A safe first-change guide for systems whose conventions and invariants are not obvious from the repository.
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Key takeaways
Map runtime behavior first.
Name invariants and traps.
Start with a reversible slice.
Draw the runtime map
Trace one user request from entry point to data or external effect. Note where validation, authorization, caching, and error handling occur.
A map is more useful than a broad orientation lecture because it gives the developer a path to inspect.
Find the invariants
Ask the system owner what must never change: ordering, tenant isolation, accounting, or compatibility. Add a focused test before changing the fragile path.
Record known traps and the reason they exist, including any temporary workaround.
Change one boundary
Choose a small, observable edit with a rollback path. Review behavior and documentation together so future work does not repeat the discovery.
Do not start with a rewrite simply because the system is difficult to explain.
