeps the plain stem; the later one is re-stemmed.
+    expect(ex.stem).toBe('Orphan');
+    expect(ey.stem).toBe('Orphan ~sy');
+  });
+
+  it('sanitizes a slugId containing a path separator before using it as a suffix', () => {
+    // A crafted slugId with "/" must NOT leak a path separator into the stem.
+    const pages: PageNode[] = [
+      { id: 'p1', title: 'Notes', slugId: 'a/b', parentPageId: null },
+      { id: 'p2', title: 'Notes', slugId: 'c/d', parentPageId: null },
+    ];
+    const layout = buildVaultLayout(pages);
+    const stem = layout.get('p2')!.stem;
+    expect(stem).not.toContain('/');
+    expect(stem).not.toContain('\\');
+    // The "/" was replaced by sanitizeTitle's dash substitution.
+    expect(stem).toBe('Notes ~c-d');
+  });
+
+  it('disambiguates two ORPHAN ancestors at the NAME pass so their children stay in sync', () => {
+    // Two orphan PARENTS share the same title but live under DIFFERENT missing
+    // parents, so sibling-scoping by raw parentPageId would never compare them.
+    // Both bucket at the vault root, so they MUST be disambiguated in the name
+    // pass (sharing the "__root__" bucket) BEFORE any child folder segment is
+    // computed from the parent name — otherwise re-stemming a parent post-hoc
+    // would desync its child's folder from the parent file.
+    const pages: PageNode[] = [
+      { id: 'p1', title: 'Dup', slugId: 's1', parentPageId: 'missing-1' },
+      { id: 'p2', title: 'Dup', slugId: 's2', parentPageId: 'missing-2' },
+      { id: 'c1', title: 'Child', parentPageId: 'p1' },
+      { id: 'c2', title: 'Child', parentPageId: 'p2' },
+    ];
+    const layout = buildVaultLayout(pages);
+    const p1 = layout.get('p1')!;
+    const p2 = layout.get('p2')!;
+    const c1 = layout.get('c1')!;
+    const c2 = layout.get('c2')!;
+
+    // The two orphan parents get DISTINCT stems, both at the root.
+    expect(p1.segments).toEqual([]);
+    expect(p2.segments).toEqual([]);
+    expect(p1.stem).toBe('Dup');
+    expect(p2.stem).toBe('Dup ~s2');
+    expect(p1.stem).not.toBe(p2.stem);
+
+    // Each child's folder segment EXACTLY equals its parent's resolved stem
+    // (no desync): the parent name is final before segments are built.
+    expect(c1.segments).toEqual([p1.stem]);
+    expect(c2.segments).toEqual([p2.stem]);
+
+    // All four full paths are unique.
+    const pathOf = (e: { segments: string[]; stem: string }) =>
+      [...e.segments, e.stem].join('/');
+    const paths = [p1, p2, c1, c2].map(pathOf);
+    expect(new Set(paths).size).toBe(paths.length);
+  });
+});
diff --git a/test/markdown-document.test.ts b/test/markdown-document.test.ts
new file mode 100644
index 0000000..c23447d
--- /dev/null
+++ b/test/markdown-document.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, it } from 'vitest';
+import {
+  serializeDocmostMarkdownBody,
+  parseDocmostMarkdown,
+  type DocmostMdMeta,
+} from 'docmost-client';
+
+describe('serializeDocmostMarkdownBody round-trip (SPEC §3)', () => {
+  it('serialize -> parse preserves meta and the trimmed body, with no comments block', () => {
+    const meta: DocmostMdMeta = {
+      version: 1,
+      pageId: 'page-123',
+      slugId: 'slug-abc',
+      title: 'My Page',
+      spaceId: 'space-1',
+      parentPageId: 'parent-9',
+    };
+    const body = 'Hello\n\nWorld';
+
+    const file = serializeDocmostMarkdownBody(meta, body);
+    const parsed = parseDocmostMarkdown(file);
+
+    expect(parsed.meta).toEqual(meta);
+    expect(parsed.body).toBe(body);
+    // No trailing docmost:comments block was emitted (SPEC §3).
+    expect(parsed.comments).toBeNull();
+  });
+
+  it('preserves a null parentPageId for a root page', () => {
+    const meta: DocmostMdMeta = {
+      version: 1,
+      pageId: 'root-1',
+      slugId: 'root-slug',
+      title: 'Root',
+      spaceId: 'space-1',
+      parentPageId: null,
+    };
+    const file = serializeDocmostMarkdownBody(meta, 'body text');
+    const parsed = parseDocmostMarkdown(file);
+    expect(parsed.meta).toEqual(meta);
+    expect(parsed.comments).toBeNull();
+  });
+
+  it('produces a parseable file for an empty/missing body', () => {
+    const meta: DocmostMdMeta = { version: 1, pageId: 'p-empty' };
+
+    // Empty string body.
+    const emptyFile = serializeDocmostMarkdownBody(meta, '');
+    expect(() => parseDocmostMarkdown(emptyFile)).not.toThrow();
+    const parsedEmpty = parseDocmostMarkdown(emptyFile);
+    expect(parsedEmpty.meta).toEqual(meta);
+    expect(parsedEmpty.body).toBe('');
+    expect(parsedEmpty.comments).toBeNull();
+
+    // Missing body (undefined) — serializer coalesces to "".
+    const missingFile = serializeDocmostMarkdownBody(
+      meta,
+      undefined as unknown as string,
+    );
+    expect(() => parseDocmostMarkdown(missingFile)).not.toThrow();
+    const parsedMissing = parseDocmostMarkdown(missingFile);
+    expect(parsedMissing.meta).toEqual(meta);
+    expect(parsedMissing.body).toBe('');
+    expect(parsedMissing.comments).toBeNull();
+  });
+});