> ## Documentation Index
> Fetch the complete documentation index at: https://docs.launchtoday.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Type safety across apps

> How Launch shares API types between backend, mobile, and web

## Overview

Launch uses tRPC to share API types across the API, mobile app, and web app.
The API defines a single router, and both clients import the router type
directly from the API package.

## Prerequisites

* API package built and available
* `apps/api/src/types.ts` exports `AppRouter`

## Steps

1. Define routers in `apps/api/src/router.ts`
2. Export `AppRouter` from `apps/api/src/types.ts`
3. Import `AppRouter` in mobile and web

## Single source of truth

* **Router definition**: `apps/api/src/router.ts`
* **Type export**: `apps/api/src/types.ts`

```ts theme={null}
// apps/api/src/types.ts
export type { AppRouter } from "./router";
```

Because `apps/api/package.json` exposes `types`, both clients can import
`AppRouter` from the `api` package without any manual codegen.

## Mobile client usage

```ts theme={null}
// apps/mobile/lib/trpc/client.ts
import type { AppRouter } from "api";
import { createTRPCReact } from "@trpc/react-query";

export const trpc = createTRPCReact<AppRouter>();
```

## Web client usage

```ts theme={null}
// apps/web/lib/trpc.ts
import type { AppRouter } from "api";
import { createTRPCReact } from "@trpc/react-query";

export const trpc = createTRPCReact<AppRouter>();
```

## Extending routers safely

1. Add a new router in `apps/api/src/routers/`
2. Register it in `apps/api/src/router.ts`
3. Use the typed hooks on mobile/web immediately

Type safety is automatic as long as:

* The client imports `AppRouter` from `api`
* You avoid `any` or casting tRPC hooks

## Common pitfalls

* **Missing router registration**: If a router isn’t added to `appRouter`,
  clients won’t see it.
* **Casting away types**: Avoid `as any` or `as unknown` at the tRPC boundary.
* **Out-of-date builds**: Restart the mobile/web dev server after adding new
  procedures to pick up updated types.

## Troubleshooting

* **Types missing**: ensure `apps/api/package.json` points to `types`
* **Type errors in mobile**: restart dev server to pick up changes

## Next Steps

* [Feature Registry](/mobile/feature-registry)