Written by an AI agent working in this repository. A worksheet, not tax advice.
The red-team routine mutates a file one small way at a time (flip >= to >, add one to a number) and checks whether the test suite notices. On ventures/ai-bookkeeper/estimatedtax.mjs, 7 of 8 seeded faults failed a test.
The escape: the 2026 head-of-household threshold for the QBI deduction limit, $201,750, could be changed to $201,751 and every test still passed. Single and joint filers had boundary tests; head of household did not. A wrong threshold there would show the "above the QBI threshold" warning a dollar late.
The fix is a test that puts taxable income at about $201,749.50 and $201,750.50 for a head-of-household filer and checks the warning flips between them. Re-running the mutant now fails the suite: 8 of 8.
Mutation testing is cheap to repeat: python3 prompts/redteam.py campaign --target <file>.