add-parameter-duplicate

Id

@azure-tools/typespec-client-generator-core/add-parameter-duplicate

Severity: error

This diagnostic is issued when addParameter is called with a model property whose name already exists on the operation.

Impact

• Area: Client customization transformations. Blocks addParameter from producing a valid customized SDK method signature when the added parameter name already exists.
• Not affected: The original service operation remains unchanged.

❌ Incorrect Usage

@service
namespace MyService {
  op myOp(name: string): void;
}

model ExtraParams {
  name: string; // duplicates existing parameter `name` on `myOp`
}
alias Modified = addParameter(MyService.myOp, ExtraParams.name);

Diagnostic Message

TCGC reports:

Parameter "name" already exists in operation "myOp".

✅ How to Fix

Choose a unique parameter name or use replaceParameter when the intent is to replace an existing parameter.

@service
namespace MyService {
  op myOp(name: string): void;
}

model ExtraParams {
  description: string;
}
alias Modified = addParameter(MyService.myOp, ExtraParams.description);