diff --git a/extensions/mattermost/src/channel.ts b/extensions/mattermost/src/channel.ts
index e0392728b41..90d24e11406 100644
--- a/extensions/mattermost/src/channel.ts
+++ b/extensions/mattermost/src/channel.ts
@@ -39,6 +39,7 @@ import {
   type ChannelPlugin,
 } from "./runtime-api.js";
 import { getMattermostRuntime } from "./runtime.js";
+import { resolveMattermostOutboundSessionRoute } from "./session-route.js";
 import { mattermostSetupAdapter } from "./setup-core.js";
 import { mattermostSetupWizard } from "./setup-surface.js";
 
@@ -348,6 +349,7 @@ export const mattermostPlugin: ChannelPlugin<ResolvedMattermostAccount> = {
   },
   messaging: {
     normalizeTarget: normalizeMattermostMessagingTarget,
+    resolveOutboundSessionRoute: (params) => resolveMattermostOutboundSessionRoute(params),
     targetResolver: {
       looksLikeId: looksLikeMattermostTargetId,
       hint: "<channelId|user:ID|channel:ID>",
diff --git a/extensions/mattermost/src/session-route.ts b/extensions/mattermost/src/session-route.ts
new file mode 100644
index 00000000000..14352708986
--- /dev/null
+++ b/extensions/mattermost/src/session-route.ts
@@ -0,0 +1,52 @@
+import {
+  buildChannelOutboundSessionRoute,
+  normalizeOutboundThreadId,
+  resolveThreadSessionKeys,
+  stripChannelTargetPrefix,
+  stripTargetKindPrefix,
+  type ChannelOutboundSessionRouteParams,
+} from "openclaw/plugin-sdk/core";
+
+export function resolveMattermostOutboundSessionRoute(params: ChannelOutboundSessionRouteParams) {
+  let trimmed = stripChannelTargetPrefix(params.target, "mattermost");
+  if (!trimmed) {
+    return null;
+  }
+  const lower = trimmed.toLowerCase();
+  const resolvedKind = params.resolvedTarget?.kind;
+  const isUser =
+    resolvedKind === "user" ||
+    (resolvedKind !== "channel" &&
+      resolvedKind !== "group" &&
+      (lower.startsWith("user:") || trimmed.startsWith("@")));
+  if (trimmed.startsWith("@")) {
+    trimmed = trimmed.slice(1).trim();
+  }
+  const rawId = stripTargetKindPrefix(trimmed);
+  if (!rawId) {
+    return null;
+  }
+  const baseRoute = buildChannelOutboundSessionRoute({
+    cfg: params.cfg,
+    agentId: params.agentId,
+    channel: "mattermost",
+    accountId: params.accountId,
+    peer: {
+      kind: isUser ? "direct" : "channel",
+      id: rawId,
+    },
+    chatType: isUser ? "direct" : "channel",
+    from: isUser ? `mattermost:${rawId}` : `mattermost:channel:${rawId}`,
+    to: isUser ? `user:${rawId}` : `channel:${rawId}`,
+  });
+  const threadId = normalizeOutboundThreadId(params.replyToId ?? params.threadId);
+  const threadKeys = resolveThreadSessionKeys({
+    baseSessionKey: baseRoute.baseSessionKey,
+    threadId,
+  });
+  return {
+    ...baseRoute,
+    sessionKey: threadKeys.sessionKey,
+    ...(threadId !== undefined ? { threadId } : {}),
+  };
+}