diff --git a/test/fixtures/test-parallel.behavior.json b/test/fixtures/test-parallel.behavior.json
index 727aef8c198..262c9fa11b9 100644
--- a/test/fixtures/test-parallel.behavior.json
+++ b/test/fixtures/test-parallel.behavior.json
@@ -101,6 +101,10 @@
       {
         "file": "extensions/telegram/src/webhook.test.ts",
         "reason": "This Telegram webhook suite hoists grammY bot and InputFile mocks; keep it in its own forked channel lane so shared workers do not reuse telegram runtime modules with mismatched constructors."
+      },
+      {
+        "file": "extensions/slack/src/action-runtime.test.ts",
+        "reason": "This Slack action runtime suite measured ~766.2 MiB RSS growth locally; keep it in its own forked channel lane so the shared channels worker can recycle immediately after the hotspot file."
       }
     ]
   },