From fecc9f4a7f432c8aeca6db76e56a740535e92ba1 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <7190048+kegsay@users.noreply.github.com>
Date: Wed, 7 Oct 2026 17:01:34 +0200
Subject: [PATCH] Ensure we build valid MSC4242 events

---
 synapse/events/builder.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index 742bb82da9..15a543cf3e 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -307,6 +307,7 @@ class EventBuilder:
             # Auth events are removed entirely on state DAG rooms
             event_dict.pop("auth_events")
             assert prev_state_events is not None
+            assert len(prev_state_events) <= 20
             event_dict["prev_state_events"] = prev_state_events
         if self.room_id is not None:
             event_dict["room_id"] = self.room_id