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. Ported from Lauren "potato" Tan's blast-radius skill (cursor/plugins → pstack, MIT — license note at the bottom).

Where it sits in this toolkit: diagnose is post-hoc (something already broke), review/slim-review are boundary reviews of a finished diff. Blast radius is the pre-ship slot — impact analysis with forced proof, run before the PR boundary.

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 commits (gh pr view, git log -p) when the change is bigger than the working tree.
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, framework-specific timing. 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; 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 the question out: parallel-explore (several intentionally different runs, merged), or a review-council pass where that MCP exists. Different models catch different real bugs.

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.

Run unslop over the writeup, 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.

Connection to other skills

• diagnose — the post-hoc mirror: it starts from a symptom, this starts from a change. If the blast-radius pass finds the bug already live, switch.
• review / slim-review — boundary reviews of the finished diff; blast radius runs earlier and hands them the proven safety fact.
• burn-down — offers this pre-PR-boundary on items whose change touches a contract, wire format, or shared surface.
• unslop — the writeup goes through it before it goes anywhere public.

Origin

Ported from Lauren "potato" Tan's blast-radius skill — pstack/skills/blast-radius/SKILL.md in the Cursor plugins monorepo (github.com/cursor/plugins; the backnotprop/pstack repo is a stale mirror). MIT-licensed; substantial portions reproduced verbatim, so the upstream copyright and permission notice is preserved at UPSTREAM-LICENSE in this directory. House deltas: pstack sibling refs remapped (how/why companions cut — no local equivalents; why step-2 pointer replaced with plain gh pr view/git log -p; arena → parallel-explore / review-council; unslop → the workit port), "Solid versus React" generalized to framework-specific timing, toolkit-positioning paragraph and Connection section added. Upstream's disable-model-invocation: true kept — this stays user-invoked.

</supporting_info>