test(threading): the agreement set has two members, and only one was pinned

backend/__tests__/unit/models/threadRootResolver.test.js

@@ -67,6 +67,30 @@ describe('explicit, the in-thread post shape', () => {
   test('explicit wins over derivation when both agree', async () => {
     expect(await resolveThreadRoot({ podId: POD, replyToMessageId: 101, threadRootId: 100 })).toBe(100);
   });
+
+  // @sprint-review 56879. "Agree" is a set with two members and this file only
+  // held one: above, the parent is a reply INSIDE the thread. The other member
+  // is the parent BEING the root, and it is the one V2PodChat's handleSend
+  // comment is written about, because it is the only shape where the reply
+  // edge pings the ROOT's author rather than some mid-thread author.
+  //
+  // It is also the one the resolver structurally cannot refuse. derived is
+  // COALESCE(parent.thread_root_id, parent.id), and a root's thread_root_id is
+  // NULL, so derived falls through to parent.id — which IS the named root. The
+  // two statements are equal by construction; thread_root_mismatch can never
+  // fire here no matter what the client sends. Nothing server-side stops it,
+  // so the client rule is the only thing that does, and this test is what
+  // makes that dependency visible rather than assumed.
+  test('replying to the root while aimed at its thread: accepted, and unrefusable', async () => {
+    expect(await resolveThreadRoot({ podId: POD, replyToMessageId: 100, threadRootId: 100 })).toBe(100);
+  });
+
+  test('CONTROL: the same pair one message over DOES disagree and 400s', async () => {
+    // Proves the acceptance above is the COALESCE fall-through and not a
+    // resolver that waves through any pair naming the same pod.
+    await expect(resolveThreadRoot({ podId: POD, replyToMessageId: 100, threadRootId: 200 }))
+      .rejects.toMatchObject({ code: 'thread_root_mismatch' });
+  });
 });
 
 describe('validation — each of these is a 400, not a silent choice', () => {

frontend/src/v2/components/V2PodChat.tsx

@@ -860,8 +860,20 @@ const V2PodChat: React.FC<V2PodChatProps> = ({ detail, firstRunVisible = false,
       // It would not: resolveThreadRoot 400s only when the two DISAGREE
       // (thread_root_mismatch) and accepts them when they agree. An in-thread
       // post must carry no addressing edge because a reply edge pings the
-      // root's author (@ux-lead 56879) — that is the rule this enforces, and
-      // it is the client's to keep.
+      // author of whatever it points at (@ux-lead 56879) — that is the rule
+      // this enforces, and it is the client's to keep.
+      //
+      // "Agree" is a set, not a point: the backend accepts the pair whenever
+      // explicit === COALESCE(parent.thread_root_id, parent.id), i.e. whenever
+      // the parent is anywhere IN the thread being aimed at. Two members, and
+      // they differ in WHO gets pinged, because resolveImplicitReplyTarget
+      // resolves the author of replyToMessageId — the parent, not the root:
+      //   parent mid-thread (reply 101 in thread 100) -> pings 101's author
+      //   parent IS the root (replyTo 100, root 100)  -> pings the root's author
+      // Only the second collapses onto "the root's author", which is why that
+      // is the shape this comment is really about — and it is structurally
+      // unrefusable: a root's COALESCE falls through to its own id, so the two
+      // statements can never disagree. @sprint-review 56879.
       const created = await sendMessage(
         text,
         'text',