diff --git a/hive-c0re/src/agent_server.rs b/hive-c0re/src/agent_server.rs
index 182e3413..9dcf468e 100644
--- a/hive-c0re/src/agent_server.rs
+++ b/hive-c0re/src/agent_server.rs
@@ -563,6 +563,31 @@ pub(crate) fn handle_send(
     // learning the label — runtime reparenting propagates for free. See
     // `docs/conventions.md::Recipient sentinels`.
     let resolved = crate::topology::resolve_recipient(agent, to);
+    // Validate that the resolved recipient is a known local agent or the
+    // special "operator" recipient. Without this check a typo in `to`
+    // silently queues a message nobody will ever read (issue #1165).
+    //
+    // Cross-hive messaging (`name@hive` qualified names) is not routed
+    // through the broker — use the Matrix MCP tools for that instead.
+    if resolved.contains('@') {
+        return AgentResponse::Err {
+            message: format!(
+                "send failed: cross-hive recipient `{resolved}` is not supported \
+                 via the broker — use Matrix MCP tools for cross-hive messaging"
+            ),
+        };
+    }
+    if resolved != hive_sh4re::OPERATOR_RECIPIENT {
+        let state_root = crate::coordinator::Coordinator::agent_state_root(&resolved);
+        if !state_root.exists() {
+            return AgentResponse::Err {
+                message: format!(
+                    "send failed: unknown recipient `{resolved}` \
+                     (no agent with that name exists on this hive)"
+                ),
+            };
+        }
+    }
     match coord.broker.send(&Message {
         from: agent.to_owned(),
         to: resolved,