Unclear The value that turns it on and the exact cases where the shortcut applies are not known.
What
There is a new environment variable, CLAUDE_CODE_PLAIN_DELTA_FAST_PATH. An environment variable is a named setting you give a program when you start it. It applies to headless sessions, meaning Claude Code runs without its interactive screen, for example in print mode or when another program drives it.
CLAUDE_CODE_PLAIN_DELTA_FAST_PATHturns on a shortcut,plainDeltasNeedOnlyEmit, in the part that runs headless turns. With it, plain streamed pieces of a reply ("deltas") go straight to output.- When the shortcut is in effect, the session logs how many streamed pieces went straight to output during the turn.
The code that reads the variable was not traced.
Why
If you run Claude Code headless, there is a new opt-in switch that changes how streamed reply text reaches the output. The log line shows how often the shortcut was used in a turn.
Names in the bundleCLAUDE_CODE_PLAIN_DELTA_FAST_PATH
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The value that turns it on and the exact cases where the shortcut applies are not known.
The name it cites is new in this build
New in this build: CLAUDE_CODE_PLAIN_DELTA_FAST_PATH