The present disclosure is directed to a modified GraphQL server that enables application developers to define custom GraphQL schema fragments. For example, a developer may provide the GraphQL server with a GraphQL standard definition language (SDL) file having annotations that indicate that a customized data-handling script, such as a custom data-fetching script or a custom type resolver script, be called for handling certain data types or fields. These customized data-handling scripts are resolved and called at runtime based on the annotations in the GraphQL SDL file. By enabling developers to define schema fragments in the SDL language and implement data-handling scripts in a suitable scripting language, schema portability can be maintained across client instances, as well as instance release versions. Additionally, the disclosed approach enables enhanced modularity and reuse of previously defined data types and data-handling scripts, which increases developer productivity and reduces errors during application development.