diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2902617108f..cc52f4a8273 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ Docs: https://docs.openclaw.ai
 ### Fixes
 
 - macOS app/chat UI: route browser proxy through the local node browser service, preserve plain-text paste semantics, strip completed assistant trace/debug wrapper noise from transcripts, refresh permission state after returning from System Settings, and tolerate malformed cron rows in the macOS tab. (#39516) Thanks @Imhermes1.
+- Mattermost replies: keep `root_id` pinned to the existing thread root when an agent replies inside a thread, while still using reply-target threading for top-level posts. (#27744) thanks @hnykda.
 
 ## 2026.3.7
 
diff --git a/extensions/mattermost/src/mattermost/monitor.test.ts b/extensions/mattermost/src/mattermost/monitor.test.ts
index ab122948ebc..1bd871714c4 100644
--- a/extensions/mattermost/src/mattermost/monitor.test.ts
+++ b/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();
+  });
+});
diff --git a/extensions/mattermost/src/mattermost/monitor.ts b/extensions/mattermost/src/mattermost/monitor.ts
index a7b692c2c74..d6f4bd9543c 100644
--- a/extensions/mattermost/src/mattermost/monitor.ts
+++ b/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,
+                }),
               });
             }
           }
diff --git a/src/auto-reply/reply/reply-plumbing.test.ts b/src/auto-reply/reply/reply-plumbing.test.ts
index 6d8a3d53232..6e039333c58 100644
--- a/src/auto-reply/reply/reply-plumbing.test.ts
+++ b/src/auto-reply/reply/reply-plumbing.test.ts
@@ -230,6 +230,46 @@ describe("applyReplyThreading auto-threading", () => {
     expect(result[0].replyToId).toBe("42");
     expect(result[0].replyToTag).toBe(true);
   });
+
+  it("resolves [[reply_to_current]] to currentMessageId when replyToMode is 'all'", () => {
+    // Mattermost-style scenario: agent responds with [[reply_to_current]] and replyToMode
+    // is "all". The tag should resolve to the inbound message id.
+    const result = applyReplyThreading({
+      payloads: [{ text: "[[reply_to_current]] some reply text" }],
+      replyToMode: "all",
+      currentMessageId: "mm-post-abc123",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("mm-post-abc123");
+    expect(result[0].replyToTag).toBe(true);
+    expect(result[0].text).toBe("some reply text");
+  });
+
+  it("resolves [[reply_to:<id>]] to explicit id when replyToMode is 'all'", () => {
+    const result = applyReplyThreading({
+      payloads: [{ text: "[[reply_to:mm-post-xyz789]] threaded reply" }],
+      replyToMode: "all",
+      currentMessageId: "mm-post-abc123",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("mm-post-xyz789");
+    expect(result[0].text).toBe("threaded reply");
+  });
+
+  it("sets replyToId via implicit threading when replyToMode is 'all'", () => {
+    // Even without explicit tags, replyToMode "all" should set replyToId
+    // to currentMessageId for threading.
+    const result = applyReplyThreading({
+      payloads: [{ text: "hello" }],
+      replyToMode: "all",
+      currentMessageId: "mm-post-abc123",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("mm-post-abc123");
+  });
 });
 
 const baseRun: SubagentRunRecord = {