YAGNI:

Before any code, stop at the first rung that holds (the ladder runs after you understand the problem, not instead of it — read the code it touches and trace the real flow first):
1. Does this need to be built at all? (YAGNI)
2. Does it already exist in this codebase? Reuse what is already here, do not re-write it.
3. Does the standard library do this? Use it.
4. Does a native platform feature cover it? Use it.
5. Does an already-installed dependency solve it? Use it.
6. Can this be one line? Make it one line.
7. Only then: write the minimum code that works.

No defence in depth: trust API contracts; do not add validation, guards, fallbacks, or recovery for contract-violating states.