# Migrating from v11

> Serve v11 clients from the new router with createV11Handler, run the codemod, then move the clients.

Move the server first, so you can deploy before every client (a mobile app in the stores, a tab left open) updates. Then move the clients to `@trpc12/trpc/client` and drop the old endpoint:

1. **Serve v11 clients from the new router.** `createV11Handler` from `@trpc12/trpc/interop` speaks every v11 link on the endpoint they already call, next to the v12 handler ([Serving v11 clients](https://v12.trpc.io/v12/docs/migrations/from-v11/serve-v11-clients.md)).
2. **Run the codemod.** `npx @trpc12/upgrade src` rewrites what has one v12 form and lists the rest as `file:line:column` steps ([The codemod](https://v12.trpc.io/v12/docs/migrations/from-v11/codemod.md)).
3. **Move the clients.** Your own v11 links keep working in a v12 chain with `fromV11Link` until you rewrite them ([v11 links](https://v12.trpc.io/v12/docs/migrations/from-v11/links.md)).
4. **Drop the old endpoint** once no v11 client calls it.

[API changes](https://v12.trpc.io/v12/docs/migrations/from-v11/api-changes.md) maps each v11 API to its v12 form.

The [`examples/v11-migration`](https://github.com/trpc/trpc/blob/v12/next/examples/v11-migration/README.md) app follows these steps. Its v11 clients (batching, SSE and WebSocket subscriptions, superjson, TanStack Query) call the new server through `createV11Handler`. Its v12 client keeps a custom v11 link through `fromV11Link`. Its smoke runs the codemod on a copy of the v11 app and checks every step it prints.