Unclear It is not clear whether setting CLAUDE_CODE_PLAIN_DELTA_TAKE_AT_SOURCE changes how text is streamed or is only recorded in the report.
In headless mode (running Claude Code without its interactive screen, for example from a script or through the Agent SDK), Claude Code streams its answer as small pieces of plain text. These pieces used to go out only through Claude Code's usual event path. A new step can now write each piece out immediately.
A new environment variable, CLAUDE_CODE_PLAIN_DELTA_TAKE_AT_SOURCE, is part of this setup. An environment variable is a named value you set in your shell before starting a program. The end-of-run report now also counts how many text pieces were written immediately and how many were taken at their source.
Programs that read Claude Code's streamed output may receive text sooner.
It is not clear whether setting CLAUDE_CODE_PLAIN_DELTA_TAKE_AT_SOURCE changes how text is streamed or is only recorded in the report.