When adding or modifying configuration settings, treat default values as part of the public API: do not silently change an existing default (e.g. bumping a parser limit from 1000 to 10000) unless the change is intentional and documented. If a default behavior must change, add a deprecation warning that fires only when the application relies on the implicit default, not when the user has explicitly set that value — track this with an internal flag rather than inferring intent from the chosen value. ```js // Track whether the user explicitly configured the setting var usingDefaultExtendedParser = true; app.set('query parser', function (val) { usingDefaultExtendedParser = false; // user made an explicit choice return parseExtendedQueryString(val); }); // Only warn when no explicit choice was made if (usingDefaultExtendedParser) { deprecate('default extended query parser is deprecated, set explicitly to override'); } ``` Additionally, always read or write configuration through the established accessor functions (`app.get()` / `app.set()`) rather than touching the internal settings object directly (e.g. `app.settings.etag`). Direct property access bypasses validation/defaults handling and was deprecated for this reason — use `app.get('etag')` instead.