diff --git a/extensions/feishu/src/bot.ts b/extensions/feishu/src/bot.ts
index ba10c803ad4..7a1ffd6191e 100644
--- a/extensions/feishu/src/bot.ts
+++ b/extensions/feishu/src/bot.ts
@@ -10,6 +10,7 @@ import type { FeishuMessageContext, FeishuMediaInfo, ResolvedFeishuAccount } fro
 import type { DynamicAgentCreationConfig } from "./types.js";
 import { resolveFeishuAccount } from "./accounts.js";
 import { createFeishuClient } from "./client.js";
+import { tryRecordMessage } from "./dedup.js";
 import { maybeCreateDynamicAgent } from "./dynamic-agent.js";
 import { downloadImageFeishu, downloadMessageResourceFeishu } from "./media.js";
 import { extractMentionTargets, extractMessageBody, isMentionForwardRequest } from "./mention.js";
@@ -23,37 +24,6 @@ import { createFeishuReplyDispatcher } from "./reply-dispatcher.js";
 import { getFeishuRuntime } from "./runtime.js";
 import { getMessageFeishu, sendMessageFeishu } from "./send.js";
 
-// --- Message deduplication ---
-// Prevent duplicate processing when WebSocket reconnects or Feishu redelivers messages.
-const DEDUP_TTL_MS = 30 * 60 * 1000; // 30 minutes
-const DEDUP_MAX_SIZE = 1_000;
-const DEDUP_CLEANUP_INTERVAL_MS = 5 * 60 * 1000; // cleanup every 5 minutes
-const processedMessageIds = new Map<string, number>(); // messageId -> timestamp
-let lastCleanupTime = Date.now();
-
-function tryRecordMessage(messageId: string): boolean {
-  const now = Date.now();
-
-  // Throttled cleanup: evict expired entries at most once per interval
-  if (now - lastCleanupTime > DEDUP_CLEANUP_INTERVAL_MS) {
-    for (const [id, ts] of processedMessageIds) {
-      if (now - ts > DEDUP_TTL_MS) processedMessageIds.delete(id);
-    }
-    lastCleanupTime = now;
-  }
-
-  if (processedMessageIds.has(messageId)) return false;
-
-  // Evict oldest entries if cache is full
-  if (processedMessageIds.size >= DEDUP_MAX_SIZE) {
-    const first = processedMessageIds.keys().next().value!;
-    processedMessageIds.delete(first);
-  }
-
-  processedMessageIds.set(messageId, now);
-  return true;
-}
-
 // --- Permission error extraction ---
 // Extract permission grant URL from Feishu API error response.
 type PermissionError = {
diff --git a/extensions/feishu/src/dedup.ts b/extensions/feishu/src/dedup.ts
new file mode 100644
index 00000000000..25677f628d5
--- /dev/null
+++ b/extensions/feishu/src/dedup.ts
@@ -0,0 +1,33 @@
+// Prevent duplicate processing when WebSocket reconnects or Feishu redelivers messages.
+const DEDUP_TTL_MS = 30 * 60 * 1000; // 30 minutes
+const DEDUP_MAX_SIZE = 1_000;
+const DEDUP_CLEANUP_INTERVAL_MS = 5 * 60 * 1000; // cleanup every 5 minutes
+const processedMessageIds = new Map<string, number>(); // messageId -> timestamp
+let lastCleanupTime = Date.now();
+
+export function tryRecordMessage(messageId: string): boolean {
+  const now = Date.now();
+
+  // Throttled cleanup: evict expired entries at most once per interval.
+  if (now - lastCleanupTime > DEDUP_CLEANUP_INTERVAL_MS) {
+    for (const [id, ts] of processedMessageIds) {
+      if (now - ts > DEDUP_TTL_MS) {
+        processedMessageIds.delete(id);
+      }
+    }
+    lastCleanupTime = now;
+  }
+
+  if (processedMessageIds.has(messageId)) {
+    return false;
+  }
+
+  // Evict oldest entries if cache is full.
+  if (processedMessageIds.size >= DEDUP_MAX_SIZE) {
+    const first = processedMessageIds.keys().next().value!;
+    processedMessageIds.delete(first);
+  }
+
+  processedMessageIds.set(messageId, now);
+  return true;
+}
diff --git a/extensions/mattermost/src/mattermost/monitor-onchar.ts b/extensions/mattermost/src/mattermost/monitor-onchar.ts
new file mode 100644
index 00000000000..c23629fbee1
--- /dev/null
+++ b/extensions/mattermost/src/mattermost/monitor-onchar.ts
@@ -0,0 +1,25 @@
+const DEFAULT_ONCHAR_PREFIXES = [">", "!"];
+
+export function resolveOncharPrefixes(prefixes: string[] | undefined): string[] {
+  const cleaned = prefixes?.map((entry) => entry.trim()).filter(Boolean) ?? DEFAULT_ONCHAR_PREFIXES;
+  return cleaned.length > 0 ? cleaned : DEFAULT_ONCHAR_PREFIXES;
+}
+
+export function stripOncharPrefix(
+  text: string,
+  prefixes: string[],
+): { triggered: boolean; stripped: string } {
+  const trimmed = text.trimStart();
+  for (const prefix of prefixes) {
+    if (!prefix) {
+      continue;
+    }
+    if (trimmed.startsWith(prefix)) {
+      return {
+        triggered: true,
+        stripped: trimmed.slice(prefix.length).trimStart(),
+      };
+    }
+  }
+  return { triggered: false, stripped: text };
+}
diff --git a/extensions/mattermost/src/mattermost/monitor.ts b/extensions/mattermost/src/mattermost/monitor.ts
index cce4d87b381..8d4f3d95e95 100644
--- a/extensions/mattermost/src/mattermost/monitor.ts
+++ b/extensions/mattermost/src/mattermost/monitor.ts
@@ -38,6 +38,7 @@ import {
   rawDataToString,
   resolveThreadSessionKeys,
 } from "./monitor-helpers.js";
+import { resolveOncharPrefixes, stripOncharPrefix } from "./monitor-onchar.js";
 import { sendMessageMattermost } from "./send.js";
 
