# NexGen

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> NexGen generates typed models, runtime validators, and Nexus Service definitions in C#, Go, Java, Python, and TypeScript from one schema file.

> **Pre-release** — Go, Java, Python, TypeScript, .NET
> APIs are experimental and may be subject to backwards-incompatible changes.

> **📝 Note:**
>
> For setup instructions, see the [NexGen GitHub project](https://github.com/temporalio/nexgen#installation).
>

[NexGen](https://github.com/temporalio/nexgen) generates the request and response types, runtime validators, and [Nexus Service](/nexus/services) definition for a Nexus Service from one schema file.
The handler and the caller each generate code from that file, in C#, Go, Java, Python, or TypeScript, so both sides use the same types even when they're written in different languages.
Without a shared schema, each team hand-writes its own copy of the types, the copies drift apart, and the mismatch shows up as a failed call.

## What NexGen generates

You write the contract as a JSON or YAML definition file.
Types use [JSON Schema 2020-12](https://json-schema.org), and a file that declares Services also lists each Operation's input and output types.
Run `nexgen` against the file for each language you need.

For each type, NexGen generates a typed model (a record, struct, class, interface, or dataclass, depending on the language) with doc comments copied from the schema, and a runtime validator.
For each Service, it generates a Service definition: the handler implements it, and the caller uses it to start Operations.

For a complete contract, see the approval contract in the [Java Microservice Development Walkthrough](/develop/java/nexus/development-walkthrough/define-the-data-contract), or the [Definition files](https://github.com/temporalio/nexgen#definition-files) section of the `nexgen` README for the file format.

## Data validation

The generated code validates each value when it's deserialized from a payload and again when it's serialized into one, so invalid data is rejected before it reaches your Workflow or Activity.
A failure produces one error that lists every violation, with the field and the constraint it broke.

When an Operation's input fails validation, the Worker rejects it with a non-retryable `BAD_REQUEST` [Nexus handler error](/nexus/error-handling) before your Operation code runs.
This requires the minimum SDK versions listed in [SDK requirements](https://github.com/temporalio/nexgen#sdk-requirements) in the `nexgen` README.

Validation behaves the same in every language.
To keep it that way, `nexgen` refuses at generation time any schema feature it can't validate the same way in all five languages.
See [Supported JSON Schema features](https://github.com/temporalio/nexgen#supported-json-schema-features) for the supported subset.

Three edge cases still differ by language: negative zero, very large fractional integers, and nanosecond timestamp precision in C# and Python.
See [Known cross-language divergences](https://github.com/temporalio/nexgen#known-cross-language-divergences) in the `nexgen` README for what each language does.