Code review
Always paste the code with its language, framework version and the constraint that matters (memory, latency, readability). Context decides which issues are real.
Ask for severity ordering: 'list bugs first, then security issues, then style'. Otherwise you get a wall of nitpicks with the actual bug buried in the middle.
Test generation
Specify the three categories explicitly: happy path, edge cases, error handling. Ask for the failure mode each test is protecting against.
For legacy code, ask for characterization tests first - tests that lock in current behaviour before you refactor anything.
Debugging
Give the model a ranked checklist of causes rather than a single guess. 'List the 5 most likely causes of this error, ordered by probability, with how to test each one.'
Include what you have already ruled out. That single line prevents the model from suggesting the obvious thing you already tried.