Type Sharing in Monorepos: The Schema-as-Contract Pattern

The most expensive bugs in multi-surface apps aren't logic errors — they're drift errors. The mobile app sends a field the API renamed. The web app reads a field the backend stopped returning.

The pattern

Generate types from your schema (Prisma, Zod, OpenAPI) and import them everywhere.

// Generated by Prisma, imported by web, mobile, and API
import type { Article } from "@repo/types";

What it eliminates

• Runtime errors from mismatched field names
• Manual type updates that miss a surface
• "It works on my machine" integration issues

When it breaks

The pattern fails if you hand-write types in one surface. The fix is a lint rule or CI check that surfaces can't define their own copy of shared types.