fix(mattermost): pass payload.replyToId as root_id for threaded replies (#27744)

Merged via squash.

Prepared head SHA: e029079872
Co-authored-by: hnykda <2741256+hnykda@users.noreply.github.com>
Co-authored-by: mukhtharcm <56378562+mukhtharcm@users.noreply.github.com>
Reviewed-by: @mukhtharcm

extensions/mattermost/src/mattermost/monitor.test.ts

@@ -3,6 +3,7 @@ import { describe, expect, it, vi } from "vitest";
 import { resolveMattermostAccount } from "./accounts.js";
 import {
   evaluateMattermostMentionGate,
+  resolveMattermostReplyRootId,
   type MattermostMentionGateInput,
   type MattermostRequireMentionResolverInput,
 } from "./monitor.js";
@@ -107,3 +108,26 @@ describe("mattermost mention gating", () => {
     expect(decision.dropReason).toBe("missing-mention");
   });
 });
+
+describe("resolveMattermostReplyRootId", () => {
+  it("uses replyToId for top-level replies", () => {
+    expect(
+      resolveMattermostReplyRootId({
+        replyToId: "inbound-post-123",
+      }),
+    ).toBe("inbound-post-123");
+  });
+
+  it("keeps the thread root when replying inside an existing thread", () => {
+    expect(
+      resolveMattermostReplyRootId({
+        threadRootId: "thread-root-456",
+        replyToId: "child-post-789",
+      }),
+    ).toBe("thread-root-456");
+  });
+
+  it("falls back to undefined when neither reply target is available", () => {
+    expect(resolveMattermostReplyRootId({})).toBeUndefined();
+  });
+});

extensions/mattermost/src/mattermost/monitor.ts

@@ -271,6 +271,17 @@ export function evaluateMattermostMentionGate(
     dropReason: null,
   };
 }
+
+export function resolveMattermostReplyRootId(params: {
+  threadRootId?: string;
+  replyToId?: string;
+}): string | undefined {
+  const threadRootId = params.threadRootId?.trim();
+  if (threadRootId) {
+    return threadRootId;
+  }
+  return params.replyToId?.trim() || undefined;
+}
 type MattermostMediaInfo = {
   path: string;
   contentType?: string;
@@ -1651,7 +1662,10 @@ export async function monitorMattermostProvider(opts: MonitorMattermostOpts = {}
               }
               await sendMessageMattermost(to, chunk, {
                 accountId: account.accountId,
-                replyToId: threadRootId,
+                replyToId: resolveMattermostReplyRootId({
+                  threadRootId,
+                  replyToId: payload.replyToId,
+                }),
               });
             }
           } else {
@@ -1662,7 +1676,10 @@ export async function monitorMattermostProvider(opts: MonitorMattermostOpts = {}
               await sendMessageMattermost(to, caption, {
                 accountId: account.accountId,
                 mediaUrl,
-                replyToId: threadRootId,
+                replyToId: resolveMattermostReplyRootId({
+                  threadRootId,
+                  replyToId: payload.replyToId,
+                }),
               });
             }
           }