See the Diff First, Then Change the Design — The Confirm Button Stays in Your Hands

When you let an AI change a design, the worst fear is that it quietly changes something you didn't notice. LensBench's approach: every change is dry-run first.

When the AI wants to change a thickness, add a variable or run an optimization, it does a dry run first:

• It gives a text diff: exactly which numbers changed
• It gives the key first-order quantities before and after: focal length EFFL, back focal length BFL, F-number FNO, total length TOTR
• It writes only after you confirm; after writing, you can undo at any time

Every change leaves a trace ​

Every modification the AI makes to a design is recorded in the change log: time, source, command and parameters, diff, and the before/after first-order quantities, with full-text snapshots of both versions. On the desktop app's timeline, the AI's changes and your manual ones are shown in different colors.

• To go back to before any change: history_restore takes you back in one step (it also shows the diff first, and is undoable)
• To find out "what changed, and did it get better or worse": compare puts any two versions side by side, comparing first-order quantities, prescription differences and image quality

Some things are left to people ​

• Sign-off is for people only. The AI cannot sign off for you.
• The direction of a metric must be asked about. When turning a sentence into an acceptance metric, the AI must first confirm the direction and value with you. Take "double the F-number": do you want it faster (the F-number gets smaller), or the F-number's value doubled? The two readings go in opposite directions, so the AI must ask.
• Long tasks don't change the design behind your back. An optimization can go to the background, with progress and cancellation at any time; it runs on a snapshot of the design as it was when it started, and when it finishes it hands back a patch for you to confirm.