The number says whether it worked. The words travel separately, and they travel on more than one channel.
stdin, stdout and stderr
Also called Standard streams, Standard output, Standard error
Three channels every process starts with. Stdin is what comes in. Stdout is its results. Stderr is for errors and remarks. On screen, stdout and stderr look alike, but they are separate and can each be sent somewhere different.
An agent reads these the way you read a screen. The split lets a program pass clean results onward while its complaints still reach someone. Agent hooks, scripts that run at set moments in an agent's work (chapter 6), speak this too: they get their data on stdin and answer on stdout or stderr.
“Send only the JSON result to stdout, and every progress and error message to stderr, so that mytool | jq . keeps working when something goes wrong.”