diff --git a/extensions/buzz/src/buzz-bus.lifecycle.test.ts b/extensions/buzz/src/buzz-bus.lifecycle.test.ts
index a7ef87040786..8e685b27a4d8 100644
--- a/extensions/buzz/src/buzz-bus.lifecycle.test.ts
+++ b/extensions/buzz/src/buzz-bus.lifecycle.test.ts
@@ -343,6 +343,22 @@ describe("Buzz bus lifecycle", () => {
     expect(relayMocks.close).toHaveBeenCalledOnce();
   });
 
+  it("closes a standalone relay when mention preflight rejects the message", async () => {
+    relayMocks.auth.mockResolvedValue("ok");
+
+    await expect(
+      sendBuzzTextOneShot({
+        relayUrl: "wss://buzz.example.com",
+        privateKey: PRIVATE_KEY,
+        channelId: CHANNEL_ID,
+        text: "Hello @Missing",
+      }),
+    ).rejects.toThrow('Buzz mention "@missing" does not match a current room member');
+
+    expect(relayMocks.publish).not.toHaveBeenCalled();
+    expect(relayMocks.close).toHaveBeenCalledOnce();
+  });
+
   it("resolves active-bus mentions for proactive sends and agent replies", async () => {
     relayMocks.auth.mockResolvedValue("ok");
     relayMocks.profileEvents = [
diff --git a/extensions/buzz/src/mentions.test.ts b/extensions/buzz/src/mentions.test.ts
index 65c74ac56516..82839000e17f 100644
--- a/extensions/buzz/src/mentions.test.ts
+++ b/extensions/buzz/src/mentions.test.ts
@@ -89,4 +89,22 @@ describe("Buzz outbound mentions", () => {
     ).toEqual([]);
     expect(hasBuzzMentionSyntax("mail user@example.com")).toBe(false);
   });
+
+  it("rejects messages above the native mention limit", () => {
+    const mentionedMembers = Array.from({ length: 51 }, (_, index) => {
+      const publicKey = (index + 1).toString(16).padStart(64, "0");
+      return {
+        publicKey,
+        displayName: `Member${index + 1}`,
+      };
+    });
+
+    expect(() =>
+      resolveBuzzMessageMentions({
+        text: mentionedMembers.map((member) => `@${member.displayName}`).join(" "),
+        members: members(...mentionedMembers),
+        senderPublicKey: BOT_PUBLIC_KEY,
+      }),
+    ).toThrow("Buzz messages support at most 50 mentions");
+  });
 });