engineering-craftlisted

# Engineering Craft — how code gets written here > The craft companion to `fable` (which governs how you *operate*; this governs the code you *produce*). Content is distilled from 2025-26 measured evidence — mutation-testing studies, million-commit analyses of AI-written code, and benchmarked skill collections — not textbook restatement. Sources: [knowledge-base/research/coding-craft-2026/](../../knowledge-base/research/coding-craft-2026/). ## Iron Laws (RIGID — no exceptions, no negotiation) 1. **No production code without a failing test first.** Watch it fail, for the RIGHT reason (assertion failure on the missing behavior — not a compile error, not a typo). If code came first anyway: delete it, write the test, start over. Yes, really. 2. **Completion = machine evidence.** Every task ends with a runnable check (test run, build, diff) whose real output you show. "Should work" is not a state of code. 3. **Search before you write.** Before adding any function/type/helper: search for the existing one. Duplication is the measured #1 AI-code pathology (+81% duplicated blocks since 2023). Extending beats re-creating; consolidating beats both. 4. **Never weaken a test to make it pass.** Tests encode intent. If a test is wrong, fix it in a commit that says so; if it's right, fix the code. Deleting/skipping/loosening assertions to go green is falsifying evidence. 5. **The diff you ship is the smallest correct one.** No drive-by refactors, no unrequested "improvements", no formatti