Prisma 7 TypeScript Error Reproduction

Minimal reproduction case for prisma/prisma#28620

Issue

When using Prisma 7 with dual schema setup (two different databases), TypeScript throws an error during npm run build:

Type error: This expression is not callable.
  Each member of the union type '(<T extends AccountFindFirstArgs>(args?: SelectSubset<T, AccountFindFirstArgs<DefaultArgs>> | undefined) => Prisma__AccountClient<GetFindResult<$AccountPayload<DefaultArgs>, T, { ...; }> | null, null, DefaultArgs, { ...; }>) | (<T extends AccountFindFirstArgs>(args?: SelectSubset<...> | undefined) => Prisma__Account...' has signatures, but none of those signatures are compatible with each other.

This error occurs at the findFirst call in src/app/actions.ts:6:40.

Setup

This project demonstrates the issue with:

• Next.js 16.1.6 with TypeScript
• Prisma 7.4.0 with dual schema configuration
• Two Prisma schemas:
  • prisma-1/ - MySQL schema
  • prisma-2/ - SQLite schema
• Unified Prisma client in src/lib/prisma.ts that switches between clients based on environment

Project Structure

prisma-7-bug/
├── prisma-1/
│   ├── schema.prisma      # MySQL schema
│   └── types/             # Generated MySQL client
├── prisma-2/
│   ├── schema.prisma      # SQLite schema
│   └── types/             # Generated SQLite client
├── src/
│   ├── app/
│   │   ├── actions.ts     # Server action with findFirst call (ERROR HERE)
│   │   └── page.tsx       # Page that calls the action
│   └── lib/
│       └── prisma.ts      # Unified client that imports both schemas
├── prisma.config.ts       # Prisma config for dual setup
└── .env                   # Environment variables

Reproduction Steps

1. Install dependencies:

   npm install

2. Generate Prisma clients:

   npx prisma generate --schema ./prisma-1/schema.prisma
   npx prisma generate --schema ./prisma-2/schema.prisma

3. Run build (reproduces the error):

   npm run build

Expected Behavior

The build should complete successfully without TypeScript errors.

Actual Behavior

TypeScript compilation fails with the error:

./src/app/actions.ts:6:40
Type error: This expression is not callable.

The error occurs at prisma.account.findFirst({...}) in the server action.

Key Files

src/lib/prisma.ts

This file creates a unified Prisma client by:

1. Importing both PrismaClient from the two generated schemas
2. Re-exporting types from prisma-1
3. Conditionally creating either client based on USE_SQLITE env var

src/app/actions.ts

Contains a minimal server action that calls prisma.account.findFirst(), which triggers the TypeScript error during build.

Environment

• Next.js: 16.1.6
• Prisma: 7.4.0
• TypeScript: 5.x
• OS: Linux (Fedora)

Notes

• The error only occurs during Next.js build, not during runtime
• The error is specific to TypeScript type checking
• Both schemas have identical Account model definitions
• The issue appears to be related to type union incompatibility when combining two Prisma clients