# Contracts, types and translators

Generate TypeScript clients and verify contract changes with adaptations for older versions.

Requires WU CLI `1.2.0` and WU Framework `4.0.0` for application APIs. Check `wu --version` and the project dependencies.

```sh
wu contracts types counter-v1.json --out client.d.ts
wu contracts types counter-v1.json counter-v2.json --out clients.d.ts
wu contracts check counter-v1.json counter-v2.json --json
wu contracts check counter-v1.json counter-v2.json --translators translators.mjs --json
```

JSON files contain the descriptor accepted by `defineService`. The CLI loads `wu-framework/services` from the project, not a global copy. Each file must be valid JSON and no larger than `1 MiB`. `types` writes TypeScript declarations to stdout or a separate `.d.ts` file with `--out`.

With multiple contracts, names are `ServiceClientV1`, `ServiceClientV2`, and so on. If they share a major, the full version is included, such as `ServiceClient_1_2_0`. `check` takes exactly the previous and next contracts; it does not accept `--out`. `types` does not accept `--json` or `--translators`.

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## Compatibility with translators

The `--translators` module exports an array as `default` or `translators`. Each entry has `definition` and `methods`, matching what `provideService` accepts. The check moves covered changes into `translated`; uncovered changes remain in `changes`. Use [versioned services](/docs/framework/service-evolution/) to implement and test translators.

| Exit | Meaning |
| --- | --- |
| `0` | Compatible contract or types generated. |
| `1` | Uncovered incompatible changes. |
| `2` | Invalid file, descriptor, dependency or option. |

A compatible result verifies contract structure, not that translator functions are correct for every value. Exercise input and output examples, validation failures and permissions against the runtime before changing the provider.