CLI Claude Code, interactive mode, seeded with flags-account. 0 added and 1 removed, of 214 lines, in the prompt 17 of 27 arms receive. 3 other prompts also changed. 1 of 31 tool descriptions changed. The appended system-reminder blocks moved: 1 line added, 1 line removed. Compared against v2.1.272.
The two arm counts on this page measure different things: 17 arms receive the prompt the headline is about, and the widest edit below reached 27, because the same changed lines sit in more than one prompt.
This arm was captured with the sandbox seeded from flags-account, this account's own GrowthBook payload, seeded into the sandbox .claude.json. 643 flags from data/flags/snapshots/flags-20260915T181004Z.json. What a seeded capture does and does not tell you.