visschemas

Visschemas is an open-source framework for defining, validating, and applying schemas to structured data. It provides a concise schema language and a validation engine designed to catch structural and semantic errors before data is consumed. Schemas are written in a JSON-like syntax that supports primitive types, nested objects, arrays, enumerations, and references to reusable components. The language emphasizes composability, allowing schemas to be built from smaller sub-schemas and extended through augmentation.

The validation engine offers strict and permissive modes, optional type coercion, and detailed error reporting that points to the exact field and constraint violated. It also supports custom validators and asynchronous checks, enabling integration with external data sources and business rules. In addition to runtime validation, the framework can generate basic API contracts and (to some extent) documentation from schemas.

Visschemas includes tooling for Python and JavaScript environments, a command-line interface, and a programmable API for embedding validation logic into applications. It supports references, default values, required and optional fields, and common constraints such as minimums, maximums, patterns, and length limits. Interoperability with JSON Schema is provided through translation helpers and schema adapters.

Common use cases include API input validation, configuration file verification, data-pipeline quality checks, and data interchange between systems. The project is maintained by an open community and released under an open-source license. See also JSON Schema, data validation libraries, and API design best practices.