diff --git a/codex-rs/mcp-server/src/message_processor.rs b/codex-rs/mcp-server/src/message_processor.rs
index 8018095d..6e63dee5 100644
--- a/codex-rs/mcp-server/src/message_processor.rs
+++ b/codex-rs/mcp-server/src/message_processor.rs
@@ -238,7 +238,7 @@ impl MessageProcessor {
                 name: "codex-mcp-server".to_string(),
                 version: env!("CARGO_PKG_VERSION").to_string(),
                 title: Some("Codex".to_string()),
-                user_agent: get_codex_user_agent(),
+                user_agent: Some(get_codex_user_agent()),
             },
         };
 
diff --git a/codex-rs/mcp-types/src/lib.rs b/codex-rs/mcp-types/src/lib.rs
index 2f862be8..75778cb2 100644
--- a/codex-rs/mcp-types/src/lib.rs
+++ b/codex-rs/mcp-types/src/lib.rs
@@ -496,7 +496,8 @@ pub struct McpServerInfo {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub title: Option<String>,
     pub version: String,
-    pub user_agent: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub user_agent: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]