diff --git a/src/services/guard.ts b/src/services/guard.ts
index 28db6dc..61dc68e 100644
--- a/src/services/guard.ts
+++ b/src/services/guard.ts
@@ -21,7 +21,10 @@
  *     pattern first, so it cannot carry grammar. The ID stays a variable.
  *   - Moment is NOT a stamp: `Moment=<an earlier moment of this asset>`
  *     selects the asset as it was then (as-of), and an update scoped by it
- *     lands on the current asset. A Moment guard guards nothing.
+ *     lands on the current asset. A Moment guard guards nothing. Proven for
+ *     writes on 2026-11-11: an update filtered on a stale Moment previews as
+ *     one command and applies, overwriting the change made in between
+ *     ("why not Moment, on a write"; versionone/VersionOne.SDK.JavaScript#51).
  * Every type has ChangeDate except the Ideas product's (Idea*), which keeps
  * no history and cannot be guarded.
  *
diff --git a/test/integration/guard.test.ts b/test/integration/guard.test.ts
index 148923b..1a39e55 100644
--- a/test/integration/guard.test.ts
+++ b/test/integration/guard.test.ts
@@ -8,7 +8,8 @@ import { describeInstance, fixtureName, instance } from "./instance";
  * afterwards by exact OID): one changed by "someone else" between the
  * preview and the send, one not. What only the server can prove: that the
  * projection's add/remove form for a scalar applies in a real update, that
- * a ChangeDate literal is a true compare-and-set (and a Moment is not), and
+ * a ChangeDate literal is a true compare-and-set (and a Moment is not, for a
+ * query or for a write), and
  * that a stale guard writes nothing.
  */
 describeInstance("guard (live)", () => {
@@ -59,4 +60,27 @@ describeInstance("guard (live)", () => {
     expect(asOf).toHaveLength(1); // stale, yet it matches —
     expect(asOf[0]!.Estimate).toBe("8"); // — as the asset was at that moment
   });
+
+  it("why not Moment, on a write: an update filtered on a stale Moment previews as one command and applies, while a stale ChangeDate literal applies nothing", async () => {
+    // Its own Story, so the result doesn't depend on what the tests above did to a and b.
+    const res = await assetApi.send(t, [{ AssetType: "Story", Name: fixtureName("guard moment write"), Scope: { _oid: "Scope:0" }, Estimate: 1 }], { throwOnFailure: true });
+    const s = assetApi.momentless(res.assetsCreated.oidTokens[0]!);
+    created.push(s);
+    const [before] = await assetApi.query<{ _oid: string; Moment: string; ChangeDate: string }>(t, { from: s, select: ["Moment", "ChangeDate"], page: { size: 1, start: 0 } });
+    await assetApi.send(t, [{ from: s, update: { Estimate: 2 } }], { throwOnFailure: true }); // "someone else" changes it
+    expect(await estimate(s)).toBe("2");
+
+    // Stale Moment, as a literal: the projection still has the asset, and the write lands on the current one.
+    const staleMoment = [{ from: "Story", filter: ["ID=$oid", `Moment='${before!.Moment}'`], with: { $oid: s }, update: { Estimate: 7 } }];
+    expect(await assetApi.preview(t, staleMoment)).toHaveLength(1);
+    const sent = await assetApi.send(t, staleMoment);
+    expect(sent.assetsModified.oidTokens.map(assetApi.momentless)).toEqual([s]);
+    expect(await estimate(s)).toBe("7"); // the other writer's 2 is overwritten: Moment guards nothing
+
+    // Stale ChangeDate, as a literal (what guard.ts sends): matches nothing, so nothing is applied.
+    const staleStamp = [{ from: "Story", filter: ["ID=$oid", `ChangeDate='${before!.ChangeDate}'`], with: { $oid: s }, update: { Estimate: 9 } }];
+    expect(await assetApi.preview(t, staleStamp)).toEqual([]);
+    expect((await assetApi.send(t, staleStamp)).assetsModified.count).toBe(0);
+    expect(await estimate(s)).toBe("7");
+  });
 });