blast-radius

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.

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. Existing tests, parity checks, and contract checks count when they run the real code and fail if the safety fact is wrong.

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 description and the commit messages for intent.
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, Solid versus React. 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.
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. Run the smallest existing check that proves it. For move-only work, normalized-body or AST parity plus route or contract proof is enough. Do not add a separate script or long report when those checks already prove the fact. If you can't prove it cheaply, mark it unproven. Don't round up.
6. For a big or wide change, spawn several parallel read-only reviewer subagents (the way the interrogate skill does), ask each the same question, and merge the answers. Independent runs catch different real bugs.