Write a Blast-Radius File Before Your First AI Patch

Your first AI patch should fail closed today. Do not ship a feature on day one. Prove one target file can revert cleanly now. You joined a messy repo this morning. The assistant wants a wide rewrite. Your job is a tiny reversible cut only. This drill gives you a blast-radius file first. You fill it before any model writes code. Then a short script checks the revert path. Why day-one AI diffs explode Cheap code is not cheap to unwind. One extra import can touch auth. One extra migration can lock deploys. You will not know the architecture yet. You also should not pretend otherwise. A blast-radius file makes unknowns explicit fast. If the file cannot name a revert, stop. You do not prompt for more code. You shrink the change until revert is boring. What you will build today You will add two artifacts on your branch. Keep both files in the first PR. blast_radius.py — the contract for this change. scripts/check_blast_radius.py — the fail-closed proof. The contract is the source of truth. The checker is the only merge gate. No green checker means no review yet. Step 1: Freeze one target file Pick one production file you can read. Do not pick a whole folder. Do not pick generated vendor code. git ls-files '*.py' '*.ts' '*.go' | head -n 40 TARGET = src/billing/invoice.py wc -l " $TARGET " git log -n 5 --oneline -- " $TARGET " Read the last five commits on that file. Write two plain sentences in notes. Pick a smaller file if you cannot yet. You now have a hard fence. Everything outside that fence is forbidden. Your assistant may not cross it. Step 2: Write the blast-radius contract Create blast_radius.py at the repo root. Keep the dict small. Fill every field with your own hands. # blast_radius.py # Day-one contract. Humans edit this. Models do not. BLAST = { " change_id " : " day-one-001 " , " intent " : " Add a fail-closed guard on invoice totals. " , " target_files " : [ " src/billing/invoice.py " , " tests/billing/test_invoice_flag_off.py " , " blast_radius.py " , " scr