Describe the bug

Hi! What a breeze to migrate from tsup to tsdown 😍

I made the migration (super simple 👌 ) on my library https://github.com/victorgarciaesgi/regle, and noticed a output problem (not breaking). I open this bug here because in the docs it says to open it here for all .d.ts related problems.

I use a mix of minify and outExtensions to output different files format.

When compiling without the outExtensions option, it correctly generated the .js and .d.ts.

But when applying the options, it will try to compile the .d.mts file, resulting in an unnecessary and wrong compiled file

In the reproduction repo (main branch)

• pnpm install
• pnpm run build
• Check ./packages/core/dist

Common config for prod build is the root tsdown.common.build.ts
Config for package core project is here packages/core/tsdown.config.ts

Reproduction

https://github.com/victorgarciaesgi/regle

System Info

System:
    OS: macOS 15.4.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 315.06 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - ~/.nvm/versions/node/v22.14.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v20.14.0/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.14.0/bin/npm
    pnpm: 9.7.0 - ~/.nvm/versions/node/v20.14.0/bin/pnpm
    bun: 1.0.35 - /opt/homebrew/bin/bun
  Browsers:
    Chrome: 137.0.7151.41
    Chrome Canary: 138.0.7202.0
    Edge: 136.0.3240.92
    Safari: 18.4
    Safari Technology Preview: 18.4

Used Package Manager

npm

Validations

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• Read the Contributing Guide.
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• The provided reproduction is a minimal reproducible of the bug.