diff --git a/extensions/mattermost/src/channel.ts b/extensions/mattermost/src/channel.ts
index b151ed708d9..401ddabf38b 100644
--- a/extensions/mattermost/src/channel.ts
+++ b/extensions/mattermost/src/channel.ts
@@ -167,15 +167,24 @@ const mattermostMessageActions: ChannelMessageActionAdapter = {
       if (account.botToken) setInteractionSecret(account.botToken);
       const callbackUrl = resolveInteractionCallbackUrl(account.accountId, cfg);
 
-      const buttons = (params.buttons as Array<Record<string, unknown>>).map((btn) => ({
-        id: String(btn.id ?? btn.callback_data ?? ""),
-        name: String(btn.text ?? btn.name ?? btn.label ?? ""),
-        style: (btn.style as "default" | "primary" | "danger") ?? "default",
-        context:
-          typeof btn.context === "object" && btn.context !== null
-            ? (btn.context as Record<string, unknown>)
-            : undefined,
-      }));
+      // Flatten 2D array (rows of buttons) to 1D — core schema sends Array<Array<Button>>
+      // but Mattermost doesn't have row layout, so we flatten all rows into a single list.
+      // Also supports 1D arrays for backward compatibility.
+      const rawButtons = (params.buttons as Array<unknown>).flatMap((item) =>
+        Array.isArray(item) ? item : [item],
+      ) as Array<Record<string, unknown>>;
+
+      const buttons = rawButtons
+        .map((btn) => ({
+          id: String(btn.id ?? btn.callback_data ?? ""),
+          name: String(btn.text ?? btn.name ?? btn.label ?? ""),
+          style: (btn.style as "default" | "primary" | "danger") ?? "default",
+          context:
+            typeof btn.context === "object" && btn.context !== null
+              ? (btn.context as Record<string, unknown>)
+              : undefined,
+        }))
+        .filter((btn) => btn.id && btn.name);
 
       const attachmentText =
         typeof params.attachmentText === "string" ? params.attachmentText : undefined;