fix: capture actual stderr in ProcessError instead of hardcoded message #529
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When the CLI subprocess exits with non-zero code, ProcessError was raised with stderr="Check stderr output for details" instead of the actual stderr content. This makes debugging impossible since real error messages (e.g. "No conversation found with session ID") are never surfaced. Changes: - Add _stderr_lines buffer to collect all stderr output - Drain stderr task group before checking exit code - Pass collected stderr content to ProcessError Fixes anthropics#437
stderr was only piped when a callback or debug mode was set. Without either, stderr went to the terminal and _stderr_lines stayed empty, making the ProcessError fix ineffective.
The CLI sends an 'errors' list in result messages (e.g. when a session cannot be found), but ResultMessage was missing this field so the error details were silently dropped during parsing.
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When the CLI subprocess exits with non-zero code, ProcessError was raised with stderr="Check stderr output for details" instead of the actual stderr content. This makes debugging impossible since real error messages (e.g. "No conversation found with session ID") are never surfaced.
Changes:
Fixes #437