Use when adding data, config, state, constants, an enum-like string, a cache, or anything that could exist in two places - or when two sources already disagree. Encodes the one-fact-one-source doctrine for code and data: derive rather than store, extend the owner, absorb duplicates. Use at the moment copying a value feels faster than referencing it.
Install
npx skills add riekelt/principal-engineer --skill keeping-one-source-of-truthnpx skills add riekelt/principal-engineer --skill keeping-one-source-of-truth -a claude-code -gnpx skills add riekelt/principal-engineer --skill keeping-one-source-of-truth -a codex -ggemini skills install https://github.com/riekelt/principal-engineer.git --path plugins/principal-engineer/skills/keeping-one-source-of-truthgh skill install riekelt/principal-engineer keeping-one-source-of-truthgit clone --depth 1 https://github.com/riekelt/principal-engineer /tmp/riekelt__principal-engineer
mkdir -p ~/.claude/skills ~/.agents/skills
cp -R /tmp/riekelt__principal-engineer/plugins/principal-engineer/skills/keeping-one-source-of-truth ~/.claude/skills/keeping-one-source-of-truth # Claude Code
cp -R /tmp/riekelt__principal-engineer/plugins/principal-engineer/skills/keeping-one-source-of-truth ~/.agents/skills/keeping-one-source-of-truth # Codex, Cursor, Gemini CLI, Copilot, OpenCode