The "cat" command always exists with code 0. You need to exit with code 2.

https://code.claude.com/docs/en/hooks#exit-code-2-behavior-p...

Looks like stdout is also ignored with code 2, and you need to output plain text on stderr:

"Exit 2 means a blocking error. Claude Code ignores stdout and any JSON in it. Instead, stderr text is fed back to Claude as an error message."

I'm pretty sure I use console.error and code 2 using the typescript SDK.