To add an npm package as a development dependency, use bun add --development.

bun add zod --dev
bun add zod -d # shorthand

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This will add the package to devDependencies in package.json.

{
  "devDependencies": {
    "zod": "^3.0.0"
  }
}

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See Docs > Package manager for complete documentation of Bun's package manager.