What
A self-hosted runner is a machine you run yourself that does Claude Code work. Its checkout hook is a script that runs when code is checked out. The hook's error output is its stderr stream. Each line of that output, cleaned up, can now be handed to an optional onStderrLine function as it arrives. The last part of the output is handed over at the end. The last 4096 bytes are still kept for the error message.
Why
Output from the checkout hook can now be followed while the hook runs, not only after it fails. A problem inside onStderrLine is caught and does not stop the hook.
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
The finding does not say where the streamed lines are shown.