blast-radius

name	blast-radius
description	Use before shipping a risky change or reviewing a diff you don't trust: finds what it breaks elsewhere, beyond the diff, and proves the one safety fact by running real code instead of writing it up.

Blast radius

Find what a change breaks somewhere else, before it ships. Use for "blast radius of X", "what could this break", or reviewing a small diff you don't trust yet.

Companion to code exploration and archaeology. A locator agent (the explorer agent, or Explore) tells you what the code does. git log, git blame, and gh pr view tell you why it's shaped that way. Blast radius tells you what it breaks somewhere else.

Listing the callers is not the job. The agent can grep those in a second. The job is the breakage grep won't show you.

Don't trust your own writeup

A blast-radius writeup that sounds right is worthless. It reads as convincing whether or not it's true, and that is the trap you are walking into. So don't hand back the writeup. Find the one or two facts the whole thing depends on and prove them by running code. Words are where you start, not what you ship.

How sure are you

For each fact the change's safety depends on, get it as far down this list as is cheap, and say where it stopped.

1. You said so. Worthless on its own.
2. You pointed at the line. A real file:line, or the library's own source.
3. You showed the bad case can't happen. You walked the failure step by step and it doesn't reach.
4. You ran it. A script or test that calls the real code and fails loud if you're wrong.
5. You reproduced it in the running app.

Any safety fact you can't get to step 4, say so out loud. Don't write it up as settled. Step 4 is usually one small script that imports the same library the app ships and calls the exact function you're worried about.

Steps

1. Read the change. The diff, the symbols it adds, changes, and deletes, and what it now does differently, including the part the diff doesn't spell out. Pull the PR and commit history yourself: git log --oneline -20 -- <file>, git blame -L <start>,<end> <file>, and gh pr view <number> --json title,body,comments,reviews for anything substantive.
2. Find the one fact it's safe because of. Most changes that look scary are safe because of a single fact, like "this call only drops already-dead cache entries and does nothing else". Find that fact. If it holds, most of the scary cases die at once. Spend your time here, not on a long list of maybes.
3. Look where grep stops. Read the source of the library you call, and check its pinned version and any local patch. Work out when things run: microtasks, unmount and teardown, one framework's lifecycle versus another's. Follow what a symbol search misses: the JSON an API returns, a DB column, a wire format, another language reading the same bytes, a feature flag, code three hops downstream. Delegate wide, read-only fan-out to the explorer agent or Explore when the search spans more files than you can hold in one pass.
4. Be honest about each risk. Give it a real chance of happening and a real cost if it does. Keep the risks you confirmed; list the ones you checked and cleared separately. Cite a real file:line, a search that finds nothing is still an answer, and never make up a caller or an API.
5. Prove the one fact. Write a script or test that runs the real code, run it, and paste what happened. If you can't prove it cheaply, mark it unproven. Don't round up.
6. For a big or wide change, fan out instead of relying on one pass: launch several Agent tool calls in a single message, each given the same question (or a distinct slice of the diff), then read every result and merge the answers by hand. Different agents catch different real bugs; merging is your job, not automatic.

What to hand back

• What it does. What changed, including the part that isn't obvious.
• The one fact it's safe because of. State it, say which step you got it to, and show the proof. If you couldn't prove it, write unproven.
• Risks. Only the real ones. Each names how it breaks, the file:line, how likely and how bad, and how to check. Paste the proof for the ones that matter.
• Cleared. What you checked and why it's fine.
• Before you merge. The cheapest test or repro that catches the real bug, including the script you wrote.

Write it through unslop, cite real code, and strip anything private before it goes anywhere public.

Reply: the writeup above, with the one safety fact either proven or marked unproven.