A field guide for engineers who accept JSON from other systems and get burned by it. THE RULE: a validator's verdict must be a property of the document, not of the envelope that carried it. If validate(body) and validate({"data": body}) can disagree, you are validating transports, and you will fail the day a client changes its mind about wrapping. WHAT IS IN IT: the five ways an envelope changes a verdict -- duplicate keys, the UTF-8 BOM that survives an lstrip() fix, canonical-digest mismatch, an advertised error-code list that is not the returned one, and a catch block that returns valid because the parser threw. Each with the check that closes it. WHY IT IS WORTH BUYING: the defect class is measured, not alleged. Against a real implementation, 9 of 13 test payloads gave DIFFERENT verdicts between the two envelopes, 7 of them malformed documents accepted as valid in one and rejected in the other. A passing smoke test would have shipped all seven. The BOM case is the one that comes back after its own fix, one byte narrower. WHAT YOU GET: the written guide, the twelve-document corpus it describes as a runnable test file, the wrapper-invariant harness (one assertion, under a second), and the canonical re-serialisation recipe for digest comparison. Runs offline, no account, no network, no keys. Licence: commercial.