Techniques are described for enabling customers of a service provider network to modify and extend the functionality of application programming interfaces (APIs) provided to customers by various services of the provider network. Services of a service provider network provide interfaces that enable customers and other users to define custom API actions representing functionality to be performed by the services upon receipt of corresponding API requests. In this manner, the functionality exposed by provider network services to customers via APIs becomes extensible and customizable to fit customers' particular use cases and without the provider network needing to contemplate and explicitly implement all possible customer-specific API action variations.