Unclear It is not clear whether setting CLAUDE_CODE_PLAIN_DELTA_FAST_PATH turns this route on, or whether it is already on by default.
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
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
It is not clear whether setting `CLAUDE_CODE_PLAIN_DELTA_FAST_PATH` turns this route on, or whether it is already on by default.
The name it cites is new in this build
New in this build: CLAUDE_CODE_PLAIN_DELTA_FAST_PATH