Some requirements are too intricate or too tacit to write down, but working code somewhere already embodies them. The best reference is not a screenshot or a description — it's source. Read it like a spec, then reimplement the semantics, not the syntax.
Steps
- Get the reference: a repo path, a vendored folder, a library name, or a site whose underlying code can be read. Ask what specifically to extract from it — behavior, structure, visual system, API shape — so you don't imitate the wrong dimension.
- Read the reference and produce a semantics summary before writing any code:
- the behaviors and guarantees it implements (timing, ordering, error handling, edge cases),
- the decisions that look deliberate versus incidental,
- anything that won't translate to the target language or stack, with a proposed equivalent.
- Have the user confirm the semantics summary. This is the moment misreadings get caught cheaply.
- Reimplement in the target stack: same semantics, native idioms. Do not transliterate line by line, and do not copy code verbatim from references whose license doesn't allow it — note the license if it's unclear.
- Close the loop: list each behavior from the summary and where the new implementation honors it, plus any place you consciously diverged and why.
Guardrails
- The reference defines what; the target codebase's conventions define how.
- If the reference itself turns out to be buggy or inconsistent, surface that instead of faithfully reproducing the bug.
- Respect licenses: extracting semantics is fine; copying incompatible code is not.