diff --git a/codex-rs/core/src/plugins/marketplace_add/metadata.rs b/codex-rs/core/src/plugins/marketplace_add/metadata.rs
index ce271b116d..d87f54cd3a 100644
--- a/codex-rs/core/src/plugins/marketplace_add/metadata.rs
+++ b/codex-rs/core/src/plugins/marketplace_add/metadata.rs
@@ -296,12 +296,22 @@ mod tests {
             r#"{"name":"debug","plugins":[]}"#,
         )
         .unwrap();
+        let mut marketplace = toml::map::Map::new();
+        marketplace.insert(
+            "source_type".to_string(),
+            toml::Value::String("local".to_string()),
+        );
+        marketplace.insert(
+            "source".to_string(),
+            toml::Value::String(source_root.display().to_string()),
+        );
+        let mut marketplaces = toml::map::Map::new();
+        marketplaces.insert("debug".to_string(), toml::Value::Table(marketplace));
+        let mut config = toml::map::Map::new();
+        config.insert("marketplaces".to_string(), toml::Value::Table(marketplaces));
         fs::write(
             codex_home.path().join(CONFIG_TOML_FILE),
-            format!(
-                "[marketplaces.debug]\nsource_type = \"local\"\nsource = \"{}\"\n",
-                source_root.display()
-            ),
+            toml::to_string(&toml::Value::Table(config)).unwrap(),
         )
         .unwrap();