diff --git a/dashcaddy-api/__tests__/atomic-write-dc099.test.js b/dashcaddy-api/__tests__/atomic-write-dc099.test.js
index 3157c0b..84e9d3b 100644
--- a/dashcaddy-api/__tests__/atomic-write-dc099.test.js
+++ b/dashcaddy-api/__tests__/atomic-write-dc099.test.js
@@ -196,3 +196,42 @@ describe('DC-099 atomic-write syscall discipline (mocked fs)', () => {
     expect(calls).toEqual(['openSync', 'unlinkSync']);
   });
 });
+
+// DC-100: invite-store migrated off its private _atomicWriteJSON copy onto
+// the canonical writer. Store-level pins: writes are durable-canonical
+// (0600, complete JSON, no temp leftovers) even under back-to-back mutations
+// — the access pattern that could collide tmp names in the naive copy.
+describe('DC-100 invite-store on canonical atomic-write (real fs)', () => {
+  let dir, store;
+
+  beforeEach(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'dc100-invite-'));
+    store = require('../src/security/invite-store').createInviteStore({ dataDir: dir });
+  });
+  afterEach(() => { try { fs.rmSync(dir, { recursive: true, force: true }); } catch (_) {} });
+
+  test('issued invite lands as complete JSON at mode 0600 with no temp leftovers', async () => {
+    const r = await store.issue({ email: 'dc100@x.com', ttlMs: 60_000 });
+    expect(r.ok).toBe(true);
+    const file = path.join(dir, 'invites.json');
+    const st = fs.statSync(file);
+    expect(st.mode & 0o777).toBe(0o600);
+    const data = JSON.parse(fs.readFileSync(file, 'utf8'));
+    expect(Object.keys(data.invites)).toHaveLength(1);
+    const leftovers = fs.readdirSync(dir).filter((f) => f !== 'invites.json');
+    expect(leftovers).toEqual([]);
+  });
+
+  test('back-to-back mutations (issue, revoke, issue) never collide on tmp names', async () => {
+    const a = await store.issue({ email: 'a@x.com', ttlMs: 60_000 });
+    const b = await store.issue({ email: 'b@x.com', ttlMs: 60_000 });
+    await store.revoke(a.id);
+    const c = await store.issue({ email: 'c@x.com', ttlMs: 60_000 });
+    expect(b.ok).toBe(true);
+    expect(c.ok).toBe(true);
+    const data = JSON.parse(fs.readFileSync(path.join(dir, 'invites.json'), 'utf8'));
+    expect(Object.keys(data.invites).sort()).toEqual([b.id, c.id].sort());
+    const leftovers = fs.readdirSync(dir).filter((f) => f !== 'invites.json');
+    expect(leftovers).toEqual([]);
+  });
+});
diff --git a/dashcaddy-api/src/security/invite-store.js b/dashcaddy-api/src/security/invite-store.js
index b4d161d..85f68d0 100644
--- a/dashcaddy-api/src/security/invite-store.js
+++ b/dashcaddy-api/src/security/invite-store.js
@@ -5,7 +5,8 @@
  * the system emails (or logs in dev) a magic-link-style URL containing the
  * raw token. The recipient clicks → accepts → becomes an authorized user.
  *
- * Storage: data/invites.json. Atomic writes via tmp+rename.
+ * Storage: data/invites.json. Atomic durable writes via the canonical
+ * shared atomic-write util (DC-099/DC-100) — fsync'd tmp+rename.
  *
  * Token shape:
  *   - 32 random bytes, base64url-encoded (256 bits of entropy).
@@ -30,6 +31,7 @@ const path = require('path');
 const fs = require('fs');
 const crypto = require('crypto');
 const platformPaths = require('../../platform-paths');
+const { atomicWriteJSON } = require('../utils/atomic-write');
 
 const DEFAULT_TTL_MS = 24 * 60 * 60 * 1000; // 24 hours
 const PRUNE_AFTER_MS = 7 * 24 * 60 * 60 * 1000; // auto-prune used/expired after 7d
@@ -37,12 +39,6 @@ const PRUNE_AFTER_MS = 7 * 24 * 60 * 60 * 1000; // auto-prune used/expired after
 function _nowMs() { return Date.now(); }
 function _nowIso() { return new Date().toISOString(); }
 
-function _atomicWriteJSON(filePath, data) {
-  const tmp = filePath + '.tmp.' + process.pid + '.' + Date.now();
-  fs.writeFileSync(tmp, JSON.stringify(data, null, 2) + '\n', { mode: 0o600 });
-  fs.renameSync(tmp, filePath);
-}
-
 function _readJSON(filePath, fallback) {
   try {
     const raw = fs.readFileSync(filePath, 'utf8');
@@ -85,7 +81,7 @@ function createInviteStore(opts = {}) {
     if (!data.invites || typeof data.invites !== 'object') data.invites = {};
     return data;
   }
-  function _save(data) { _atomicWriteJSON(file, data); }
+  function _save(data) { atomicWriteJSON(file, data); }
 
   function _prune(data) {
     const cutoff = _nowMs() - PRUNE_AFTER_MS;