 export type MonitorMattermostOpts = {
@@ -75,7 +76,6 @@ const RECENT_MATTERMOST_MESSAGE_TTL_MS = 5 * 60_000;
 const RECENT_MATTERMOST_MESSAGE_MAX = 2000;
 const CHANNEL_CACHE_TTL_MS = 5 * 60_000;
 const USER_CACHE_TTL_MS = 10 * 60_000;
-const DEFAULT_ONCHAR_PREFIXES = [">", "!"];
 
 const recentInboundMessages = createDedupeCache({
   ttlMs: RECENT_MATTERMOST_MESSAGE_TTL_MS,
@@ -103,30 +103,6 @@ function normalizeMention(text: string, mention: string | undefined): string {
   return text.replace(re, " ").replace(/\s+/g, " ").trim();
 }
 
-function resolveOncharPrefixes(prefixes: string[] | undefined): string[] {
-  const cleaned = prefixes?.map((entry) => entry.trim()).filter(Boolean) ?? DEFAULT_ONCHAR_PREFIXES;
-  return cleaned.length > 0 ? cleaned : DEFAULT_ONCHAR_PREFIXES;
-}
-
-function stripOncharPrefix(
-  text: string,
-  prefixes: string[],
-): { triggered: boolean; stripped: string } {
-  const trimmed = text.trimStart();
-  for (const prefix of prefixes) {
-    if (!prefix) {
-      continue;
-    }
-    if (trimmed.startsWith(prefix)) {
-      return {
-        triggered: true,
-        stripped: trimmed.slice(prefix.length).trimStart(),
-      };
-    }
-  }
-  return { triggered: false, stripped: text };
-}
-
 function isSystemPost(post: MattermostPost): boolean {
   const type = post.type?.trim();
   return Boolean(type);