diff --git a/packages/mosaic/src/lease-broker/broker-test-client.spec.ts b/packages/mosaic/src/lease-broker/broker-test-client.spec.ts
index b4a4278..f1c6d68 100644
--- a/packages/mosaic/src/lease-broker/broker-test-client.spec.ts
+++ b/packages/mosaic/src/lease-broker/broker-test-client.spec.ts
@@ -24,8 +24,13 @@ async function scriptedBroker(
     connections += 1;
     socket.once('end', () => {
       if (chunks === 'hang') return;
-      for (const chunk of chunks) socket.write(chunk);
-      socket.end();
+      void (async () => {
+        for (const chunk of chunks) {
+          socket.write(chunk);
+          await new Promise<void>((resolve) => setTimeout(resolve, 5));
+        }
+        socket.end();
+      })();
     });
     socket.resume();
   });
@@ -105,6 +110,40 @@ describe('newline-framed broker test client', () => {
     expect(broker.connections()).toBe(1);
   });
 
+  test('rejects trailing bytes delivered after a complete frame in a later data event', async () => {
+    const broker = await scriptedBroker([[Buffer.from('{"ok":true}\n'), Buffer.from('extra')]]);
+
+    const failure = await requestBrokerReply(broker.socketPath, { action: 'probe' }).catch(
+      (error: unknown) => error,
+    );
+
+    expect(failure).toBeInstanceOf(BrokerTransportError);
+    expect(failure).toMatchObject({ kind: 'malformed-reply', responseLength: 17 });
+    expect(failure).toHaveProperty(
+      'message',
+      expect.stringMatching(/bytes after the newline terminator/i),
+    );
+  });
+
+  test('redacts security tokens from typed transport diagnostics', async () => {
+    const token = 'a'.repeat(64);
+    const reply = Buffer.from(`{"ok":true,"promotion_token":"${token}`);
+    const broker = await scriptedBroker([[reply]]);
+
+    const failure = await requestBrokerReply(broker.socketPath, { action: 'probe' }).catch(
+      (error: unknown) => error,
+    );
+
+    expect(failure).toBeInstanceOf(BrokerTransportError);
+    expect(failure).toMatchObject({
+      kind: 'early-close',
+      responseLength: reply.length,
+      responsePreview: '<redacted-sensitive-reply>',
+    });
+    expect(String((failure as Error).message)).not.toContain(token);
+    expect(JSON.stringify(failure)).not.toContain(token);
+  });
+
   test.each([
     ['trailing bytes', Buffer.from('{"ok":true}\nextra'), /bytes after the newline/i],
     ['non-object JSON', Buffer.from('[]\n'), /JSON value is not an object/i],