diff --git a/config/services.yaml b/config/services.yaml
index e161f008..efe01cbb 100644
--- a/config/services.yaml
+++ b/config/services.yaml
@@ -9,6 +9,14 @@
 parameters:
     default_api_ir_base_url: 'https://s.api.ir'
 
+    # Optional ISR webhook for the public site. Without defaults, a deploy that
+    # does not define these variables makes BlogCacheInvalidator un-instantiable,
+    # which takes the WHOLE blog module down with HTTP 500 — reads included.
+    # BlogCacheInvalidator is fail-open on an empty string: no webhook, no cache
+    # invalidation, everything else keeps working.
+    env(REVALIDATE_WEBHOOK_URL): ''
+    env(REVALIDATE_WEBHOOK_SECRET): ''
+
 services:
     # default configuration for services in *this* file
     _defaults: