NestJS "nest build" error TS with decorators / emitDecoratorMetadata in CI

nest build ran tsc and the compiler rejected the decorators, or compiled without emitting the type metadata Nest needs. The fix is enabling the two decorator options in tsconfig.

What this error means

nest build fails with a decorator-related "error TS" (for example TS1241 or TS1240), or the build passes but DI later fails because parameter type metadata was not emitted.

NestJS

src/cats/cats.service.ts:7:3 - error TS1241: Unable to resolve signature of parameter
decorator when called as an expression.
  Argument of type 'undefined' is not assignable to parameter of type 'Object'.

Common causes

experimentalDecorators is not enabled

Without experimentalDecorators, tsc treats the legacy decorator syntax Nest uses as invalid and errors out.

emitDecoratorMetadata is off

Nest DI reads design-time type metadata; without emitDecoratorMetadata no metadata is emitted, so injection cannot infer types.

How to fix it

Enable both decorator options in tsconfig

Open the tsconfig.json that nest build uses.
Set experimentalDecorators and emitDecoratorMetadata to true.
Rebuild so decorators compile and metadata is emitted.

tsconfig.json

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "target": "ES2021"
  }
}

Confirm nest build reads the right tsconfig

nest build uses the path in nest-cli.json; make sure it points at the tsconfig that has the decorator options.

nest-cli.json

{ "compilerOptions": { "tsConfigPath": "tsconfig.build.json" } }

How to prevent it

Keep experimentalDecorators and emitDecoratorMetadata true in every tsconfig used to build.
Extend a single base tsconfig so build and test configs stay in sync.
Run nest build in CI on every PR so config drift is caught early.