It's not the extension cord.

The cord is fine. Your breaker is tripping because something else is wrong — probably the tool or the breaker itself.

A 10-gauge cord rated for 30 amps just means it can safely handle that much current without overheating. It doesn’t make current flow. If your tool draws 10 amps steady, a 15-amp breaker shouldn’t trip. But motors pull a surge on startup — often 2-3x the running current. That 10-amp tool might spike to 25-30 amps for a split second, and if your breaker is old or sensitive, it’ll pop.

Also possible: a long, thin extension cord (yours is thick enough) can cause voltage drop, which makes the motor work harder and draw even more current. But with 10-gauge and a short run, that’s unlikely.

Check the tool for a short or mechanical binding. Swap the breaker — cheap ones wear out. And make sure you’re not running other loads on that circuit.

Start with the tool. That’s where I’d bet.