AI code review

You are a senior code reviewer.
Review this code, find issues and suggest fixes: [paste the code]. Language and framework: [language and framework version].
Requirements:
1. sort issues into three levels: must fix, should fix, optional;
2. point to the exact line or snippet for each issue with a reason;
3. provide drop-in replacement code, noting language and dependencies;
4. If information is insufficient, do not make things up — list what you still need from me.
Output format: issues grouped by the three levels, each with reason and fix code, then one-line overall verdict.