diff --git a/codex-rs/core/src/tools/spec.rs b/codex-rs/core/src/tools/spec.rs
index bfad3cdb3531..188be42ba179 100644
--- a/codex-rs/core/src/tools/spec.rs
+++ b/codex-rs/core/src/tools/spec.rs
@@ -263,6 +263,15 @@ pub(crate) fn build_specs_with_discoverable_tools(
             }
         }
     }
+    if let Some(deferred_mcp_tools) = deferred_mcp_tools.as_ref() {
+        for (name, _) in deferred_mcp_tools.iter().filter(|(name, _)| {
+            !mcp_tools
+                .as_ref()
+                .is_some_and(|tools| tools.contains_key(*name))
+        }) {
+            builder.register_handler(name.clone(), mcp_handler.clone());
+        }
+    }
     builder
 }
 
diff --git a/codex-rs/core/src/tools/spec_tests.rs b/codex-rs/core/src/tools/spec_tests.rs
index da76a1bb28f2..b0195e4563d5 100644
--- a/codex-rs/core/src/tools/spec_tests.rs
+++ b/codex-rs/core/src/tools/spec_tests.rs
@@ -909,6 +909,8 @@ fn search_tool_registers_namespaced_mcp_tool_aliases() {
     assert!(registry.has_handler(&ToolName::plain(TOOL_SEARCH_TOOL_NAME)));
     assert!(registry.has_handler(&app_alias));
     assert!(registry.has_handler(&mcp_alias));
+    assert!(registry.has_handler(&ToolName::plain("mcp__codex_apps__calendar_create_event")));
+    assert!(registry.has_handler(&ToolName::plain("mcp__rmcp__echo")));
 }
 
 #[test]