Problem

Module augmentation is usefull for declaration merging https://www.typescriptlang.org/docs/handbook/declaration-merging.html#module-augmentation
When using tsup to generate .d.ts using experimentalDts, it's ignore module augmentation. Change to dts will resolve this problem.

Version Info

• @microsoft/api-extractor 7.38.3
• tsup 8.0.1
• typescript 5.3.2

Reproduce

• clone this repo https://github.com/axmad386/repro-tsup-augmentation
• install node_modules (pnpm install)
• build using tsup (pnpm build)
• try to change tsup.config between dts and experimentalDts

Expectation

experimentalDts should generate module augmentation like dts do. This line should be generated on .d.ts file

declare module "axios" {
    interface Axios {
        dummy(): string;
    }
}