v4.3.1 CLI fails with [Error: Invalid handle] when @source present

[alesvaupotic]

What version of Tailwind CSS are you using?

v4.3.1

What build tool (or framework if it abstracts the build tool) are you using?

tailwindcss -i ./css/app.css -o ./static/app-dev.css --watch --verbose

What version of Node.js are you using?

v26.1.0

What browser are you using?

N/A

What operating system are you using?

Windows

Reproduction URL

https://github.com/reepolee/tw-431

Describe your issue

@import "tailwindcss";
@source "src/**/*";

When running CLI on css file with @source in it, build fails with error message

$ tailwindcss -i ./css/app.css -o ./static/app-dev.css --watch --verbose
≈ tailwindcss v4.3.1

node:internal/modules/run_main:107
    triggerUncaughtException(
    ^

[Error: Invalid handle]

Node.js v26.1.0
error: script "css:watch" exited with code 1

[RobinMalfait]

While this definitely shouldn't crash, the auto content detection starts from where you run the CLI, which is the root of the project so the src/ folder should already be included.

That said, @source directives are relative to the file they are in, so that means that @source "src/**/*"; is relative to the css/ folder. Therefore css/src/ can't be found and crashes.

I'll keep this open until I fix the crash, but the @source "src/**/*"; can be dropped and it should work as expected.