Not all of a recipe's fields are available in this list endpoint, for performance reasons. To get all information related to a recipe, use the Get Recipe Details endpoint.

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Recipe modification dates
The modification date gets updated only when there is a change in one of the recipe's native data fields. Attributes that are a calculated result from an aggregation of related objects do not update the recipe's modified_date. These are the financial, nutrition_info, nutrition_info_per_portion and dietary_info objects. Changes to a related object's fields (such as changing an ingredient's names) will also not update the modified_date of the recipes that contain it.
To track these changes, you need to track the modification dates of the ingredients contained in these recipes by querying the Get Ingredients endpoint with the modification_date filter.
For more info about keeping local copies of data objects up to date, consult the guides.