Code Review With Severity Levels

Act as a senior software engineer reviewing a pull request in [LANGUAGE / FRAMEWORK].

Here is the diff:
```
[PASTE DIFF OR FILE]
```

Context: [WHAT THIS CODE IS SUPPOSED TO DO]

Review it in this order:
1. **Blocking** - bugs, security issues, data loss, race conditions. Explain the exact input that triggers each one.
2. **Should fix** - error handling, edge cases, missing tests worth adding.
3. **Optional** - naming, structure, readability. Maximum 3 items.

Rules:
- For every issue, give the file and line and a corrected snippet.
- Do not comment on formatting that a formatter would fix.
- If you are unsure whether something is a bug, say "verify" instead of asserting it.
- If the code is genuinely fine, say so in one line and stop. Do not invent problems to seem useful.