Blast radius

When the prompt asks “delete this file?” the right question isn’t will this delete a file? (yes, that’s the command). The right question is what does that delete actually touch?

Blast radius: how far the consequences of an action can reach if it goes wrong.

Same word the military uses for explosions. Same idea. A small one stays in the room; a big one takes down the block.

The five-step ladder

Every action you approve sits somewhere on this ladder, from smallest blast to largest:

Your terminal session. Setting a variable, printing some text. Goes away when you close the window.
A scratch folder. Creating a file in /tmp or a notes directory you alone use. Touches nothing else.
Your project. Editing source files, running npm install, anything inside one folder you can rebuild from git.
Your machine. Touching your home directory, your shell config, system files, anything outside the project that other apps depend on.
Other people. Pushing code, sending an email, calling an API that bills your team, posting to Slack. The internet doesn’t forget.

The top rung is fine to click through. The bottom rung is where you stop and actually read.

The trick: same command, different blast

This is the part that trips people up.

rm file.txt looks identical in two different folders. In a scratch directory you made yesterday, it’s rung 2 — nothing of value lost. In a project you haven’t pushed yet, it’s rung 3 — you may have just deleted an hour of work. In your home directory, it’s rung 4 — something else may have depended on that file.

The command doesn’t tell you the blast radius. The where does. That’s why the last lesson told you to read the directory line before the command line.

Reading sensitive files is blast too

Worth naming: blast isn’t only about changing things. If Claude reads your .env file, those secrets are now in the conversation — and conversations can be copied, shared, logged. Reading sensitive files is rung 4 or 5, not rung 1, even though “read” sounds harmless.

What’s next

Blast radius answers “how far?” The next lesson answers a sharper question: not how far, but whether you can take it back at all.