# Nexus Client Code Generator

> For the complete documentation index, see [llms.txt](https://docs.temporal.io/llms.txt).
> Any documentation page is available as raw Markdown by appending `.md` to its URL.

> The Nexus Client Code Generator turns one schema into typed models, runtime validators, and Nexus Service definitions for Go, Java, Python, and TypeScript.

> **⚠️ Caution:**
>
> `nexgen` is pre-release software and may not retain backwards compatibility with previous versions of the tool.
> It is not yet published to any package registry, so you build it from source.
>

A [Nexus Service](/nexus/services) is a contract meant to be shared across team boundaries.
Those teams often work in different languages, so the same request and response types get hand-written for each SDK.
Hand-written copies drift: a field is required on one side and optional on the other, a bound is enforced by the caller but not the handler.

The **[Nexus Client Code Generator](https://github.com/temporalio/nexgen)** removes those copies.
You describe your types and [Nexus Operations](/nexus/operations) once in a definition file, and the generator emits the equivalent library code for Go, Java, Python, and TypeScript.
The generator is a command-line tool named `nexgen`, distributed from the [temporalio/nexgen](https://github.com/temporalio/nexgen) repository.

## What the generator produces

The generator produces a client library for the inputs and outputs of your Nexus Operations. The generated types check values against the contract as they are sent and received, so a violation surfaces as an error rather than as bad data.

That client library contains three things:

- **A typed model.** An idiomatic struct, class, interface, or dataclass, with doc comments carried over from the schema.
- **A runtime validator.** One validator per type, applied when a value is parsed off the wire and again when it is serialized onto it.
- **A [Nexus Service Contract](/glossary#nexus-service-contract) definition**, for a file that declares Services. These are the Service and Operation declarations you register on a Worker and call from a caller Workflow. A pure JSON Schema file declares none, so it produces only the models and their validators.

Constraint failures are aggregated into a single native error listing every violation, each naming the offending field and the bound it broke.
A handler maps that error to a `BAD_REQUEST` [Nexus error](/nexus/error-handling), so a malformed request tells the caller everything that was wrong with it in one response.

## Related

- How to use the Nexus Client Code Generator:
  [Go](/develop/go/nexus/client-code-generator) ·
  [Java](/develop/java/nexus/client-code-generator) ·
  [Python](/develop/python/nexus/client-code-generator) ·
  [TypeScript](/develop/typescript/nexus/client-code-generator)
- [Temporal Operation Handler](/nexus/temporal-operation-handler) — implement generated Service Operations
- [Microservice Development Walkthrough](/develop/java/nexus/development-walkthrough) — end-to-end Java guide