How good should a story be for "tell me a time you solved a difficult bug"

It depends on the exact bug you fixed. Without more context regarding the specific problem you faced and your thought process in solving the bug, it would be difficult to determine whether it would be an appropriate example for a mid level position.

Since you are interviewing for a mid level position, the bar is likely going to be fairly high. I do recall a section in Alex's amazing E3 to E4 video series mentioning mid-level engineer expectations for debugging. Ideally, you would have an example involving a bug that was either:

• performance related
• hard to reproduce
• outside of the codebase

Hope this helps!

Those seem... fine? It's hard to tell without additional context. With behavioral interviews, it's more about behavior than impact (though impact is important too). FAANG understands that many candidates haven't worked with gigantic impact as they have never worked for Big Tech before - In those cases, they are looking more for behavior.

In terms of what behavior to show, I recommend going through the project credit section of the promotion course: https://www.jointaro.com/course/nail-your-promotion-as-a-software-engineer/its-not-just-about-impact/

Here are ways a bug can be more impressive outside of impact (largely expanding on what Helpful Tarodactyl said as they are correct):

• Hard to repro - Points for technical complexity
• Outside of your team's immediate codebase - Points for organizational complexity and expanding your scope
• Low observability - You need to make a lot of smart, educated guesses instead of simply following the logs
• Performance related - In other words, the bug isn't purely binary of feature clearly works or doesn't work and is more like "The feature works, but it has 1 extra second of delay that is very noticeable"