From 8c81f4ba0f18c11731f9d13ec552f6a2e984c55c Mon Sep 17 00:00:00 2001
From: Anton <vikorenman@icloud.com>
Date: Wed, 22 Jul 2026 14:26:51 +1000
Subject: [PATCH] Fix Synapse crash on empty experimental_features (rendered as
 null) when no experimental features are enabled

Fixes #5455
---
 .../templates/synapse/homeserver.yaml.j2           | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2 b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
index ec3b3a3c6..abecb636a 100644
--- a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
+++ b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
@@ -3003,7 +3003,19 @@ matrix_authentication_service:
   secret: {{ matrix_synapse_matrix_authentication_service_secret | to_json }}
 {% endif %}
 
-experimental_features:
+{% set experimental_features_enabled =
+       matrix_synapse_experimental_features_msc2409_to_device_messages_enabled
+    or matrix_synapse_experimental_features_msc3202_transaction_extensions_enabled
+    or matrix_synapse_experimental_features_msc3266_enabled
+    or matrix_synapse_experimental_features_msc4108_enabled
+    or matrix_synapse_experimental_features_msc4140_enabled
+    or matrix_synapse_experimental_features_msc4143_enabled
+    or matrix_synapse_experimental_features_msc4222_enabled
+    or matrix_synapse_experimental_features_msc4306_enabled
+    or matrix_synapse_experimental_features_msc4354_enabled
+    or matrix_synapse_experimental_features_msc4429_enabled
+%}
+experimental_features:{{ ' {}' if not experimental_features_enabled else '' }}
   {% if matrix_synapse_experimental_features_msc2409_to_device_messages_enabled %}
   msc2409_to_device_messages_enabled: true
   {% endif %}