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

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.