Migration from trpc-to-openapi

If you're using trpc-to-openapi, migrating is straightforward. The API is intentionally similar.

1. Swap the package

pnpm remove trpc-to-openapi
pnpm add trpc-rest

2. Update imports

// Before
import { createOpenApiHttpHandler } from 'trpc-to-openapi';
import { generateOpenApiDocument } from 'trpc-to-openapi';

// After
import { createOpenApiFetchHandler } from 'trpc-rest';
import { generateOpenApiDocument } from 'trpc-rest';

3. Replace the handler

The main difference: createOpenApiFetchHandler takes a Fetch Request and returns a Response. No framework-specific adapters.

// Before (Express adapter)
app.use('/api', createOpenApiHttpHandler({ router: appRouter, createContext }));

// After (any framework — just pass a Request)
const response = await createOpenApiFetchHandler({
  router: appRouter,
  endpoint: '/api',
  req: request,
  createContext,
});

Next.js App Router

app/api/[...trpc]/route.ts

import { createOpenApiFetchHandler } from 'trpc-rest';

const handler = async (req: Request) => {
  return createOpenApiFetchHandler({
    router: appRouter,
    endpoint: '/api',
    req,
    createContext: async ({ req }) => createContext(req),
  });
};

export { handler as GET, handler as POST, handler as PUT, handler as PATCH, handler as DELETE };

4. Update the generator

The generator API is the same. errorSchemas and extensions are new features you can optionally adopt.

5. Remove patches

If you had pnpm patches for trpc-to-openapi, remove them:

rm patches/trpc-to-openapi*.patch

Key differences

Behavior	trpc-to-openapi	trpc-rest
Handler input	Framework-specific	Fetch `Request`
Procedure invocation	Internal calling	`createCaller` (full middleware)
Bodyless POST	415 error	Works correctly
Error response caching	Shared by status code	No caching issue
Optional param descriptions	Stripped by unwrapping	Preserved
Error format	Nested `data.error`	Flat `{ message, code, issues? }`

What's NOT supported

Express/Fastify adapters — use their Fetch compatibility layer instead
Content negotiation beyond JSON and form-urlencoded

Migration from trpc-to-openapi

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