Systems and methods are described for modifying input and output (I/O) to an object storage service by implementing one or more owner-specified functions to I/O requests. A function can implement a data manipulation, such as filtering out sensitive data before reading or writing the data. The functions can be applied prior to implementing a request method (e.g., GET or PUT) specified within the I/O request, such that the data to which the method is applied my not match the object specified within the request. For example, a user may request to obtain (e.g., GET) a data set. The data set may be passed to a function that filters sensitive data to the data set, and the GET request method may then be applied to the output of the function. In this manner, owners of objects on an object storage service are provided with greater control of objects stored or retrieved from the service.