diff --git a/libs/hackbot-runtime/hackbot_runtime/actions/bugzilla.py b/libs/hackbot-runtime/hackbot_runtime/actions/bugzilla.py
index 0cd5d4d2e9..1c31fc8d7d 100644
--- a/libs/hackbot-runtime/hackbot_runtime/actions/bugzilla.py
+++ b/libs/hackbot-runtime/hackbot_runtime/actions/bugzilla.py
@@ -226,7 +226,6 @@ async def create_bug(
         "summary": summary,
         "version": version,
         "description": description,
-        "is_markdown": True,
     }
     for k, v in (extra or {}).items():
         body.setdefault(k, v)
diff --git a/libs/hackbot-runtime/tests/test_bugzilla_actions.py b/libs/hackbot-runtime/tests/test_bugzilla_actions.py
index f9bccc86ac..02c1e969f5 100644
--- a/libs/hackbot-runtime/tests/test_bugzilla_actions.py
+++ b/libs/hackbot-runtime/tests/test_bugzilla_actions.py
@@ -58,7 +58,8 @@ async def test_create_bug_merges_extra_top_level_wins():
     body = rec.actions[0]["params"]
     assert body["severity"] == "S3"
     assert body["product"] == "Core"  # explicit arg wins over extra
-    assert body["is_markdown"] is True
+    # BMO's POST /bug rejects `is_markdown` with a 400, so it is never recorded.
+    assert "is_markdown" not in body
 
 
 async def test_handlers_return_confirmation_string():