Unclear The finding does not say where the streamed lines are shown.
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.
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The finding does not say where the streamed lines are shown.