Choose by input, operation, and destination
Start with the representation you have, the exact change you need, and the system that will consume the result. Preserve an untouched source and record any format, size, privacy, or compatibility rule before opening a tool. A technically valid result can still be wrong for its destination.
Structured data must remain exact
For JSON, CSV, YAML, SQL, URLs, and timestamps, decide which parser or importer is authoritative. Preserve identifiers, duplicate keys, number precision, quoting, time zones, and empty values instead of judging only by readable output.
Text or encoding must preserve intended units
For cleanup, case conversion, Unicode, Base64, and line-ending work, distinguish visible characters from code points, bytes, separators, and binary data. Compare a reversible operation with a round trip whenever the format allows it.
Security output requires an external authority
Decoded JWT claims, password estimates, hashes, CSP reviews, and response-header checks are diagnostic evidence, not authorization or a security guarantee. Confirm the final decision with the issuing service, deployed response, or approved security process.
Documents and images need a reopened result
For PDF and image work, keep the original, check page or image order, dimensions, orientation, transparency, readability, and output size, then reopen the downloaded file in a separate viewer before sharing it.
Generated output needs a real destination test
QR codes, markup, types, tables, fixtures, and formatted code should be tested with the scanner, compiler, renderer, schema, or importer that will actually use them. Treat browser output as a reviewable draft until that check passes.