When the modal asks “delete this file?” the right question isn’t will this delete a file? (yes, that’s the action). 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:
- Just this chat. Claude thinks something through, writes a note, sets something up in memory for the conversation. Nothing outside the chat moves.
- A scratch file. Creating a throwaway file in a notes folder you alone use. Touches nothing else.
- Your project. Editing files, installing a package, anything inside one folder you can roll back (you’ll see how in Lesson 5.8).
- Your machine. Touching files outside the project — your settings, your documents, anything other apps depend on.
- Other people. Pushing code, sending an email, calling a paid service, posting to Slack. The internet doesn’t forget.
The top rung you can approve and move on. The bottom rung is where you stop and actually read.
The trick: same action, different blast
This is the part that trips people up.
“Delete notes.txt” looks identical in two different folders. In a scratch folder you made yesterday, it’s rung 2 — nothing of value lost. In a project you haven’t backed up yet, it’s rung 3 — you may have just deleted an hour of work. Somewhere in your documents, it’s rung 4 — something else may have depended on that file.
The action doesn’t tell you the blast radius. The where does. That’s why the last lesson told you to read the location before the action.
Reading sensitive files is blast too
Worth naming: blast isn’t only about changing things. If Claude reads a file full of secrets — passwords, keys, customer data — those secrets are now in the conversation, and conversations can be copied, shared, logged. Reading a sensitive file is rung 4 or 5, not rung 1, even though “just reading” 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.