diff --git a/hive-matrix-mcp/src/protocol.rs b/hive-matrix-mcp/src/protocol.rs
index efc5affd..d5982620 100644
--- a/hive-matrix-mcp/src/protocol.rs
+++ b/hive-matrix-mcp/src/protocol.rs
@@ -68,8 +68,11 @@ pub enum DaemonRequest {
     #[serde(rename = "read_room")]
     ReadRoom { room: String, limit: Option<usize> },
 
-    /// Liveness probe used by the stdio MCP bridge on connect — fast
-    /// "are you up?" round-trip that doesn't touch matrix-sdk.
+    /// Liveness probe — fast "are you up?" round-trip that doesn't
+    /// touch matrix-sdk. Not used by the in-tree stdio MCP bridge
+    /// (which surfaces a daemon-down condition as a normal tool-call
+    /// connect error); reserved for external clients that want an
+    /// explicit health check without doing real work.
     #[serde(rename = "ping")]
     Ping,
 }
@@ -86,10 +89,14 @@ pub enum DaemonResponse {
 
 impl DaemonResponse {
     /// Convenience: build an Ok response from any serializable value.
+    /// Serialisation failure (impossible for the small handler structs
+    /// we use, but the API is generic) surfaces as a structured
+    /// `{"serialise_error": "..."}` payload so callers see WHY the
+    /// data is missing instead of a silent `null`.
     pub fn ok<T: Serialize>(payload: &T) -> Self {
-        Self::Ok {
-            payload: serde_json::to_value(payload).unwrap_or(serde_json::Value::Null),
-        }
+        let payload = serde_json::to_value(payload)
+            .unwrap_or_else(|e| serde_json::json!({ "serialise_error": e.to_string() }));
+        Self::Ok { payload }
     }
 
     /// Convenience: build an Error response from any `Display` value.