diff --git a/.changeset/delivery-not-eligible-ack-claim-ownership.md b/.changeset/delivery-not-eligible-ack-claim-ownership.md
new file mode 100644
index 0000000000..2576ea92d5
--- /dev/null
+++ b/.changeset/delivery-not-eligible-ack-claim-ownership.md
@@ -0,0 +1,27 @@
+---
+'@objectstack/spec': patch
+---
+
+`DELIVERY_NOT_ELIGIBLE`'s `INotificationOutbox.ack` gloss now describes the post-#11859 refusal set
+
+The `ERROR_CODE_LEDGER` entry for `DELIVERY_NOT_ELIGIBLE` enumerates `ack`'s
+refusal cases per surface. #11859 widened that set on the notification surface —
+the claim credential now rides the record `claim()` returns and `ack()` takes it
+back — and two statements in the bullet stopped being the whole truth:
+
+- **The refusal set is wider than "not `in_flight`".** `ack` now also refuses a
+  row that **is** `in_flight` but is no longer held by the claim being
+  completed: ownership joined the predicate, and the credential is the
+  (`claimed_by`, `claimed_at`) **pair**, so a claim lost to the `claimTtlMs`
+  reap plus a re-claim by any node — including the caller's own later claim —
+  matches nothing and nothing is written. It additionally refuses a record
+  handed back carrying no claim credential at all.
+- **"Also raised by `SqlNotificationOutbox`" was too narrow.**
+  `MemoryNotificationOutbox` raises the lost-claim refusal too; both backends
+  are pinned on one table in `outbox-ack-claim-ownership.integration.test.ts`,
+  and both route through the shared refusal messages in `outbox.ts` so the two
+  cannot drift into different wordings for one contract violation.
+
+Comment only. No code is registered or removed, no wire value changes, and no
+acceptance or refusal behaviour moves — `packages/spec` publishes
+`src/**/*.zod.ts`, so the corrected gloss ships to consumers reading the ledger.
diff --git a/packages/spec/src/api/error-code-ledger.zod.ts b/packages/spec/src/api/error-code-ledger.zod.ts
index f804174b4f..09df76ca40 100644
--- a/packages/spec/src/api/error-code-ledger.zod.ts
+++ b/packages/spec/src/api/error-code-ledger.zod.ts
@@ -569,13 +569,20 @@ export const ERROR_CODE_LEDGER = {
     //     re-claimed the row mid-call — both `SqlHttpOutbox` and
     //     `MemoryHttpOutbox` report that miss instead of a false success
     //     (#11009).
-    //   - `INotificationOutbox.ack` (`NotificationAckError`; #11453) refuses a
-    //     row that is not `in_flight` — which covers BOTH an unclaimed
-    //     `pending` row (the ack-as-cancel trap) and an already-terminal one,
-    //     because `ack` records the outcome of a delivery the caller CLAIMED.
-    //     Also raised by `SqlNotificationOutbox` when its compare-and-set
-    //     read-back shows the claim was lost mid-ack (a slow send outrunning
-    //     `claimTtlMs`), so nothing was written.
+    //   - `INotificationOutbox.ack` (`NotificationAckError`; #11453, #11859)
+    //     refuses a row that is not `in_flight` — an unclaimed `pending` row
+    //     (the ack-as-cancel trap) or an already-terminal one, because `ack`
+    //     records the outcome of a delivery the caller CLAIMED — AND, since
+    //     #11859, an `in_flight` row no longer held by the claim being
+    //     completed: `ack` takes back the record `claim()` returned and the
+    //     compare-and-set binds its (`claimed_by`, `claimed_at`) credential,
+    //     so a claim lost to the `claimTtlMs` reap plus a re-claim (by ANY
+    //     node, including the caller's own later claim) matches nothing and
+    //     nothing is written. Also raised for a record handed back carrying
+    //     no claim credential at all. BOTH backends raise every one of these
+    //     refusals — `SqlNotificationOutbox` and `MemoryNotificationOutbox`,
+    //     pinned on one table in
+    //     `outbox-ack-claim-ownership.integration.test.ts`.
     // Distinct from DELIVERY_NEVER_SENT: this one says "wrong state for THIS
     // operation, try when it settles"; that one says "never, fix the
     // configuration instead".