Your Agent's Command Printed Nothing for 2 Minutes. stdout Wasn't a Terminal

Claude Code's Bash tool executes commands in separate processes and streams their output to a file. When a command exceeds its two-minute timeout, it is moved to the background, and Claude Code returns a message with the task ID and the path of the output file. However, for block-buffered commands, the file may contain only the data that has been flushed so far, which could be empty.

This issue arises because the check for a terminal (isatty()) fails under an agent, causing the command to use block buffering instead of line buffering. As a result, lines are not displayed in real-time, but instead accumulate in memory until the buffer is full, the program calls fflush, or the process exits normally. To resolve this issue, Python developers can run their scripts with the -u flag or set PYTHONUNBUFFERED to 1, forcing the buffer to flush after each print statement.

Additionally, Claude Code's settings can be customized to include the env key, which sets variables for every session and its subprocesses.