I've tried --tsconfig on the cmdline, tsconfig and esbuildOptions(options) options.tsconfig in the tsup.config.ts file, nothing works.

In your test if I add

"target": "es5"

to compilerOptions, and

'--format','cjs',

it should fail with

✘ [ERROR] Transforming const to the configured target environment ("es5") is not supported yet

But it doesn't.

So I think the test is too simple and doesn't actually test that the custom tsconfig file is used.